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Mineralogical Applications of Crystal Field Theory
Abstract
This book provides an updated look at crystal field theory, one of the simplest models of chemical bonding, and its applications. After an introductory section chapters go on to describe: an outline of crystal field theory; energy level diagrams and crystal field spectra of transition metal ions; measurements of absorption spectra of minerals; crystal field spectra of transition metal ions in minerals; crystal chemistry of transition metal-bearing minerals; thermodynamic properties influenced by crystal field energies; trace element geochemistry in the crust and mantle; remote sensing compositions of planetary surfaces; and covalent bonding of transition metals. -A.W.Hall
... However, we cannot determine whether the Ni 2+ is octahedrally or tetrahedrally coordinated or perhaps both. The absorption features also cannot be interpreted fully using Tanabe-Sugano (T-S) diagrams for either d 8 or d 2 electronic configurations [d 2 corresponds to Ni 2+ in a tetrahedral crystal field (Sviridov et al. 1976;Burns 1993)]. We analyze the various issues. ...
... Burns (1993, Table 5.19), who refers to the results of Reinen (1970) 2 , gives a similar value of 9800 cm -1 for this band. A second spin-allowed transition of VI Ni 2+ , 3 A 2g → 3 T 1g ( 3 F), is assigned to a band at ~16 000 cm -1 (Sakurai et al. 1969;Burns 1993) and here Δ = 9800 and B = 907 cm -1 (Burns op. cit.). ...
... Finally, the third possible spin-allowed transition of VI Ni 2+ , 3 A 2g → 3 T 1g ( 3 P) should have a wavenumber of ~27 009 cm -1 . Burns (1993) and Sakurai et al. (1969) assigned this transition to a band at 27 000 and 27 500 cm -1 , respectively. Assuming that the first transition 3 A 2g → 3 T 2g ( 3 F) does have an energy of 10 000 cm -1 (i.e., band a in Fig. 3), our measured intense band c at ~16 790 cm -1 could be assigned to 3 A 2g → 3 T 1g ( 3 F). ...
Synthetic flux-grown end-member gahnite, ZnAl2O4, and several different colored crystals doped with one or more transition metals including Mn, Ni, Cr, Co, and Fe were studied by electron microprobe methods and UV/Vis/NIR single-crystal optical absorption spectroscopy. The first major objective was to measure and assign the various electronic absorption features. The second was to analyze quantitatively the crystal colors using the experimental spectra and the CIE 1931 color-space-chromaticity diagram. The microprobe results show that the doped gahnites have transition metal concentrations between about 0.001 and 0.1 cations per formula unit. The spectrum of colorless, nominally pure ZnAl2O4 displays no absorption in the visible region. Microprobe analysis of a light-blue gahnite crystal reveals small amounts of Ni and Mn. The UV/Vis/NIR spectrum does not indicate any dd-electronic transitions relating to Mn. All absorption features also cannot be fully interpreted using Tanabe-Sugano diagrams for Ni2+ in either octahedral or tetrahedral coordination. A series of seven slightly different colored gahnites with differing concentrations of Cr3+ and most also containing smaller amounts of Ni was investigated. The spectrum of a one pink crystal shows two intense absorption features in the visible region. They are assigned to spin-allowed 4A2g → 4T2g (4F) and 4A2g → 4T1g (4F) transitions of VICr3+. Other spectra display additional weak bands and lines that are most probably spin-forbidden dd-transitions of Ni2+. These gahnites with Ni and Cr show varying purple colorations depending on the concentrations of both metals. Two more deeply blue gahnites contain Co2+ as demonstrated by their UV/Vis spectra but not by microprobe analysis. Two intense absorption features at ~7440 and ~16 850 cm–1 are observed and assigned to the spin-allowed transitions 4A2 → 4T1 (4F) and 4A2 → 4T1 (4P) of Co2+, respectively. Complex absorption fine structure, caused by spin-orbit and/or vibronic interactions, is also observed. Three different gahnites with yellow to orange colorations contain measurable Mn. Their spectra are similar in appearance and display several weak IVMn2+ spin-forbidden transitions located above 20 000 cm–1. The spectra of two green gahnites show several Fe spin-forbidden electronic transitions arising from single, isolated IVFe2+ and VIFe3+ cations between 10 000 and 25 000 cm–1. The intensities of some of the VIFe3+-related bands can be increased through exchange-coupled interactions with next nearest IVFe2+ neighbors. The colors of various doped gahnites and the end-member galaxite are analyzed using their single-crystal absorption spectra in the visible region. Their dominant wavelength, λk, and hue saturation, pc, values are given on the CIE 1931 color-space-chromaticity diagram and are discussed. The Hex colors of all crystals are calculated and can be compared to those of the studied crystals.
... Note also, that the diagram in Figure 5 differs from that, for example, of andradite, where B = 613 and C = 3308 cm -1 giving C/B = 5.4 (Platonov and Taran 2018). Therefore, the energies of bands l, m, and r measured in the spectra of the almandine-pyrope garnets (Table 1) are lower than those obtained from Figure 5. Burns (1993) gives, without explanation, the relative energies of Ligand → Metal C-T transitions for different cations as Cr 3+ > Ti 3+ > Fe 2+ > tet. Fe 3+ > oct. ...
... The X cation in garnet has, however, D 2 point symmetry, and thus, the use of these diagrams is a simple approximation when attempting to assign Fe 2+ and Mn 2+ electronic transitions. Figure 4 shows the various transition energies for Fe 2+ of electronic configuration d 6 as a function of the crystal field strength, Dq (i.e., the bond length in a crystal or ligand field model is given by where 〈r 4 〉 is the mean value to the fourth power of the 3delectron radius, Z L is the effective ligand charge, R is the mean metal to ligand distance in the coordination polyhedron, and е is the charge of the electron (Dunn et al. 1965;Marfunin 1979;Burns 1993). Most of the transitions from the ground 5 E g state to the different excited levels, originating from the 3 H-, 3 P-, 3 F-, and 3 G-terms of free ions, do not indicate any large variation in energy as a function of Dq, with the exception of the 5 E g → 3 T 1g ( 3 H) transition. ...
... We start by considering the Tanabe-Sugano diagram for a cation with a d 5 configuration (Fig. 5). The local field bond strength for Mn 2+ in spessartine is given by Dq cubic = 8/9Dq octahedral in the crystal-field model (e.g., Marfunin 1979;Burns 1993). Therefore, one may assume, as a starting point and in a simple approximation, that the energies of the Mn 2+ bands for both coordination environments should not be too different. ...
