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Insight into understanding structural relaxation dynamics of [NH2NH3][Mn(HCOO)3] metal-organic formate
Abstract
We report the synthesis, thermal and dielectric measurements of [NH2NH3][Mn(HCOO)3] (HyMn) compound. Above room temperature this polymeric material undergoes two phase transitions at ∼360 K and ∼298 K, as observed via DSC and BDS spectra. The first high temperature phase transition is associated with the paraelectric to ferroelectric transition of perovskite HyMn. The low temperature phase transition corresponds to the paraelectric to antiferroelectric transition of chiral HyMn. However, mechanisms hidden behind the observed two polymorphic phases are not completely clear yet. Dielectric spectroscopy measurements have revealed formation of clusters and superclusters as well as the relaxor-like behavior of this compound in wide temperature range resulting from both chiral and perovskite phases. Analysis of dielectric permittivity spectra obtained for the investigated material showed the generalized Mittag-Leffler two-power-law relaxation pattern that was interpreted by means the stochastic scenario of correlated-clusters. The proposed approach brought into light the presence of polar nanoregions in the material suggesting the relaxor nature of the existing ferroelectricity.
... Thus, a relaxor-like nature of these materials was suggested [12,177], although absence of the typical relaxor properties (compositional disorder and Vogel-Fulcher freezing of relaxation time) makes these claims highly speculative and probably incorrect. Amongst successful attempts to synthesize HOIP formates with various built-in cations (excluding those mentioned above) from the amine family, only a few have been dielectrically tested: AC + , GA + , FA + , and Hy + [7,22,25,58,[178][179][180]. In all these compounds, the dielectric measurements showed changes in the vicinity of the PT temperatures. ...
... The frequency changes of the dielectric permittivity were used to explain the mechanism of the PTs associated with the ordering (freezing) of the movements of the cations. The observed relaxation processes allowed for a quantitative description of the dynamics of cation movements (translation and rotation), including the determination of the relaxation times and E a [22,25,178]. ...
Hybrid organic–inorganic perovskites (HOIPs) constitute unique coordination polymers with a huge potential of transforming modern materials chemistry due to their wide applications, mainly in electronics, optoelectronics and photovoltaics. Yearly, thousands of works on HOIPs are published in various fields of science, technology and industry. Many of these studies are devoted to structural and structure-related properties. Molecular spectroscopy methods (optical and broadband dielectric spectroscopy, IR, Raman, EPR, NMR) are often used to understand their magnetic, optical and luminescent properties; energy conversion; basic molecular dynamics; hydrogen bond-associated guest molecule-framework interactions; stability; flexibility; mechanisms of temperature- and pressure induced phase transitions; and other more sophisticated physical phenomena, such as polar properties, ferroelectricity, multiferroicity, etc. The goal of this review is to create a comprehensive and versatile phase-related guide for researchers working in the field of HOIPs using a wide range of molecular spectroscopy techniques. This review presents a systematical description of very recent achievements obtained using modern molecular spectroscopy methods. It also summarizes the current state of knowledge on HOIPs, including metal formates, hypophosphites, azides, cyanides, dicyanamides, and halides templated by protonated amines and phosphines that adopt a perovskite architecture.
... Another interesting metal-formate framework is a structure with the general formula [ [36] 0.30 [35] 0.28 [63] 187 [48,64] 165 [36] 270 [35] 156 [63] 160 [27] 180 [65] Multiferroicity [36] Dielectricity [35] Magnetism [ [75] 367 [74] Multiferroicity [74] C 3 N 2 H 5 + Mn 2+ Monoclinic [77] 0.83 [77] 438 [77,78] Ferroelectricity [78] that the low-temperature phase is ferroelectric. The conclusions were also based on structural studies showing that non-centrosymmetric and spontaneous polarization in the ferroelectric phase results from the ordering of ethylammonium cations. ...
The metal–organic frameworks (MOFs) crystallizing in a perovskite-like architecture became extremely interesting for scientists due to a variety of applications including memory devices, energy conversion and drug delivery. These compounds are constructed from a metal–oxygen or metal–nitrogen octahedral coordinated by organic ligands. They exhibit various interesting properties due to their hybrid organic–inorganic nature. However, ferroelectric MOFs still remain scarce and the topic of ferroelectricity raises a lot of controversies. In this article, we will discuss the actual state of knowledge of these specific compounds with a focus on ferroelectric properties. We will try to create an order out of the current confusion that followed attributing ferroelectric properties to metal–formate frameworks without a direct proof.
... As the templates of the synthesis of AMFFs, the ammonium cations A n+ (n = 1-4) are mainly derived from the protonation of aliphatic organic amines [2]. Metal cations ranging from monovalent to trivalent include major group metal ions and 3d transition metal ions [2][3][4][5]. Among them, Cu(II) with Jahn-Teller effect and Co(II) with large single-ion anisotropy have received more attention from researchers of magnetic materials [6][7][8][9][10]. ...
The synthesis, crystal structures, magnetic and dielectric properties of four ammonium bimetallic formate frameworks [AI][GaIIICoII(HCOO)6] for AI = CH3NH3⁺ (1), (CH3)2NH2⁺ (2), CH3CH2NH3⁺ (3) and (CH3CH2)2NH2⁺ (4) have been reported. They are isomorphic and crystallize in the trigonal P3¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}1c space group at both 110 and 293 K. The structures possess [GaIIICoII(HCOO)6]⁻ anionic metal formate frameworks of binodal (4¹²·6³)(4⁹·6⁶) topology, in which the disordered ammonium cations are located in the cavities. The magnetic data of complex 2 was fitted by the one-ion approximation for CoII ion with spin–orbit coupling in Oh symmetry giving λ = − 102.7 cm⁻¹. The dielectric studies of 1 and 2 showed that the dielectric abnormality was caused by the weakening of the movement of the ammonium cations with the decrease in temperature. The activation energies for the motions of ammonium cations in 1 and 2 were determined to be 3.00 and 3.19 × 10³ K, respectively.