The UV/Vis single-crystal absorption spectra of two almandine-bearing and several spessartine garnets were measured and their respective Fe 2+ and Mn 2+ spin-forbidden electronic transitions analyzed. Spin-forbidden bands of Fe 3+ are also considered, because many aluminosilicate garnets contain some Fe 3+. The spectra of the almandine-bearing garnets were recorded at room temperature between about 10 000 and 30 000 cm-1. The spectrum of a nearly end-member spessartine (97 mol% Mn 3 2+ Al 2 Si 3 O 12) was measured between about 15 000 cm-1 and 30 000 cm-1 at room temperature and 78 K, the latter for the first time. The 78 K spectrum shows absorption features not observed at room temperature. Five additional spessartine-rich garnets with different Mn 2+ /(Mn 2+ + Fe 2+) ratios, and two with unusual chemistries, were recorded up to 26 000 cm-1. The spectra of the two almandine-bearing garnets agree well with published results and show several overlapping Fe 2+/3+ bands located between about 14 000 and 25 000 cm-1. The spectra were deconvoluted to gain more insight into the electronic transition behavior. These results, together with an analysis of other measured spectra, reveal several absorption features that were previously unrecognized or misassigned. The spectrum of spessartine shows several Mn 2+ bands, and most are clearly spaced from one another. A synthesis of various UV/Vis spectroscopic results is made and assignments for the Fe 2+/3+ and Mn 2+ bands are attempted. The intensities of the Mn 2+ spin-forbidden bands and the ligand → metal charge edge observed in the various spessartine spectra are discussed. Spectra of almandine and spessartine have been interpreted using Tanabe-Sugano diagrams that are constructed for cations in octahedral coordination, point symmetry O h. However, such analysis does not appear to be fully successful because Fe 2+ and Mn 2+ in garnet have triangular dodecahedral coordination with point symmetry D 2. The interpretation of the spectrum of spessartine is especially problematic. An analysis shows that published model calculations of Fe 2+ electronic transition energies in garnet are not in good agreement with each other and are also not in full agreement with experimental spectra. First principles calculations are needed to better understand the spin-forbidden transitions of Fe 2+ , Fe 3+ , and Mn 2+ in garnet.
... This is commonly observed in emerald, which is the Cr and V rich variety of beryl (Ollier et al. 2015;O'Bannon and Williams 2016b). The intense and relatively sharp emission bands are commonly referred to as the R-lines and are associated with the spin-forbidden 2 E-4 A 2 transition, with the 4 A 2 state being the ground state and the 2 E state being split in non-cubic environments (Syassen 2008), and the broad band emission is assigned to the spin-allowed 4 T 2 -4 A 2 transition (Sugano and Tanabe 1958;Burns 1993;Gaft et al. 2005). Dravite has two crystallographically unique octahedral sites, Al and Mg, and emission bands from both sites would likely be observed in steady state luminescence spectra. ...
... Of these, the most well-characterized material is ruby (Al 2 O 3 :Cr), whose role as a pressure calibrant is well-known (Piermarini et al. 1975;Mao et al. 1986). Ruby is also the model system through which much of crystal field theory was developed (Sugano and Tanabe 1958;Burns 1993). The materials that have had their luminescent behavior probed under pressure share a few general properties: they are simple oxide systems, they are not hydrated, and they all have Al-sites that are close to ideal octahedra. ...
... The emission of Cr 3+ and V 2+ is typically characterized by relatively narrow intense R-lines associated with the spinforbidden 2 E-4 A 2 transition, with the 4 A 2 state being the ground state and the 2 E state being split in non-cubic environments (Syassen 2008). At high field strengths, the narrow band 2 E-4 A 2 emission typically dominates the fluorescence spectrum, while at low crystal-field strengths the generally broader (and spin-allowed) 4 T 2 -4 A 2 emission dominates the emission spectrum, and at intermediate field strengths both can occur together (Sugano and Tanabe 1958;Burns 1993;Gaft et al. 2005). In samples with significant Cr 3+ or V 2+ substitution, sharp neighbor lines (N-lines) generated by shared excitations between neighboring Cr 3+ or V 2+ centers can also be observed. ...
... The VNIR-active minerals absorptions are mainly caused by electronic processes, like the Charge Transfer Feature (CTF) and the Crystal Field Absorption Feature (CFA) in, for example, iron oxides (Curtiss, 1985;Morris et al., 1985;Burns, 1993;Cudahy and Ramanaidou, 1997). Three other processes, instead, cause the absorption features in the SWIR region (e.g., Laukamp et al., 2021): (1) first overtones of fundamental stretching vibrations of hydroxyl groups (2νOH) in hydroxylbearing minerals or a combination band of the OH-related stretching fundamental and a first overtone of a H 2 O-related bending vibration (2δH 2 O); (2) combinations (ν + δOH) of hydroxyl-related fundamental stretching (νOH) and bending vibrations (δOH), and (3) CO 3 -related combinations (e.g., 2ν 3 + ν 1 CO 3 ) or overtones (e.g., 3ν 3 CO 3 ) of fundamental stretching vibrations. ...
... The hyperspectral products obtained using the band ratio method allow the visualization of wavelength position variation (chemical composition) and absorption depth (absolute intensity) of diagnostic features shown along the spectral signatures of rock-forming and alteration minerals defining each pixel of the hypercube acquired in the study area. The minerals features considered for this study for mineral mapping are referenced in Table 1 (Burns, 1993;Cudahy and Ramanaidou, 1997;Laukamp et al., 2021 and references therein). ...
... The M 3 data are hyperspectral images acquired with a resolution of about 140 m/pixel in 85 narrow spectral channels in the range between 540 and 2950 nm, covering over 95% of the lunar surface. The absorption bands of the main chromophore elements (i.e., Fe, Ti, Cr), which are produced either by the charge-transfer mechanism or the crystal-field splitting, are located in this range (Burns, 1993). The positions, depths and profiles of these bands significantly depend not only on the presence/concentration of certain chromophores but also on the type of crystal structure and the presence of additional non-chromophore elements. ...
... Some types of high-calcium pyroxenes (e.g., see Figs. 1 and 15 in Cloutis, 2002) produce a very similar double-minima band at 1000 nm. The mechanisms of the formation of such bands in the case of high-Ca pyroxenes is ferrous-ferric (Fe 2+ -Fe 3+ ) intervalence charge transfer (Burns, 1993). Although, the Fe 2+ -Fe 3+ band is placed near 780-800 nm according to laboratory studies, this band may be shifted by overlapping with the crystal-field band at 950 nm, as well as the influence of space weathering effects, primarily due to the generation of nano-sized particles of pure iron (npFe 0 ). ...
... This structural transition is characterized by a significant drop in its excess volume (Figure 2). Based on simple crystal field arguments (Burns, 1993) it can be expected that a decrease in GB excess volume intensifies the crystal or ligand field effects on iron at the interface. This correlation between excess volume and bonding environment is likely to trigger the HS-LS transition of iron in the GB. ...