... With these cations, two new metal-free hexagonal perovskite high-energetic materials, (H2dabco)B(ClO4)3 (B = NH3OH + and NH2NH3 + for DAP-6 and DAP-7, respectively, Fig. 1), were prepared with a one-step self-assembly process in aqueous solution under ambient conditions. To the best of our knowledge, DAP-6 and DAP-7 represent the first examples of perovskite compounds with NH3OH + and NH2NH3 + , respectively, as B-site cations rather than A-site cations [45][46][47][48][49]. The structures, thermal stabilities, and energetic performances of DAP-6 and DAP-7 were studied experimentally and theoretically. ...
Designing and synthesizing more advanced high-energetic materials for practical use via a simple synthetic route are two of the most important issues for the development of energetic materials. Through an elaborate design and rationally selected molecular components, two new metal-free hexagonal perovskite compounds, which are named as DAP-6 and DAP-7 with a general formula of (H2dabco)B(ClO4)3 (H2dabco²⁺ = 1,4-diazabicyclo[2.2.2]octane-1,4-diium), were fabricated via an easily scaled-up synthetic route using NH3OH⁺ and NH2NH3⁺ as B-site cations, respectively. Compared with their NH4⁺ analog ((H2dabco)(NH4)(ClO4)3; DAP-4), which has a cubic perovskite structure, DAP-6 and DAP-7 have higher crystal densities and enthalpies of formation, thus exhibiting higher calculated detonation performances. Specifically, DAP-7 has an ultrahigh thermal stability (decomposition temperatures (Td) = 375.3 °C), a high detonation velocity (D = 8.883 km·s⁻¹), and a high detonation pressure (P = 35.8 GPa); therefore, it exhibits potential as a heat-resistant explosive. Similarly, DAP-6 has a high thermal stability (Td = 245.9 °C) and excellent detonation performance (D = 9.123 km·s⁻¹; P = 38.1 GPa). Nevertheless, it also possesses a remarkably high detonation heat (Q = 6.35 kJ·g⁻¹) and specific impulse (Isp = 265.3 s), which is superior to that of hexanitrohexaazaisowurtzitane (CL-20; Q = 6.23 kJ·g⁻¹; Isp = 264.8 s). Thus, DAP-6 can serve as a promising high-performance energetic material for practical use.
The field of relaxor ferroelectrics has long been dominated by ceramic oxide materials exhibiting large polarisations with temperature and frequency dependence. Intriguingly, the dense metal-organic framework (MOF) [NH 3 NH 2 ]Mg(HCO 2 ) 3 was reported...
Phase transition materials that can exist two energetically stable states are suitable for a breadth of applications in electronics and physics. In this work, we report a new organic–inorganic hybrid...
In this work, we compare the dielectric properties of single-crystal and pelletized perovskite-like formamidinium-templated inorganic-organic hybrid [NH2CHNH2][Mn(HCOO)3] (FAMn) at ambient pressure. Furthermore, we report pressure-dependent dielectric measurements of pelletized FAMn allowing us to determine the equation of state, as well as changes in the dielectric properties and the rotational motion of formamidinium (FA) cations in the crystal lattice under mechanical stress. Our experiments show that the mechanism of FA cation motion is complex, encompassing six different reorientation paths around three two-fold symmetry axes. Multiple reorientation possibilities of FA cations inside the crystal lattice leads to their irregular molecular dynamics. High-pressure experiments allowed observing that formate anions together with manganese cations form a rigid skeleton, which is stable over a wide temperature and pressure range. Consequently, the hydrostatic pressure exerts a surprisingly small influence on the FA dynamics compared to the related compounds containing the same type of molecular cations. Finally, we reveal a different role of interactions between the A-site organic and B-site metal cations with X-site likers in controlling the dynamics of the A-site moieties under various pressure-temperature conditions. Our results are supported by the molecular dynamics simulations.
Correlating electromechanical and dielectric properties with nanometre-scale order is the defining challenge for the development of piezoelectric oxides. Current lead (Pb)-based relaxor ferroelectrics can serve as model systems with which to unravel these correlations, but the nature of the local order and its relation to material properties remains controversial. Here we employ recent advances in diffuse scattering instrumentation to investigate crystals that span the phase diagram of PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) and identify four forms of local order. From the compositional dependence, we resolve the coupling of each form to the dielectric and electromechanical properties observed. We show that relaxor behaviour does not correlate simply with ferroic diffuse scattering; instead, it results from a competition between local antiferroelectric correlations, seeded by chemical short-range order, and local ferroic order. The ferroic diffuse scattering is strongest where piezoelectricity is maximal and displays previously unrecognized modulations caused by anion displacements. Our observations provide new guidelines for evaluating displacive models and hence the piezoelectric properties of environmentally friendly next-generation materials.
The correspondence between the temperature dependence of phase structures and the experimental physical properties was made clear for the first time in the Bi1/2Na1/2TiO3 system according to in situ XRD, in situ Raman and impedance spectroscopy. XRD profiles show pseudo-cubic symmetry independent of temperature, while one of the Raman models disappears near 620 K, indicating that the local symmetry increases. The temperature dependence of the main relaxation time of the electric modulus spectra can be divided into four regions, characterizing the thermal evolution of polar nanoregions (PNRs) with different symmetries and a phase transition between rhombohedral and tetragonal symmetry. Therefore, the relaxation times of the electric modulus provide a way to estimate the local phase transition and the thermal evolution of R3c and P4bm PNRs.