Polycrystalline (Mg,Fe)O ferropericlase is the second most abundant mantle constituent of the Earth and possibly of super‐Earth exoplanets. Its mechanical behavior is expected to accommodate substantial plastic deformation in Earth's lower mantle. While bulk properties of ferropericlase have been extensively studied, the thermodynamics of grain boundaries and their role on mechanical response remain largely unexplored. Here, we use density functional theory calculations to investigate mechanical behavior and thermodynamics of the {310}[001] grain boundary—a representative proxy for high‐angle {hk0}[001] tilt grain boundaries—at relevant mantle pressures of the Earth and super‐Earth exoplanets. Our results provide evidence that shear‐coupled migration and grain boundary sliding are the dominant mechanisms of (Mg,Fe)O grain boundary mobility. We show that pressure‐induced structural transformations of grain boundaries can trigger a change in the mechanism and direction of grain boundary motion. Significant mechanical weakening of the grain boundary is observed under multi‐megabar pressures, caused by a change in the grain boundary transition state structure during motion. Our results identify grain boundary weakening in periclase as a potential mechanism for viscosity reductions in the mantle of super‐Earths. We further demonstrate that structural grain boundary transitions control the spin crossover of Fe²⁺ in the grain boundaries. We model iron partitioning behavior between bulk and grain boundaries and predict equipartitioning to occur in μm size ferropericlase grains. Our findings suggest that the iron spin crossover pressure in ferropericlase may increase several tens of GPa by pressure‐induced structural grain boundary transitions in dynamically active fine‐grained lower mantle regions.
... Importantly, Mastcam can collect multispectral data over broad spatial areas and expand upon stratigraphic units defined from orbit. The filter positions used by Mastcam makes it sensitive to iron oxides, due to electronic transitions and charge transfers from crystal field theory (Burns, 1993) and some hydrated minerals, due to stretching and bending vibrations (Bishop et al., 2019). Both iron oxides and hydrated minerals are important indicators of alteration (Rice et al., 2013) and thus their identification aids in the reconstruction of Mars' geologic history (e.g., Horgan et al., 2020). ...
Since leaving Vera Rubin ridge (VRr), the Mars Science Laboratory Curiosity rover has traversed though the phyllosilicate‐bearing region, Glen Torridon, and the overlying Mg‐sulfate‐bearing strata, with excursions onto the Greenheugh Pediment and Amapari Marker Band. Each of these distinct geologic units were investigated using Curiosity's Mast Camera (Mastcam) multispectral instrument which is sensitive to iron‐bearing phases and some hydrated minerals. We used Mastcam spectra, in combination with chemical data from Chemistry and Mineralogy, Alpha Particle X‐ray Spectrometer, and Chemistry and Camera instruments, to assess the variability of rock spectra and interpret the mineralogy and diagenesis in the clay‐sulfate transition and surrounding regions. We identify four new classes of rock spectra since leaving VRr; two are inherent to dusty and pyroxene‐rich surfaces on the Amapari Marker Band; one is associated with the relatively young, basaltic, Greenheugh Pediment; and the last indicates areas subjected to intense aqueous alteration with an amorphous Fe‐sulfate component, primarily in the clay‐sulfate transition region. To constrain the Mg‐sulfate detection capabilities of Mastcam and aid in the analyses of multispectral data, we also measured the spectral response of mixtures with phyllosilicates, hydrated Mg‐sulfate, and basalt in the laboratory. We find that hydrated Mg‐sulfates are easily masked by other materials, requiring ≥90 wt.% of hydrated Mg‐sulfate to exhibit a hydration signature in Mastcam spectra, which places constraints on the abundance of hydrated Mg‐sulfates along the traverse. Together, these results imply significant compositional changes along the traverse since leaving VRr, and they support the hypothesis of wet‐dry cycles in the clay‐sulfate transition.
... Hence the reduction in band gap energy of the co-substituted samples occurs due to distortion induced in FeO 6 octahedral as well as rearrangement of molecular orbitals. [66][67][68][69] The XRD analysis also confirms the increase in strain with the increase in co-substitution concentration. ...
One of the most recent developments in the area of modern optics arises due to perovskite materials. The ABO3 perovskite materials like bismuth ferrite is an appealing alternative for various optical devices. Hence, in the present study, Praseodymium (Pr) and Manganese (Mn) co-substituted bismuth ferrite ceramics are prepared by the modified sol-gel method and the correlation between structural and optical properties with the variation in co-substitution concentration has been investigated. Rietveld refinement of X-ray diffraction patterns demonstrates a structural phase change in crystal symmetry from rhombohedral to orthorhombic phase with the increase in concentration for x ≥ 0.025. Lattice deformation in the co-substituted samples is confirmed by Fourier transform infrared and Raman spectra. UV-Vis diffuse reflectance spectra confirm the strong absorption of light in the visible region. Furthermore, optical energy band gap decreases from 1.95 to 1.50 eV with increase in concentration from x = 0.000 to x = 0.100. The refractive index attains the maximum value i.e. 6.16 for x = 0.025 whereas it drops to minimum value of 4.06 for x = 0.100. This demonstrates that the structural modification in bismuth ferrite can be used to tune its optical parameters for the device applications.
... Absorption band depth is usually sensitive to mineralogical abundances (e.g., pyroxenes, olivine and opaque minerals) as well as to grain size and space weathering (Adams, 1974;Burns, 1970;Clark, 1999;Serventi et al., 2013). Indeed, the band depth also depends on the maturity of the soils, as occurrence of iron nanophases (formed due to the prolonged exposure time) tend to reduce the reflectance and the absorption band depths (Clark, 1983;Pieters et al., 2012). ...
Mare Ingenii is a site of great interest for lunar geology as it is one of the few basaltic plains on the farside of the Moon. It is located within the outer edge of the South Pole‐Aitken basin, the largest and oldest impact basin in our Solar System. Mare Ingenii includes two large craters, Thomson and Thomson M, and a prominent swirl, a high‐albedo sinuous feature whose origin is still debated. We conducted spectral analysis on 28 selected regions of interest within Mare Ingenii, with the aim of inferring its mineralogy. We considered reflectance data acquired in the visible to near‐infrared spectral range by the Moon Mineralogy Mapper (M³) imaging spectrometer onboard the Chandrayaan‐1 mission, to derive a set of spectral parameters. Our results show wide compositional variability, with the dark material of the mare basaltic floor showing the centers of the Fe²⁺ absorption bands of the pyroxenes shifted toward long wavelengths (0.96–0.99 and 2.03–2.12 μm, respectively), consistent with the spectral characteristics of high‐Ca pyroxenes (as well as swirl material and intermediate albedo regions). In contrast, the bright material of the small surrounding craters shows Fe²⁺ absorption bands shifted toward short wavelengths (0.91–0.94 and 1.91–2.04 μm, respectively), more consistent with low‐Ca or Ca‐free pyroxenes. The obtained results suggest a mafic signature throughout the surface of Mare Ingenii, probably representative of the composition of the lower lunar crust.