We report the synthesis of four perovskite-type metal formate frameworks, [CH3NH2NH2][M(HCOO)3] (MHyM) with M=Mn, Mg, Fe, Zn. These compounds exhibit two structural phase transitions. The first transition temperature depends weakly on a type of divalent metal and is observed at 310-327 K on heating. X-ray diffraction, DSC and vibrational studies revealed that it has a second-order character. It is associated with partial ordering of the methylhydrazinium (MHy+) cations and change of symmetry from non-polar R c to polar R3c. Pyroelectric measurements suggest ferroelectric nature of the room-temperature phase. The second, low-temperature phase transition has a first-order character and is associated with further ordering of the MHy+ cations and distortion of the metal formate framework. Magnetic susceptibility data shows that MHyMn and MHyFe exhibit ferromagnetic-like phase transitions at 9 and 21 K, respectively. Since the low-temperature phase is polar, these compounds are possible multiferroic materials. MHyFe shows additional magnetic anomaly in the magnetically ordered state, which most likely manifests some blocking of magnetic moments. We also report high-pressure Raman scattering studies of MHyMn that revealed a pressure-induced reversible phase transition between 4.8 and 5.5 GPa. Analysis of the data indicates that the transition leads to significant changes in both manganese formate framework and the MHy+ structure.
Hybrid organic–inorganic materials are mechanically soft, leading to large thermoelastic effects which can affect properties such as electronic structure and ferroelectric ordering. Here we use a combination of ab initio lattice dynamics and molecular dynamics to study the finite temperature behavior of the hydrazinium and guanidinium formate perovskites, [NH2NH3][Zn(CHO2)3] and [C(NH2)3][Zn(CHO2)3]. Thermal displacement parameters and ellipsoids computed from the phonons and from molecular dynamics trajectories are found to be in good agreement. The hydrazinium compound is ferroelectric at low temperatures, with a calculated spontaneous polarization of 2.6 μC cm–2, but the thermal movement of the cation leads to variations in the instantaneous polarization and eventually breakdown of the ferroelectric order. Contrary to this the guanidinium cation is found to be stationary at all temperatures; however, the movement of the cage atoms leads to variations in the electronic structure and a renormalization in the bandgap from 6.29 eV at 0 K to an average of 5.96 eV at 300 K. We conclude that accounting for temperature is necessary for quantitative modeling of the physical properties of metal–organic frameworks.
The fundamental aspects of relaxation dynamics in nicollite-type mixed valance metal-organic framework multiferroic [(CH3)2NH2][Fe3+Fe2+(HCOO)6] single crystals have been reported using dielectric relaxation spectroscopy covering eight decades in frequency (10-2 ≤f≤106) and temperature range(120 K ≤ T≤ 250K). The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxations processes in the experimental window. The variable range hopping model of small polaron explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA+ cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA+ cations occurs well below phase transition temperature. These experimental observations have fundamental importance on the theoretical explanation of relaxation dynamics in nicollite-type metal-organic frameworks.
We have investigated the multiferroic and glassy behaviour of metal-organic framework (MOF) material (CH3)2NH2Co(CHOO)3. The compound has perovskite–like architecture in which the metal–formate forms a framework. The organic cation (CH3)2NH2+ occupies the cavities in the formate framework in the framework via N–H···O hydrogen bonds. At room temperature, the organic cation is disordered and occupies three crystallographically equivalent positions. Upon cooling, the organic cation is ordered which leads to a structuralphase transition at 155 K. The structuralphase transition is associated with a para-ferroelectric phase transition and is revealed by dielectric and pyroelectricmeasurements. Further, a PE hysteresis loop below 155 K confirms the ferroelectric behaviour of the material. Analysis of dielectric data reveal large frequency dispersion in the values of dielectric constant and tanδ which signifies the presence of glassy dielectric behaviour. The material displays a antiferromagnetic ordering below 15 K which is attributed to the super-exchange interaction between Co2+ ions mediated via formate linkers. Interestingly, another magnetic transition is also found around 11 K. The peak of the transition shifts to lower temperature with increasing frequency, suggesting glassy magnetism in the sample.
We report the synthesis, single crystal x-ray diffraction, thermal, dielectric, Raman and infrared studies of novel heterometallic formate [C2H5NH3][Na0.5Fe0.5(HCOO)3] (EtANaFe). The thermal studies show that EtANaFe undergoes a second-order phase transition at about 360 K. X-ray diffraction data revealed that the high-temperature structure is monoclinic, space group P21/n, with dynamically disordered ethylammonium (EtA+) cations. EtANaFe possesses polar low temperature structure with the space group Pn, and in principle, is ferroelectric below 360 K. Dielectric data show that the reciprocal of the real part of dielectric permittivity above and below phase transition temperature follows the Curie-Weiss, as expected for a ferroelectric phase transition. Based on theoretical calculations, we estimated polarization as (0.2, 0, 0.8) μC/cm2, i.e., lying within the ac plane. The obtained data also indicate that the driving force of the phase transition is ordering of EtA+ cations. However, this ordering is accompanied by significant distortion of the metal formate framework.
Dense formate frameworks with a perovskite-like architecture exhibit multiferroic behavior and tunable mechanical properties. In such materials, interactions between the protonated amine and the metal-formate cavity have a large impact on the mechanical properties. We use complementary single-crystal X-ray diffraction and 1H solid state nuclear magnetic resonance spectroscopy to investigate amine-cavity interactions in [NH3NH2]Zn(HCOO)3. The results suggest that these interactions can be described as salt bridges similar to those in proteins and artificially synthesized helical polymers, where ionic interactions and hydrogen bonds are present at the same time. Nanoindentation and high-pressure single-crystal X-ray diffraction were used to study the mechanical properties of [NH3NH2]Zn(HCOO)3, yielding elastic moduli of E001 = 26.5 GPa and E110 = 24.6 GPa and a bulk modulus of K = 19 GPa. The mechanical properties suggest that, despite the relatively low packing density of [NH3NH2]Zn(HCOO)3, the amine-cavity interactions strengthen the framework significantly in comparison with related materials.