... In the year 2008, Satish Dhawan Space Centre, Sriharikota, India, through India's Polar Satellite Launch Vehicle PSLV-C11, launched the first Lunar mission of India, known as Chandrayaan-1. The scientists, through a NASA instrument known as Moon Mineralogy Mapper, measured the amount of water that exists on the upper surface of the lunar and indicated the presence of a small amount of water and hydroxyl molecules [2]. Chandrayaan-1 carrying the Lunar Prospector neutron spectrophotometer has revealed that there is more amount of hydrogen present in the polar regions of the moon, which are permanently shadowed regions of the moon [3]. ...
Successfully facing challenges that other countries of the World failed to cross and ultimately resulted in the failure of their Lunar missions of targeting the polar regions has consolidated India's place in successive moon missions as India has achieved this outstanding engineering and astronomical milestone. The Chandrayaan-3 mission is single-handedly driven and managed by Indian scientists to land on the South pole of the moon and study the soil and rock composition, presence of water ice, and amount of hydroxyl ions, and minerals on the Lunar South pole being carried out by the instruments sent with rover Pragyan. The data that will be sent to the earth will open the gates for the various possibilities and opportunities on the moon such as management and fulfillment of the need for water on future lunar missions, useful minerals present there, etc. In Chandrayaan-3, industrial IoT played an inevitable role in the whole of the mission as scientists needed to remotely monitor and control it accordingly through the commands on the basis of what IoT sensors and IoT devices were sending information and pictures regarding the conditions present on the moon.
... The color intensifies with increasing chromium concentration, and as soon as the latter exceeds a few 0.7at% for both binary and ternary systems, the compounds acquire a strong red component. The color change of compounds is mainly due to changes in the parameter of crystal field 0 [28,29]. Chromium dopant involving in the coloration being essentially in the form Cr 3+ in more or less distorted octahedral symmetry. ...
Cr -doped Al2O3-YAG and Al2O3-YAG-ZrO2 eutectic ceramics rods of 3mm in diameter were solidified from the melt by micro-pulling down (µ-PD) technique. The Cr dopant affect the microstructure and the...
Transition metal complexes have historically played a pivotal role in creating vibrant pigments utilized across artistic mediums such as ceramics, paintings, and glass mosaics. Despite their extensive historical use, our understanding of the mechanisms governing transition metal complex behavior has predominantly emerged in recent times, leaving numerous aspects of this process ripe for exploration. These complexes exhibit striking color variations under diverse conditions when employed in pigment formulations. This review utilizes a bottom-up scientific approach, spanning from microscopic to macroscopic scales, to unravel the molecular origins of the colors generated by transition metal complexes in pigments and ceramic glazes. Advanced spectroscopy techniques and computational chemistry play pivotal roles in this endeavor, highlighting the significance of understanding and utilizing analytical data effectively, with careful consideration of each technique’s specific application. Furthermore, this review investigates the influence of processing conditions on color variations, providing valuable insights for artists and manufacturers aiming to enhance the precision and quality of their creations while mitigating environmental impact.
Characterising the physico-chemical properties of dust-emitting sediments in arid regions is fundamental to understanding the effects of dust on climate and ecosystems. However, knowledge regarding high-latitude dust (HLD) remains limited. This study focuses on analysing the particle size distribution (PSD), mineralogy, cohesion, iron (Fe) mode of occurrence, and visible–near infrared (VNIR) reflectance spectra of dust-emitting sediments from dust hotspots in Iceland (HLD region). Extensive analysis was conducted on samples of top sediments, sediments, and aeolian ripples collected from seven dust sources, with particular emphasis on the Jökulsá basin, encompassing the desert of Dyngjunsandur. Both fully and minimally dispersed PSDs and their respective mass median particle diameters revealed remarkable similarities (56 ± 69 and 55 ± 62 µm, respectively). Mineralogical analyses indicated the prevalence of amorphous phases (68 ± 26 %), feldspars (17 ± 13 %), and pyroxenes (9.3 ± 7.2 %), consistent with thorough analyses of VNIR reflectance spectra. The Fe content reached 9.5 ± 0.40 wt %, predominantly within silicate structures (80 ± 6.3 %), complemented by magnetite (16 ± 5.5 %), hematite/goethite (4.5 ± 2.7 %), and readily exchangeable Fe ions or Fe nano-oxides (1.6 ± 0.63 %). Icelandic top sediments exhibited coarser PSDs compared to the high dust-emitting crusts from mid-latitude arid regions, distinctive mineralogy, and a 3-fold bulk Fe content, with a significant presence of magnetite. The congruence between fully and minimally dispersed PSDs underscores reduced particle aggregation and cohesion of Icelandic top sediments, suggesting that aerodynamic entrainment of dust could also play a role upon emission in this region, alongside saltation bombardment. The extensive analysis in Dyngjusandur enabled the development of a conceptual model to encapsulate Iceland's rapidly evolving high dust-emitting environments.
Incorporation of ions into the crystal structure of beryl (Be3Al2[Si6O18]) can take place by direct ion-to-ion substitution of the framework components Al3+, Be2+ and Si4+ or by occupation of interstitial or structural channel sites. The most common impurities in beryl include transition metals, alkalis and H2O. It is accepted that the transition metals Mn, Cr and V directly substitute for Al at the octahedral site and induce colour. Similarly, the octahedral site can host Fe instead of Al. Nevertheless, it is shown that it remains disputed whether Fe can also be present at the tetrahedral, interstitial, or channel sites, and opposing hypotheses exist regarding these possibilities. However, in the case of Fe, not only the possible occupation of these sites remains under debate, but also their influence on the subsequent colour of beryl. Similarly, the residence of Li in the channels and at the Be tetrahedral or interstitial tetrahedral sites is still under debate. The presence of more than two types of H2O (type I and type II) in the structural channels of beryl is also unclear. This article aims to give an overview on the consensus and on the current debates found in the literature regarding these aspects. It mainly concentrates on the substitution by and the role of Fe ions and on channel occupancy by H2O.
In this work, the suitability of the spinel material ZnFe2O4, which has already been widely investigated in the context of its magnetic and photocatalytic properties, for use as active material for the cathode side in zinc ion batteries is presented. In addition to pure ZnFe2O4, part of the Fe³⁺ was doped with Ti⁴⁺ to achieve stabilization of Zn vacancies in the material and increase ionic conductivity as indicated by previous modelling results. Ceramic samples with the composition ZnFe2−xTixO4 (x = 0 to 0.25) were prepared via a Pechini synthesis route and investigated regarding their optical, structural and electrochemical characteristics. It has been successfully demonstrated that both pure and Ti doped ZnFe2O4 can be used as active material in the positive electrodes of zinc metal batteries or in an “anode-free” setup with Sn metal. Cells with calcined ZnFe2xTixO4 (x = 0.09)|0.5 M zinc triflate in acetonitrile|Zn showed a stable cycling behavior over 1000 cycles and an average initial specific capacity of 55 mA h g⁻¹.
On Mars, the well-known crustal dichotomy marks the boundary between the old southern highlands and the younger northern lowlands. Among these lowlands, Chryse Planitia resembles a quasi-circular basin surrounded by several highlands, and blends into Acidalia Planitia, another flat lowland located farther north. The transition area between these highlands and the Chryse basin is often designated as “circum-Chryse Planitia”, and is the terminus for many outflow channels. Infrared datasets display several sites therein with extensive clay-bearing outcrops, further testifying for aqueous activity on early Mars – notably around Mawrth Vallis, Oxia Planum and Xanthe Terra. In this study, we investigate clay-bearing outcrops identified along the western margins of circum-Chryse basin, often overlooked in the Martian literature. We also compare them with outcrops found in other regions along the crustal dichotomy and relevant in the Martian literature, such as Oxia Planum, Mawrth Vallis and Nili Fossae. Investigating such deposits is crucial for astrobiological perspectives, as they are appealing targets to search for organic compounds possibly stored throughout the rocks and soils. Fe,Mg-rich clays generally result from the interaction of liquid water with rocks under low temperatures, moderate pH levels and neutral to reducing conditions, factors favorable for life. Here, the clay minerals detected in west Chryse Planitia are consistent with either ferrosaponites or vermiculites associated with hydrobiotite, as recently inferred in Oxia Planum and north Xanthe Terra. Diverse alteration pathways might be involved based on either of these clay species. The clay-bearing rocks crop out in isolated hills in Lunae Planum, and along inverted channels and small craters in Tempe Terra. Further geologic investigations in circum-Chryse Planitia should certainly provide new clues on their origin and weathering conditions, while supporting the upcoming ExoMars rover mission and other future explorations.
The chemical distribution on the lunar surface results from the combined effects of both endogenic and exogenic geological processes. Exploring global maps of chemical composition helps to gain insights into the compositional variation among three major geological units, unraveling the geological evolution of the Moon. The existing oxide abundance maps were obtained from spectral images of remote sensing and geochemical data from samples returned by Apollo and Luna, missing the chemical characteristics of the Moon’s late critical period. In this study, by adding geochemical data from Chang’e (CE)-5 lunar samples, we construct inversion models between the Christiansen feature (CF) and oxide abundance of lunar samples using the particle swarm optimization–extreme gradient boosting (PSO-XGBoost) algorithm. Then, new global oxide maps (Al2O3, CaO, FeO, and MgO) and Mg# with the resolution of 32 pixels/degree (ppd) were produced, which reduced the space weathering effect to some extent. The PSO-XGBoost models were compared with partial least square regression (PLSR) models and four previous results, indicating that PSO-XGBoost models possess the capability to effectively describe nonlinear relationships between CF and oxide abundance. Furthermore, the average contents of our results and the Diviner results for 21 major maria demonstrate high correlations, with R² of 0.95, 0.82, 0.95, and 0.86, respectively. In addition, a new Mg# map was generated, which reveals different magmatic evolutionary processes in the three geologic units.
Texturally and chemically sector-zoned garnet crystals in two contiguous metapelitic rocks from the Danba dome, eastern Tibetan Plateau (SW China) were investigated. A petrographic boundary in one of the rocks (sample 21DB103) separates a thin section into two zones. Whereas one zone containing sector-zoned garnet and fined-grained matrix is enriched in graphite and quartz, the other zone encompasses garnets with relatively regular habit in a coarse-grained matrix poor in graphite and quartz. The two zones are distinct with regards to the chemical compositions of biotite and plagioclase, as well as the major and trace element zoning patterns of garnet. Electron back-scattered diffraction analysis shows that all the investigated garnet crystals in this sample are single crystals. Relatively higher P-T conditions are estimated for the initial growth of sector-zoned garnet (~ 5.0 kbar / ~540 ℃) compared to the regular garnet (~ 3.8 kbar / ~510 ℃) in this rock, possibly indicating that growth of the sector-zoned garnet postdates growth of the regular garnet. Texturally and chemically radial sectors with garnet-quartz intergrowths and irregular sectors of garnet are preserved in the other graphite-rich rock (sample 21DB104). Isopleth thermobarometry applied to the core of the largest garnet crystal exhibiting sector zoning in this sample reveals P-T conditions of initial garnet crystallization (~ 4.4 kbar / ~512 ℃) that deviate far (~ 0.8 kbar/~45 ℃) from equilibrium, potentially indicating significant overstepping required for garnet nucleation. Plagioclase inclusions in garnet display varying trace element abundances, indicating their replacements of different preexisting phases. These results suggest that abundant graphite may play a pivotal role in changing fluid conditions and reducing the solubility of SiO2 to grow sector-zoned garnet, as well as impeding matrix coarsening. Development of sector-zoned core and dodecahedral faces of garnet may be related to rapid growth with changes in crystal morphology. Irregular sectors may have developed through fluid infiltration and local chemical adjustments.
In the last two decades copper metaborate CuB2O4 with a unique noncentrosymmetric crystal structure has become the subject of active research due to its unusual magnetic and optical properties. We consider the propagation and absorption of light in CuB2O4 based on the solution of Maxwell’s equations. We present an overview of the main results on the investigation of the phonon spectrum using infrared and Raman spectroscopy. Studies in the region of electronic transitions in Cu2+ ions in the crystal field have allowed the separation of contributions to the optical absorption from copper ions in inequivalent positions. A splitting of zero-phonon absorption lines in a magnetic field has been detected, and these results have received a theoretical explanation in terms of the exciton model. A rich structure of exciton–magnon states has been observed in the photoluminescence spectra. We have carried out a spectroscopic study of the optical second harmonic generation in the region of excitonic transitions, which has allowed the contribution of the toroidal moment and the Fano resonance to the observed signals to be revealed.
Physical parameters and characteristics of asteroids as solid atmosphereless celestial bodies are traditionally studied with the same methods as those used for investigating most of the other celestial objects, though they have certain specific features. The main attention is paid to spectrophotometry, as the most effective tool to study remotely the composition, evolution, and origin of asteroids. However, very important information about asteroids was also obtained by other observational methods, such as photometry, polarimetry, radiometry, and radar. Because of this, in addition to spectrophotometry, we discuss here photometry, polarimetry, and radiometry, which, on the one hand, are very close in methodology and, on the other hand, there has been a trend to their integrated use. In connection with the discovery of sublimation–dust activity on a number of asteroids and the periodic formation of a dust exosphere around these asteroids near perihe lion (see, e.g., Busarev et al., 2021), we also consider a methodologically new approach to estimating the chemical and mineralogical composition of particles in the exosphere of these asteroids and, indirectly, of their surface material.