Polymorphism in formate-based dense metal-organic frameworks with the general formula ABX3 is predicted by quantum chemical calculations and confirmed experimentally. In particular [NH3NH2]Zn(HCOO)3 crystallizes in two different polymorphs, a perovskite-like framework and a chiral structure with hexagonal channels. A detailed thermodynamic analysis reveals that both structures are very close in free energy and that entropy driven effects are responsible for stabilizing the channel structure. This journal is
In this study, the dielectric properties of a polycrystalline metal organic framework [(CH3)2NH2][Na0.5Fe0.5(HCOO)3] (DMNaFe) sample were investigated. The DMNaFe sample exhibited a typical relaxor-like relaxation response in addition to a ferroelastic order-disorder phase transition. Analysis of the frequency dependence of the complex permittivity revealed the characteristic two-power-law dipolar glass relaxor behavior of the DMNaFe, indicating complex cluster formation in the material. Moreover, an unusual transformation associated with the ferroelastic phase transition from the generalized Mittag-Leffler relaxation pattern (low-temperature ordered phase) to the Havriliak- Negami one (high-temperature disordered phase) was detected. The relaxation data obtained for the investigated sample were interpreted based on the stochastic approach to relaxation processes.
Relaxor ferroelectrics were discovered almost 50 years ago among the complex oxides with perovskite structure. In recent years this field of research has experienced a revival of interest. In this paper we review the progress achieved. We consider the crystal structure including quenched compositional disorder and polar nanoregions (PNR), the phase transitions including compositional order-disorder transition, transition to nonergodic (probably spherical cluster glass) state and to ferroelectric phase. We discuss the lattice dynamics and the peculiar (especially dielectric) relaxation in relaxors. Modern theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented. (c) 2006 Springer Science + Business Media, Inc.
In this work we explore the overall structural behaviour of the [(CH3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly prove the presence of a structural phase transition at Tt ~ 187 K (the temperature at which the dielectric transition occurs) that involves a symmetry change from Rc to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order–disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 ± 0.1 K kbar−1) in that it should be feasible to shift it to room temperature under adequate thermodynamic conditions, for instance by application of an external pressure.
In this work we further the structural characterization of the recently discovered (C3N2H5)[Mn(HCOO)3] metal-organic framework with perovskite-like structure, and we present its magnetic and dielectric properties up to 350 K. At low temperature, the C3N2H5+ imidazolium cations, that sit oblique within the cavities of the [Mn(HCOO)3]- framework structure, show a cooperative order resulting in an antiparallel arrangement of their electrical dipole moments. Very interestingly, it is only above 220 K that thermal energy seems to be able to break this antiferroelectric order, resulting in a linear increase of its dielectric constant with temperature. In addition, this Mn(ii) compound is antiferromagnetic below TN = 9 K, with a slightly non-collinear arrangement of its magnetic moments, yielding to a weak ferromagnetism. Therefore, this is a new multiferroic material which exhibits coexistence of magnetic and electric ordering.
Vermiculite crystals from Santa Olalla, Spain, were first Na exchanged and then intercalated with monomethylammonium (= NH 3 (CH 3 ) ⁺ , MMA) and dimethylammonium (= NH 2 (CH 3 ) 2 ⁺ , DMA) molecules, respectively, by immersion in 1 M ammonium-chloride solutions at 65°C for 2–3 wk. MMA-and DMA-exchange with vermiculite resulted in crystals with near perfect three-dimensional stacking, suitable for single crystal X-ray diffraction analysis. Unit cell parameters are: a = 5.353(2) Å, b = 9.273(3) Å, c = 11.950(6) Å, and ß = 98.45(4)° for MMA-exchanged vermiculite and a = 5.351(2) Å, b = 9.268(4) Å, c = 12.423(8) Å, and ß = 98.33(5)° for DMA-exchanged vermiculite. Refinement results are R = 0.059 and w R = 0.073 (MMA-exchanged vermiculite) and R = 0.059 and w R = 0.064 (DMA-exchanged vermiculite). The results are based on structures which show substitutional disorder, and thus the presented models are derived from average structures.
There are two distinct sites for the MMA molecule in MMA-exchanged vermiculite. One crystallo-graphically unique MMA is oriented such that the N-C axis of the molecule is perpendicular to the basal oxygen plane, with the N ion offset from the center of the interlayer by 1.04 Å. The other MMA is located such that the N ion is at the center of the interlayer between adjacent 2:1 layers, presumably with the N-C axis of the molecule oriented parallel to the basal oxygen plane. This represents the first known occurrence of an organic molecule located exactly between the two adjacent 2:1 layers. Both sites are located between hexagonal cavities of adjacent layers. DMA molecules in DMA-exchanged vermiculite are located such that the N ion is offset from the central plane in the interlayer by 0.95 A. A static model is proposed with two orientations of DMA to produce a DMA “zigzag” orientation of molecules parallel to the (001) plane. The plane defined by the C-N-C atoms in the molecule is perpendicular to the (001) plane. An alternate model is more dynamic, and it involves the rotation of DMA molecules about one C-N axis.
Identical starting material was used in previous studies on tetramethylammonium (TMA)-exchanged vermiculite and tetramethylphosphonium (TMP)-exchanged vermiculite. The effect of onium-ion substitutions on the 2:1 layer shows that the tetrahedral rotation angle, α, is significantly smaller for MMA-and DMA-exchanged vermiculite vs. TMA and TMP-exchanged vermiculite. Tetrahedral and octahedral bond distances of the 2:1 layer of the TMA, TMP, MMA, and DMA-exchanged structures may be explained by the location of the organic cation relative to the basal oxygen atom plane and by the differences in the geometries of the organic molecule. Thus, the 2:1 layer is affected by the interlayer molecule, and the 2:1 layer is not a rigid substrate, but interacts significantly with the onium ions.
Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg1/3Nb2/3O3), which crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, similar to that which has been studied for diluted antiferromagnets. If a relaxor is cooled from a high temperature, then the Burns temperature is a measure of when slow fluctuations become evident. These fluctuations are connected with the disorder and are known as nano-domains. The Burns temperature is not a well-defined transition temperature. At a lower temperature, there is a well-defined boundary to a so-called random field state when the nano-domains become static but there is no long-range periodic order. This phase may have both history-dependent properties and a skin effect in which the surface of the sample is different from that of the bulk material, as also found in experiments on magnetic systems. Section 1 is an introduction to the review, to ferroelectricity and to relaxors. Section 2 gives a description of the results obtained by dielectric, optical, specific heat and other macroscopic properties. These long-wavelength properties give a variety of different characteristic temperatures and do not directly probe the random field state. In Section 3, we describe the results of neutron and X-ray scattering and show that they strongly support the interpretation that relaxors have a random field state. In Section 4, we briefly describe the results for other relaxor systems such as (PMN)1−x(PT)x for which PMN is mixed with different amounts of the ferroelectric lead titanate (PT), and we show that the existence of a random field state enables us also to describe the experimental results for these mixed materials. We hope that this review will inspire further theoretical and experimental work to understand the nature of the random field states and to compare the experimental results more satisfactorily with theory.
Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.
Transitions associated with orientational order-disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and (1)H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH(3))(2)NH(2)]Zn(HCOO)(3). This compound, which possesses a perovskite-type architecture, undergoes a remarkable order-disorder phase transition at 156 K. The (DMA(+)) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─HO ∼ 2.9 Å) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA(+) is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65-150 K, as a result of a "memory effect." This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.
In this paper the complex dielectric permittivity of gallium doped
Cd0.99Mn0.01Te mixed
crystals is studied at different temperatures. We observe a two-power-law relaxation pattern with
m and
n, the low- and high-frequency power-law exponents respectively, satisfying the relation
m < 1 − n. To interpret the empirical result we propose a correlated-cluster relaxation
mechanism. This approach allows us to find origins of both power-law exponents,
m and
n.
The Letter suggests treating the infrared reflectivity spectra of single crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally frozen polarization makes the dielectric function strongly anisotropic in the phonon frequency range and the random orientation of the polarization at nanoscopic scale requires taking into account the inhomogeneous depolarization field. Employing a simple effective medium approximation (the Bruggeman symmetrical formula) turns out to be sufficient for reproducing all principal features of room temperature reflectivity of Pb(Mg(1/3)Nb(2/3))O3. One of the reflectivity bands is identified as a geometrical resonance entirely related to the nanoscale polarization inhomogeneity. The approach provides a general guide for systematic determination of the polar mode frequencies split by the inhomogeneous polarization at the nanometer scale.
The high-frequency fractional power law of relaxation, seen in a
wide range of materials, yields a constant ratio of the macroscopic
energy lost per radian to the energy stored in the system, in the
corresponding frequency range. For almost two decades, the above energy
criterion has been supposed to imply the existence of similar
microscopic properties which determine the observed power-law exponent.
Here, a rigorous formulation of the energy-criterion argument is
proposed in the frame of a new probabilistic approach to derive the
Havriliak-Negami (HN) and Kohlraush-Williams-Watts (KWW) responses. In
this approach the commonly observed macroscopic laws are related to the
microscopic scenario of relaxation, and the energy-criterion
interpretation is applied to the physical basis of the relation. The
presented considerations reinforce the physical significance of the
empirically found forms of relaxation, and open a new line of analysis
of relaxation phenomena
Unlike the classical exponential relaxation law, the widely prevailing universal law with its fractional power-law dependence of susceptibility on frequency cannot be explained in the framework of any intuitively simple physical concept. The resulting constancy of the ratio of the imaginary to the real parts of the complex susceptibility, known as the ``energy criterion'', has a pleasing simplicity but the understanding of its origins needs a special theoretical treatment. A fresh light on the stochastic nature of the dielectric relaxation has been shed by a novel stochastic approach introduced in the last decade. Since the theoretical analysis involved is rather unfamiliar, the aim of this paper is to give some useful comments and suggestions which should help to follow in details the proposed stochastic scheme of relaxation leading to the well-known empirical responses. We justify the universality of the power-law macroscopic response as well as Jonscher's screening and energy criterion ideas, and we give a new basis to the research into the significance of relaxation processes.
We report the synthesis and structural characterisation of three mixed-metal formate perovskite families [C(NH$_2$)$_3$]M$_{1-x}$Cu$_x$(HCOO)$_3$ (M = Mn, Zn, Mg). Using a combination of infrared spectroscopy, non-negative matrix factorization, and reverse Monte Carlo refinement, we show that the Mn- and Zn-containing compounds support compositional nanodomains resembling the polar nanoregions of conventional relaxor ferroelectrics. The M = Mg family exhibits a miscibility gap that we suggest reflects the limiting behaviour of nanodomain formation.
The [(CH3)2NH2]Mn(HCOO)3 perovskite metal-organic framework exhibits a first-order ferroelectric phase transition with a high polarization at Tc ∼ 192 K, induced by the order-disorder transition of hydrogen bonds. Accompanying this sharp phase transition, a huge pyroelectric coefficient with a peak value of 5.16 × 10⁻²C/m² K is detected. In addition, there is a large lattice expansion along the [012] direction at Tc, resulting in a giant linear thermal expansion coefficient as high as 35 000 ppm/K. These striking results indicate that ferroelectric metal-organic frameworks combing both merits of inorganic and organic compounds hold a great potential in generating superior pyroelectric and thermal expansion properties.
We present a continuous wave electron paramagnetic resonance (EPR) study of Mn²⁺ doped [(CH3)2NH2][Zn(HCOO)3] hybrid dense metal-organic framework (MOF) that exhibits an order-disorder structural phase transition at Tc = 163 K. The W-band EPR measurements of powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn²⁺ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn²⁺ probe ions are susceptible to the phase transition, as the spectrum changes drastically at Tc. The angular dependent EPR spectra of Mn²⁺ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO6 octahedra in the ordered phase that originate from a severe crystal twinning of [(CH3)2NH2][Zn(HCOO)3] MOF. The possible ferroelectric origin of the crystalline twins is investigated by performing the single crystal EPR measurements with an applied external electric field. No significant effect of the electric field on the spectra is observed. The EPR results are supported by the measurements of the electric field dependence of the macroscopic electric polarization. Analogous EPR measurements are performed on a single crystal sample of ferroelectric Mn²⁺ doped [NH4][Zn(HCOO)3] MOF. Contrarily to the dimethylammonium framework, the EPR signal and electric polarization of the ammonium compound demonstrate clear ferroelectric behavior.