The formation of basalts is a global stage in the evolution of differentiated cosmic body (planet or asteroid) of the Solar System. The paper presents the main chemical and mineralogical features of basaltic meteorites of the SNC, HED group, angrites and lunar mare basalts based on literature data. Despite the differences in the products of basaltic volcanism on different cosmic bodies and significant compositional variations in major minerals of basaltic rocks, most of them belong to low-alkaline basalts, suggesting the prevalence of this type of rocks at least among small bodies of the Solar System. All of them are characterized by the presence of such rock-forming minerals as pyroxene, olivine, and plagioclase, and their spectral characteristics can be used to search for basalts on exoplanets. The main factors affecting the spectral characteristics of atmosphere-free bodies and larger planets with an atmosphere are shown, and the possibility of searching for products of basalt volcanism on exoplanets during future missions is considered.
Pitted impact deposits on Vesta show higher reflectance and pyroxene absorption band strengths compared to their immediate surroundings and other typical Vestan materials. We investigated whether heating to different temperatures for different durations of Vestan regolith analog materials can reproduce these spectral characteristics using mixtures of HEDs, the carbonaceous chondrite Murchison, and terrestrial analogs. We find no consistent spectral trend due merely to temperature increases, but observed that the interiors of many heated samples show both higher reflectance and pyroxene band I strength than their heated surfaces. With electron probe microanalysis, we additionally observe the formation of hematite, which could account for the higher reflectance. The presence of hematite indicates oxidation occurring in the sample interiors. In combination with heat, this might cause the increase of pyroxene band strengths through migration of iron cations. The effect grows larger with increasing temperature and duration, although temperature appears to play the more dominant role. A higher proportion of Murchison or the terrestrial carbonaceous chondrite analog within our mixtures also appears to facilitate the onset of oxidation. Our observations suggest that both the introduction of exogenic material on Vesta as well as the heating from impacts were necessary to enable the process (possibly oxidation) causing the observed spectral changes.
The exposure to irradiation from high‐energy particles alters the reflectance properties of asteroid surfaces and is referred to as space weathering . This process leads to an increase in spectral slope in visible and near‐infrared wavelengths. However, changes in the regolith particle size, which can vary dramatically among the asteroid population, are known to influence the spectral properties of meteorites and asteroids. In this context, we investigate the changes in spectral slope and absorption band depths of fresh and irradiated ordinary chondrite meteorites to quantitatively compare the effects of space weathering and grain size variations. To do so, we develop and employ the Spectral Analysis for Asteroid Reflectance Investigation routine that calculates the band parameters of reflectance spectra. We then formulate a parameter called the Space Weathering Index ( SWI ) that is designed to encapsulate spectral changes due to space weathering. We find that the SWI is strongly dependent on the spectral slope which complicates the interpretation of asteroid spectra in the context of grain size variations and space weathering. We also show that a second parameter, the Band Depth Index, is indicative of petrologic type. Finally, we use a linear discriminant analysis to classify asteroid reflectance spectra into H, L, LL, and unequilibrated ordinary chondrites.
The Co1-xZnxAl2O4 spinel (x = 0, 0.25, 0.5, 0.75 and 1) was synthesized using moringa oleifera gum by the sol gel chemical route. The characterization of the investigated samples was performed by TG-DTA, XRD, XPS, FTIR, FESEM, and VSM techniques. The XRD pattern reveals cubic spinel structure with space group Fd
̅
m and a good crystallinity. Lattice parameter values linearly increase with increase in Zn concentration. The EDAX spectrum confirms purity of the prepared samples with nonexistence of the elements other than Co, Zn, Al and O. The FTIR spectrum confirms the cubic spinel structure of the aluminates. Magnetic studies showed that the saturation magnetization as well as coercivity of the investigated samples is decreasing with the increasing Zn substitution. The band gap energies obtained from the UV vis absorption spectra are increasing due to phase purity and systematic variation of the lattice parameter. Moreover, the values of band gap energies suggest the possible use of the aluminates as the optical and photoluminescence materials.
Predicting the defect levels of transition metal (TM) dopants in the band gap of crystals is critical in determining the charge states of TM dopants and explaining their electronic and optical properties. By analyzing the calculated charge transition levels and the crystal-field strengths of all the 3d-TM ions in several insulators, we demonstrate that the variation trend of the 3d-TM dopants in a crystal is a scaling of the variation of 3d-electron binding energies (ionization potential) of the free TM ions corrected by adding the contribution of the 3d-orbital's crystal-field splitting. We therefore develop a model to predict the relative location of TM ions' defect levels in the band gap from the defect level and crystal-field splitting of a reference TM ion in the host of concern. The model is applied to predict the defect levels of the series of TM ions in β-Ga 2 O 3 and ZnO, which have moderate to small band gaps, making some of the levels fall in the conduction or valence bands. These results show that the model may serve as a quick reference for related material design and optimization.
Demantoid is the green variety of andradite [Ca3Fe2(SiO4)3], an exceptionally rare and precious gemstone worldwide. In recent years, a small amount of gem-quality demantoid has been found in Pakistan. This research focuses on nine demantoids sourced from Muslim Bagh, Baluchistan, Pakistan, presenting a comprehensive analysis of the spectral characteristics and inclusions of Pakistani demantoid using classical gemological methods, energy dispersive X-ray fluorescence (EDXRF) chemical analyses, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and ultraviolet and visible (UV-vis) spectroscopy. The results show that the content of Cr and V in most samples is lower than the detection line of EDXRF, with only one sample containing a Cr2O3 content of 0.032%. The extremely low Cr content sets Pakistani demantoid apart from demantoid of the serpentinite type found in other regions. Notably, the UV-vis spectrum reveals characteristic absorption at 443 nm due to Fe3+, while a further contribution from Cr3+ would be highly likely, and weak absorption at 550 nm caused by Fe3+. This suggests that iron (Fe) is the primary chromogenic element of Pakistani demantoid, but the role of Cr3+ cannot be ignored. The FTIR spectrum of Pakistani demantoid displays the absorption peaks associated with [SiO4]4− groups at 937 cm−1, 848 cm−1, and 817 cm−1, while the absorption peaks resulting from trivalent cations appear at 481 cm−1 and 442 cm−1, which are the characteristic FTIR spectra of demantoid. Raman spectroscopy further reveals absorption peaks are displayed near 994 cm−1, 843 cm−1, 818 cm−1, associated with (Si–O)Str vibrations (Si–O stretching vibration), and absorption peaks are displayed near 350 cm−1 and 310 cm−1, related to the rotation of SiO4–R(SiO4)4−, and the peaks near 514 cm−1 and 494 cm−1 are related to (Si–O)bend vibrations (Si–O bending vibration). Additionally, related absorption peaks near 168 cm−1 are attributed to the translation of SiO4–T(SiO4)4−, and absorption peaks near 234 cm−1 are associated with the translation of X2+–T(X2+) (X2+ represents divalent ions). The common dark opaque inclusions found in Pakistani demantoid consist of a combination of magnetite and hematite. Additionally, some samples of Pakistani demantoid display inclusions of calcite. This unique combination of inclusions differentiates Pakistani demantoid from demantoids sourced from other regions. It signifies that Pakistani demantoid has a distinctive geological origin resulting from the interplay of serpentinization and skarnization processes. This geological formation distinguishes it from demantoids solely hosted in serpentinite or skarn environments in other origins. The identification of these characteristics holds significant importance for accurately determining the origin of Pakistani demantoid.