We report the synthesis, crystal structure, thermal , dielectric, phonon and magnetic properties of [CH3C(NH2)2][Mn(HCOO)3] (AceMn) compound. Our results show that this compound crystallizes in the perovskite-like orthorhombic structure, space group Imma. It undergoes a structural phase transition at 304 K
into a monoclinic structure, space group P21/n. X-ray diffraction, dielectric, IR and Raman studies show that ordering of the acetamidinium cations triggers the phase transition. Low-temperature magnetic studies show that this compound exhibits weak ferromagnetic properties below 9.0 K.
We report synthesis, X-ray diffraction, magnetic and vibrational studies of three novel heterometallic MOFs, [C2H5NH3][FeIIIMII(HCOO)6] with M = Ni (EtFeNi) and Mn (EtFeMn) as well as [C2H5NH3][CrIIIMnII(HCOO)6] (EtCrMn) crystallizing in the niccolite type architecture (space group P 1c) with disordered ethylammonium (EtA⁺) cations located in the large cavities. Magnetic studies show that EtFeNi and EtFeMn exhibit ferromagnetic order below 43 and 38 K, respectively, whereas EtCrMn remains paramagnetic at least down to 2 K. Analysis of the Raman and IR data allowed us to propose assignment of the observed bands to the respective internal and lattice modes. These data give also evidence for weaker amine-cavity interactions compared to the perovskite analogues and show that this difference has significant effect on structure of the EtA⁺ cation. We also report that synthesis of Mg-analogues resulted in formation of ethylammonium magnesium formate with partially substituted Mg²⁺ (by Cr³⁺ or Fe³⁺) and EtA⁺ cations (by HCOOH molecules). These mixed-metal compounds have most likely the same R symmetry as the high-temperature phase of undoped perovskite-type [C2H5NH3][Mg(HCOO)3].
Dimethylammonium zinc formate ([(CH3)2NH2]Zn(HCOO)3 or DMZnF) is a model system for the study of hybrid perovskite-like dielectrics. It undergoes a phase transition from the paraelectric to ferroelectric phase at ∼166 K, as observed via NMR spectra. The mechanism of this phase transition has been shown to have contributions from ordering of the hydrogen bonds between [(CH3)2NH2]+ (DMA+) and the formate groups as well as buckling of the metal-formate framework, but the transition dynamics and atomistic mechanism are not fully clear. This work presents dielectric constant measurements as evidence of cluster formation of the low-temperature phase and the relaxor-like behavior of this metal–organic framework above the phase transition temperature. 13C CP-MAS is used to track the evolution of the chemical shift, T1, and T2 of the dimethylammonium cation and formate groups from room temperature to 120 K. 2D 13C–13C correlation measurements provide evidence of the formation of pretransitional clusters above the ...
We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH2NH3][M(HCOO)3] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna21, P63 to P212121 and P63 to P21. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.
Multiferroics and magnetoelectrics with coexisting and coupled multiple ferroic orders are materials promising new technological advances. While most studies have focused on single-phase or heterostructures of inorganic materials, a new class of materials called metal–organic frameworks (MOFs) has been recently proposed as candidate materials demonstrating interesting new routes for multiferroism and magnetoelectric coupling. Herein, we report on the origin of multiferroicity of (CH3)2NH2Mn(HCOO)3 via direct observation of ferroelectric domains using second-harmonic generation techniques. For the first time, we observe how these domains are organized (sized in micrometer range), and how they are mutually affected by applied electric and magnetic fields. Calculations provide an estimate of the electric polarization and give insights into its microscopic origin.
The novel formate-based framework [CH3CH2NH3][Cd(HCOO)3] has been prepared under solvothermal conditions. The X-ray diffraction data reveals that this compound crystallizes in the polar Pna21 space group and is isostructural with the compounds [CH3CH2NH3][M(HCOO)3], where MII = Mg, Mn and Cu, reported previously. The ethylammonium cations located in the cavities of the metal-formate framework are ordered at room temperature. Polarized infrared and Raman spectra of [CH3CH2NH3][Cd(HCOO)3] single crystal have been measured at room temperature and the assignments of the observed bands to the respective external and internal vibrations are proposed. Additionally, the previously reported [CH3CH2NH3][Mn(HCOO)3] has also been synthesized and polycrystalline IR and Raman spectra of both metal-formate frameworks have been compared in order to better understand the aspects of their structures.
We calculate the structural, electronic and magnetic properties of the subgroup of Metal-Organic-Frameworks (MOFs) [AmH][M(HCOO)3] (in which AmH+ = organic ammonium cation, M = divalent metal ion) using density functional theory with GGA+U approximation. The optimized structures and magnetic ground states are in good agreement with available experimental results. The electronic structures of these MOFs are obtained at their magnetic ground states. Using hybrid functional method (HSE06), the band gap is 4.33 eV, 4.12 eV, 4.15 eV and 4.78 eV for NH2NH3+, HONH3+, CH3CH2NH3+ and NH4+ compounds, respectively. The band gap of NH2NH3+ varies from 2.63 eV (-5% compressive strain) to 3.50 eV (+5% tensile strain) at Ueff = 4 eV. It is demonstrated that the band gap of such MOFs can be easily tuned by applying external strain and the AmH+ ligand for the first time. These MOFs all show insulating properties. In addition, such strain engineering may also be useful for enhancing the Neel temperature by changing the distance of magnetic Mn ions. Interestingly, Bader charge analysis indicates that AmH+ is fully ionic suggesting that appropriate arrangement may give rise to polar order associated with the magnetic ordering, these MOFs materials can be considered as potential multiferroics. Finally, this work reveals that both strain and chemical modification are efficient approaches for designing improved and novel MOFs for future applications in photocatalytic, optoelectronic, ferroelectric or multiferroic and electronic device.