Spectral indexes are tools widely used to analyze the composition of the lunar surface. Many indexes have been formulated over the years, but there is no unified database for them. In this work we describe an Open-Source Python package called MoonIndex, that recreates thirty-nine indexes compiled from the literature and derived from several instruments, using data from the Moon Mineralogy Mapper (M3). The processing started with the filtration of the data cubes to reduce the noise, the continuum of the spectrum was then removed using the convex hull and linear-fit methods. Later, the indexes were calculated, following as possible the original formulations. The results on spectral indexes calculated on the original spectra were similar to those already published. Conversely, the results obtained for spectral indexes calculated after the continual removal were not always coherent, some indexes, like the band depth, are especially sensitive to the removal method, as well as the derived band areas and asymmetries. We also recreated RGB composite maps, our results highlight the compositional patterns is a similar way as the ones in the literature, even if the color ramps can differ. The products of MoonIndex are open, ready for interpretation, versatile, consistent, and cross-comparable.
In high-sensitive laser interferometers, such as the gravitational-wave detector KAGRA, ultra-high-quality mirrors are essential. In the case of KAGRA, where cavity mirrors are cooled down to 20 K, large-size Sapphire crystals are used as the substrate for the main mirrors to achieve both a good optical quality (i.e., low absorption and uniform refractive index) and optimized thermal behavior under cryogenic temperatures. To implement the very tight optical specifications required by this demanding application, it is mandatory to test the optical homogeneity of different substrates. In order to characterize refractive-index inhomogeneities of large-size uniaxial samples such as the KAGRA Sapphire test masses, we developed a dedicated setup, allowing to resolve birefringence changes with a sensitivity in the order of $$\Delta n\approx 2 \times 10^{-10}$$ Δ n ≈ 2 × 10 - 10 and a spatial resolution of $$1\,{{{\hbox {mm}}}}^{2}$$ 1 mm 2 . Moreover, the same setup allows us to simultaneously record residual absorption maps, thus allowing for a comparison between birefringence and absorption features. In this paper, we will present for the first time measurements on a KAGRA-sized Sapphire substrate which has been characterized in terms of absorption already in an earlier work. Both birefringence inhomogeneities and absorption distributions will be compared and correlations discussed.
The jadeite jade in Guatemala exerts remarkable commercial quality, which has attracted wide attention. Guatemalan jadeite jade displays a rich variety of colors; however, the color formation of this jadeite jade has not been systematically investigated to date. In this paper, we study different colors of jade samples to trace the compositions and color-causing mechanisms through petrography, X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), and UV-visible absorption spectroscopy (UV-Vis), as well as electron probe microanalysis (EPMA). The results show that jadeite and omphacite are the main mineral compositions of Guatemalan jadeite jade, together with minor albite and other impurities. The color of Guatemala jadeite jade is mainly related to Cr3+, Fe2+, and Fe3+, of which a small amount of Cr3+ causes the jadeite jade to be emerald green. Moreover, 1~2% FeO contents can lead to the blue or gray color of the samples, while the Fe3+ makes the sample dark green. The green color of some Cr3+-free jadeite is caused by the electron transition between bands of Fe3+, and the green color is related to the iron content. Moreover, the chemical composition analysis shows that some metallic elements existed in Guatemalan jadeite jade, such as Ca, Ti, Al, Si, Ni, Fe, Mn, Cr, Na, Mg, and Sr, and some trace elements were lost or unevenly distributed, which may lead to the heterogeneity of the color of the samples. Our present investigation provides insights into color discrimination, quality evaluation, and identification of Guatemala jadeite jade.
To overcome the challenges related to diluted magnetic semiconductors (DMS), significant research efforts are currently underway to develop functional materials to fulfil the requirements of next-generation spintronic devices. This study was planned to investigate a potential DMS material based on Al2CO using first-principles methods. Al2CO is a direct band gap semiconductor with the energy gap of 1.77 eV in such a way that the conduction band minimum is composed of Al 3 s states, whereas the valence band maximum is contributed by p states of carbon and oxygen atoms. The phonon dispersion curves revealed the dynamical stability of Al2CO. The effect of 3d transition metal (TM) dopants doping into the Al2CO on the electronic and magnetic properties was systematically investigated. The cationic site near oxygen is found energetically feasible when compared with the site adjacent to a carbon atom in the host. The dopants produced spin-polarized gap states in an otherwise non-magnetic host. The hybridization of the p states of the host with the d states of dopant pointing to p-d exchange interactions is observed. The doping of Cr in the host exhibited half-metallic nature of the material. To check the magnetic ground state, double doping with the spin-flip configuration of the host was carried out. The results revealed that the entire 3d dopants prefer to adopt antiferromagnetic coupling in Al2CO. The finding demonstrates that Al2CO is a promising host material for creating new DMS to be used in spintronics.
The infrared hydroxyl bands and first hydroxyl combination bands of glaucophane are characterized under pressure. In this weakly hydrogen-bonded mineral, the anharmonicity parameter, as determined from the difference between combinations and the fundamentals, is nearly constant with pressure to 15 GPa, indicating that the ambient pressure value of hydroxyl-bond anharmonicity closely reflects its value at high pressures. Given this near-constancy, the Grüneisen parameters of the hydroxyl stretching vibrations of a wide range of minerals, as derived from the pressure dependence of their O–H stretching frequencies, are correlated with the anharmonic parameter of each vibration, as determined from the ambient pressure offset of the summed frequencies of the fundamental n = 0 to 1 transitions and the frequency of the hydroxyl combination or overtone band corresponding to the n = 0 to 2 transition. This correlation is motivated by (1) the anharmonic origin of the Grüneisen parameter; and (2) the grossly similar form of the interatomic potential governing weak- and medium-strength hydrogen bonding in many minerals. This possible correlation provides a means through which the likely pressure-induced hydroxyl mode shifts of phases might be estimated from ambient pressure near-infrared measurements and emphasizes the importance of near-infrared combination/overtone band measurements. In this context, the combination/overtone bands of high-pressure hydrous phases are almost completely uncharacterized, and thus one probe of their anharmonicity has been neglected. Such information directly constrains the nature of hydrogen bonding in these phases, and hence provides possible insights into both their retention of hydrogen and its mobility. Deviations from the anharmonicity-Grüneisen parameter correlation, when observed (as may be the case in prehnite), could provide insights into anomalous effects on the hydroxyl potential well induced by bifurcated H-bonds, pressure-dependent Davydov splitting, or the influence of neighboring cations.