We report the synthesis and characterisation of magnesium formate framework templated by protonated imidazole. Single-crystal x-ray diffraction data show that this compound crystallizes in the monoclinic structure with P21/n space group and lattice parameters a=12.1246(4) Å, b=12.2087(5) Å, c=12.4991(4) Å and b=91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests an antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics is: a=12.261(7) Å, b=12.290(4) Å, c=6.280(4) Å, β=90.62(5)º. Raman and IR data are consistent with x-ray diffraction data. They also indicate that disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to the strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.
The paraelectric-ferroelectric phase transition in two isostructural metal-organic frameworks (MOFs) [NH4 ][M(HCOO)3 ] (M=Mg, Zn) was investigated by in situ variable-temperature (25) Mg, (67) Zn, (14) N, and (13) C solid-state NMR (SSNMR) spectroscopy. With decreasing temperature, a disorder-order transition of NH4 (+) cations causes a change in dielectric properties. It is thought that [NH4 ][Mg(HCOO)3 ] exhibits a higher transition temperature than [NH4 ][Zn(HCOO)3 ] due to stronger hydrogen-bonding interactions between NH4 (+) ions and framework oxygen atoms. (25) Mg and (67) Zn NMR parameters are very sensitive to temperature-induced changes in structure, dynamics, and dielectric behavior; stark spectral differences across the paraelectric-ferroelectric phase transition are intimately related to subtle changes in the local environment of the metal center. Although (25) Mg and (67) Zn are challenging nuclei for SSNMR experiments, the highly spherically symmetric metal-atom environments in [NH4 ][M(HCOO)3 ] give rise to relatively narrow spectra that can be acquired in 30-60 min at a low magnetic field of 9.4 T. Complementary (14) N and (13) C SSNMR experiments were performed to probe the role of NH4 (+) -framework hydrogen bonding in the paraelectric-ferroelectric phase transition. This multinuclear SSNMR approach yields new physical insights into the [NH4 ][M(HCOO)3 ] system and shows great potential for molecular-level studies on electric phenomena in a wide variety of MOFs.
We show that a turnover from the classical Debye to the two-power-law relaxation behavior, observed in the majority of physical systems, is associated with a new type of a coupled memory continuous-time random walk driving a fractional dynamics. We derive a general class of the two-powerlaw relaxation responses which is able to reproduce all of the observed relaxation patterns, given by the low- and high-frequency power-law exponents falling in the range (0,1].
Vibrational properties and the temperature-induced phase transition mechanism have been studied in [NH4][Zn(HCOO)3] and [ND4][Zn(DCOO)3] metal organic frameworks by variable-temperature dielectric, IR, and Raman measurements. DFT calculations allowed proposing the detailed assignment of vibrational modes to respective motions of atoms in the unit cell. Temperature-dependent studies reveal a very weak isotopic effect on the phase transition temperature and confirm that ordering of ammonium cations plays a major role in the mechanism of the phase transition. We also present high-pressure Raman scattering studies on [ND4][Zn(DCOO)3]. The results indicate the rigidity of the formate ions and strong compressibility of the ZnO6 octahedra. They also reveal the onset of a pressure-induced phase transition at about 1.1 GPa. This transition has strong first-order character, and it is associated with a large distortion of the metal formate framework. Our data indicate the presence of at least two nonequivalent formate ions in the high-pressure structure with very different C-D bonds. The decompression experiment shows that the transition is reversible.
We report here a new class of ammonium metal–formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn2+, Zn2+, Co2+ and Mg2+) incorporating hydrazinium as the cationic template and component. The perovskite Mn and Zn members possess anionic 412·63 metal–formate frameworks with cubic cavities occupied by the NH2NH3+ cations, while the Co and Mg members have chiral 49·66 metal–formate frameworks, with chiral hexagonal channels accommodating NH2NH3+ cations. On heating, the Mn and Zn members undergo phase transitions around 350 K. The structures change from low temperature (LT) polar phases in Pna21 to high temperature (HT) non-polar phases in Pnma, due to the thermally activated librational movement of the NH2 end of the NH2NH3+ in the cavity and significant framework regulation. The Co and Mg members in LT belong to non-polar P212121, are probably antiferroelectric, and they show phase transitions at 380 K (Co) and 348 K (Mg), and the structures change to polar HT phases in P63, triggered by the order–disorder transition of the cation from one unique orientation in LT to three of trigonally-disorder state in HT. Accompanying the phase transitions, which are ferro- to para-electric for Mn and Zn members while antiferro- to ferro-electric for Co and Mg, prominent anisotropic thermal expansions including negative ones, and dielectric anomalies, are observed. The spontaneous polarization values are estimated at 3.58 (Mn, 110 K), 3.48 (Zn, 110 K), 2.61 (Co, 405 K) and 3.44 (Mg, 400 K) μC cm−2, respectively, based on the positive and negative charge separations in the polar structures. The structure–property relevance is established based on the order–disorder transitions of NH2NH3+ and the conformity and adaptability of the metal–formate frameworks to match such order–disorder alternations. The Mn and Co members show spin-canted antiferromagnetic long-range-ordering, with Néel temperatures of 7.9 K and 13.9 K, respectively. Therefore, the two members show coexistence of electric and magnetic orderings in the low temperature region, and they are possible molecule-based multiferroics.
We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH(+)-NH2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3(-)] is counterbalanced by formamidinium (FMD(+)) cations located in the cavities of the framework. These cations form extensive N-H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD(+) cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD(+) cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD(+) cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K.
Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), synthesized by assembling metal ions with organic ligands have recently emerged as a new class of crystalline porous materials. The amenability to design as well as fine-tunable and uniform pore structures makes them promising materials for a variety of applications. Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites/hybrids, which exhibit new properties that are superior to those of the individual components through the collective behavior of the functional units. This is a rapidly developing interdisciplinary research area. This review provides an overview of the significant advances in the development of diverse MOF composites reported till now with special emphases on the synergistic effects and applications of the composites. The most widely used and successful strategies for composite synthesis are also presented.