Surface mineralogy records the primary composition, climate history and the geochemical cycling between the surface and atmosphere. We have not yet directly measured mineralogy on the Venus surface in situ, but a variety of independent investigations yield a basic understanding of surface composition and weathering reactions in the present era where rocks react under a supercritical atmosphere dominated by CO2, N2 and SO2 at ∼460 °C and 92 bars. The primary composition of the volcanic plains that cover ∼80% of the surface is inferred to be basaltic, as measured by the 7 Venera and Vega landers and consistent with morphology. These landers also recorded elevated SO3 values, low rock densities and spectral signatures of hematite consistent with chemical weathering under an oxidizing environment. Thermodynamic modeling and laboratory experiments under present day atmospheric conditions predict and demonstrate reactions where Fe, Ca, Na in rocks react primarily with S species to form sulfates, sulfides and oxides. Variations in surface emissivity at ∼1 μm detected by the VIRTIS instrument on the Venus Express orbiter are spatially correlated to geologic terrains. Laboratory measurements of the near-infrared (NIR) emissivity of geologic materials at Venus surface temperatures confirms theoretical predictions that 1 μm emissivity is directly related to Fe²⁺ content in minerals. These data reveal regions of high emissivity that may indicate unweathered and recently erupted basalts and low emissivity associated with tessera terrain that may indicate felsic materials formed during a more clement era. Magellan radar emissivity also constrain mineralogy as this parameter is inversely related to the type and volume of high dielectric minerals, likely to have formed due to surface/atmosphere reactions. The observation of both viscous and low viscosity volcanic flows in Magellan images may also be related to composition. The global NIR emissivity and high-resolution radar and topography collected by the VERITAS, EnVision and DAVINCI missions will provide a revolutionary advancement of these methods and our understanding of Venus mineralogy. Critically, these datasets must be supported with both laboratory experiments to constrain the style and rate weathering reactions and laboratory measurements of their NIR emissivity and radar characteristics at Venus conditions.
Development of cost-effective oxide catalysts holds the key to the removal of toluene, one of the most important volatile organic compounds. However, the catalysts follow varied working mechanisms at different reaction temperatures, posing a challenge to achieving efficient toluene removal over a wide temperature range. Here we report an agitation-assisted molten salt method, which achieves the rational doping on a two-dimensional Co3O4 catalyst and forms two different structures of active sites to enhance catalytic oxidation of toluene in specific temperature intervals, enabling a facile tandem design for working in a wide temperature range. Specifically, Co3O4 is doped with Cu at the octahedral site (Cu-Co3O4) and Zn at the tetrahedral site (Zn-Co3O4) to form CuOh-O-CoTe and ZnTe-O-CoOh structures on the surface, respectively. Mechanistic studies reveal the different working mechanisms of these two active sites toward remarkable performance enhancement at specific temperature intervals, and the improved performance derived from accelerated consumption of intermediates adsorbed on the catalyst surface. Taken together, Cu-Co3O4 and Zn-Co3O4 achieve excellent toluene purification performance over a wide temperature range. This work provides insights into the mechanism-oriented design of active sites at the atomic level.
We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.
Plagioclase feldspar is a common mineral in terrestrial rocks and has recently been detected on Mars surface with visible near‐infrared (VNIR) spectroscopy. The presence of plagioclase using this method is determined through the identification of an absorption band centered around 1.3 μm on reflectance spectra, which requires the incorporation of Fe²⁺ in plagioclase lattice. Previous laboratory studies of powder mixtures showed that plagioclase should only be detectable if present in amounts >90% as its absorption band can be masked by those of mafic minerals. Plagioclase composition, but also the size of the grains and the associated minerals in a rock, influence the spectral signature of plagioclase feldspars. Thus, the analysis of whole, uncrushed rocks appears to be relevant for comparisons with Mars remote sensing observations that have shown plagioclase‐like signatures. In the present work, we performed laboratory measurements on five feldspar‐bearing terrestrial rocks of various nature chosen because they reflect the first order terrestrial magmatic variability. The mineralogical composition of these samples, the chemical composition of feldspar crystals, and total rock spectra as well as spectra of each mineral were determined using microscopy, electron microprobe, and VNIR spectroscopy. Our study shows that plagioclase signature is visible on the spectra of macroscopic rocks containing between 30% and 80% plagioclase of different compositions (An25‐An67). Our findings have strong implications for the interpretation of feldspar signatures on Mars, which can belong to a range of feldspar‐bearing rocks and could thus provide information about the formation of Mars' crust.
In nature, the elements of the inorganic part of the periodic table are found in three forms: metals, ions in salts & minerals, and ions in solution. The ions may be coordinated to simple or complicated ligands. They may form purely electrostatic or partially covalent bonds. A common trend is that the more covalent bonds an element form, the more we know of its physicochemical properties. The rare earths form purely electrostatic bonds, thus, our understanding of the solution chemistry of these elements is limited-yet important. Most rare earth elements used today pass through hydrometallurgical processes that rely on the solution chemistry of these elements, even through the critical applications are in alloys and functional materials. Through developments in optical spectroscopy, total X-ray scattering, and quantum chemical methods we are posed to remedy this situation: we are ready to create predictive structure-property relationships in the field of lanthanide solution chemistry. The scope of this review is to summarise the state-of-the-art for neodymium(III), to go through the structure-property relationships that are in use. In the form of NdFeB magnets, neodymium plays a crucial role in green energy production and electric propulsion. As a 4f3 ion in solution it is also one of the simpler rare earth ions, and the Nd(III) ion has characteristic optical properties that can be exploited as a handle in physicochemical studies. Here, we start with a critical review of the current concepts used to relate structure and electronic energy levels. We follow with our suggested approach of using the methodology from molecular photophysics to relate optical properties and structure, and conclude with selected literature examples.
The use of a sacrificial cathode additive that contains a large amount of lithium is one potential solution to compensate for the irreversible capacity loss associated with next‐generation anodes such as silicon. Antifluorite‐type Li6CoO4 has attracted attention as a potential cathode additive owing to its remarkably high theoretical lithium extraction capacity. However, the complex mechanism of lithium extraction as well as the oxygen loss from Li6CoO4 is not well understood. A generalizable computational thermodynamics and experimental framework is presented to understand the lithium‐extraction pathway of Li6CoO4. It is found that one lithium per formula unit can be topotactically extracted from Li6CoO4, followed by an irreversible and nontopotactic phase transformation to Li2CoO3 or LiCoO2 depending on the temperature. The results show that peroxide species may form to charge‐compensate for Li extraction which is undesirable as this can lead to gas release during battery operation. It is suggested that charging Li6CoO4 at an elevated temperature that the electrolyte can withstand, redirects the reaction pathway and prevents the formation of intermediate peroxide species making it an effective and stable sacrificial cathode additive.
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