We perform Density Functional Theory calculations on a recently synthesized metal-organic framework (MOF) with a perovskite-like topology ABX3, i.e. [CH3CH2NH3]Mn(HCOO)3 and predict a multiferroic behaviour, \i.e.} a coexistence of ferroelectricity and ferromagnetism. A peculiar canted ordering of the organic A-cation dipole moments gives rise to ferroelectric polarization of ≈2 μC/cm(2). Starting from these findings, we show that by choosing different organic A-cations, it is possible to tune the ferroelectric polarization and increase it up to 6 μC/cm(2). The possibility of changing the magnitude and/or the canting of the organic molecular dipole opens new routes towards engineering ferroelectric polarization in the new class of multiferroic metal-organic frameworks.
This review presents a wide-ranging broad-brush picture of dielectric relaxation in solids, making use of the existence of a `universality' of dielectric response regardless of a wide diversity of materials and structures, with dipolar as well as charge-carrier polarization. The review of the experimental evidence includes extreme examples of highly conducting materials showing strongly dispersive behaviour, low-loss materials with a `flat', frequency-independent susceptibility, dipolar loss peaks etc. The surprising conclusion is that despite the evident complexity of the relaxation processes certain very simple relations prevail and this leads to a better insight into the nature of these processes.
In the frame of a new probabilistic approach to relaxation, the scenario of relaxation leading to the Havriliak– Negami and Kohlrausch–Williams–Watts responses of complex systems is presented. In this approach the macroscopic laws are related to the micro/mesoscopic stochastic characteristics of the relaxing systems. This provides a rigorous formulation of the energy-criterion argument, introduced by Jonscher to explain the commonly observed high-frequency fractional power law. The presented considerations reinforce the physical significance of the empirically found forms of relaxation, and open a new line of analysis of relaxation phenomena. Ó 2002 Elsevier Science B.V. All rights reserved.
A three-dimensional chiral metal formate framework compound, [NH(4)][Zn(HCOO)(3)], undergoes a paraelectric-ferroelectric phase transition at 191 K triggered by the disorder-order transition of NH(4)(+) cations within the structure.
Multiferroic behavior in perovskite-related metal-organic frameworks of general formula [(CH(3))(2)NH(2)]M(HCOO)(3), where M = Mn, Fe, Co, and Ni, is reported. All four compounds exhibit paraelectric-antiferroelectric phase transition behavior in the temperature range 160-185 K (Mn: 185 K, Fe: 160 K; Co: 165 K; Ni: 180 K); this is associated with an order-disorder transition involving the hydrogen bonded dimethylammonium cations. On further cooling, the compounds become canted weak ferromagnets below 40 K. This research opens up a new class of multiferroics in which the electrical ordering is achieved by means of hydrogen bonding.
Relaxor ferroelectrics, with their strong dependence of polarization on the applied electric field, are of considerable technological importance. On a microscopic scale, however, there exists competition as well as coexistence between short-range and long-range polar order. The conventional picture is that the polar nano-regions (PNRs) that appear at high temperatures beyond the Curie transition, form nuclei for the field-induced long-range order at low temperatures. Here, we report high-energy X-ray diffuse-scattering measurements on the relaxor Pb(Zn(1/3)Nb(2/3))O(3) (PZN) to study the short-range polar order under an electric field applied along the [111] direction. In contrast to conventional expectations, the overall diffuse-scattering intensity is not suppressed. On the other hand, the field induces a marked change on the shape of the three-dimensional diffuse-scattering intensity pattern, corresponding to a redistribution of PNRs in real space. We show that these surprising results are consistent with a model in which the PNRs with [110]-type polarizations, orthogonal to that of the surrounding environment, are embedded and persist in the [111]-polarized ferroelectric order of the bulk.
A new series of chiral metal formate frameworks of [HONH3
- B Liu
- R Shang
- K L Hu
- Z M Wang
- S Gao
B. Liu, R. Shang, K.L. Hu, Z.M. Wang, S. Gao, A new series of chiral metal formate
frameworks of [HONH3] [MII(HCOO)3] (M=Mn Co, Ni, Zn and Mg): synthesis,
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Metal-Organic perovskites: synthesis, structures, and magnetic properties of
- K.-L Hu
- M Kurmoo
- Z Wang
- S Gao
K.-L. Hu, M. Kurmoo, Z. Wang, S. Gao, Metal-Organic perovskites: synthesis,
structures, and magnetic properties of [C(NH 2 ) 3 ][M II (HCOO) 3 ] (M=Mn, Fe Co, Ni,
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Electron paramagnetic resonance and electric characterization of a
- M Šimenas
- S Balčiunas
- M Trzebiatowska
- M Ptak
- M Mączka
- G Völkel
- A Pöppl
- J Banys
M. Šimenas, S. Balčiunas, M. Trzebiatowska, M. Ptak, M. Mączka, G. Völkel,
A. Pöppl, J. Banys, Electron paramagnetic resonance and electric characterization
of a [CH 3 NH 2 NH 2 ][Zn(HCOO) 3 ] perovskite metal formate framework, J. Mater.
Chem. C 5 (2017) 4526-4536.
Dielectric relaxation and anhydrous proton conduction in
- A Sieradzki
- S Pawlus
- S N Tripathy
- A Gągor
- M Ptak
- M Paluch
- M Mączka
A. Sieradzki, S. Pawlus, S.N. Tripathy, A. Gągor, M. Ptak, M. Paluch, M. Mączka,
Dielectric relaxation and anhydrous proton conduction in [C 2 H 5 NH 3 ][Na 0.5 Fe 0.5
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An A-site mixed-ammonium solid solution perovskite series of [(NH 2 NH 3 ) x (CH 3 NH 3 ) 1-x
- S Chen
- R Shang
- B.-W Wang
- Z.-M Wang
- S Gao
S. Chen, R. Shang, B.-W. Wang, Z.-M. Wang, S. Gao, An A-site mixed-ammonium
solid solution perovskite series of [(NH 2 NH 3 ) x (CH 3 NH 3 ) 1-x ][Mn(HCOO) 3 ]
(x=1.00-0.67), Angew. Chem. Int. Ed. 54 (2015) 10987-11288.