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Theoretical and experimental study of the structural stability of TbPO_ {4} at high pressures
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We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at ∼9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon→monazite→scheelite→SrUO4-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO4-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO4.
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... Xenotime DyPO 4 is also tested in the "none" condition to build on our prior study [9]. DyPO 4 and TbPO 4 have B 0 values of 144 GPa and 134 GPa, respectively, which are an order of magnitude higher than the B 0 of KCl [9,19]. Our TbPO 4 experiment using neon PTM shows a xenotime-monazite P onset of 8.7(6) GPa, which is lower than other reported XRD-based hydrostatic P onset values of 9.8 GPa and 9.9 GPa [19]. ...
... DyPO 4 and TbPO 4 have B 0 values of 144 GPa and 134 GPa, respectively, which are an order of magnitude higher than the B 0 of KCl [9,19]. Our TbPO 4 experiment using neon PTM shows a xenotime-monazite P onset of 8.7(6) GPa, which is lower than other reported XRD-based hydrostatic P onset values of 9.8 GPa and 9.9 GPa [19]. In addition, TbPO 4 experiments in this study show a systematic lowering of the xenotime-monazite P onset when changing the PTM from neon to KCl to "none" (i.e., as PTM bulk modulus increases). ...
... Data collection began at pressures higher than 0 GPa due to some initial compression required to confirm membrane engagement. There are no reported TbPO 4 or DyPO 4 phase transitions below these starting pressures (as corroborated by prior work); therefore, the initial jump does not preclude any material insight [7,9,19]. The maximum pressure in each experiment (P max ) was limited by the performance of the gasket. ...
The pressure-induced phase transformations of rare earth orthophosphates (REPO4s) have become increasingly relevant in ceramic matrix composite (CMC) research; however, understanding of the shear-dependence of these transformations remains limited. This study employs diamond anvil cell experiments with three pressure media (neon, KCl, sample itself/no medium) to systematically assess the effect of shear on the phase transformations of TbPO4. Results show a lowering of the TbPO4 transformation onset pressure (Ponset) as well as an extension of the xenotime–monazite phase coexistence range under non-hydrostatic conditions. The TbPO4 Ponset under no medium (4.4(3) GPa) is the lowest REPO4 Ponset reported to date and represents a ~50% drop from the hydrostatic Ponset. Enthalpic differences likely account for lower Ponset values in TbPO4 compared to DyPO4. Experiments also show scheelite may be the post-monazite phase of TbPO4; this phase is consistent with observed and predicted REPO4 transformation pathways.
... Prior studies and reviews have reported phase diagrams showing REPO 4 transformation pressures based on a variety of computational and experimental techniques [5,[15][16][17][18]. Recent advancements in in situ diamond anvil cell (DAC) X-ray diffraction (XRD) experiments require updating the high-pressure REPO 4 phase map [12,[18][19][20][21][22][23][24][25]. In contrast to Raman spectroscopy and ab initio calculations, XRD provides more direct, crystallographic proof of the existence of REPO 4 phases and phase transformations. ...
... Prior studies and reviews have reported phase diagrams showing REPO4 transformation pressures based on a variety of computational and experimental techniques [5,[15][16][17][18]. Recent advancements in in situ diamond anvil cell (DAC) X-ray diffraction (XRD) experiments require updating the high-pressure REPO4 phase map [12,[18][19][20][21][22][23][24][25]. In contrast to Raman spectroscopy and ab initio calculations, XRD provides more direct, crystallographic proof of the existence of REPO4 phases and phase transformations. ...
... Under hydrostatic conditions, the xenotime monazite transformation has been reported in ErPO4, HoPO4, YPO4, DyPO4, and TbPO4 with onset pressures (Ponset) of 17.3 GPa, 17.7 GPa, 14.6 GPa, 9.1(1) GPa, and 9.9 GPa, respectively [12,[20][21][22][23]. We note that for any number followed by a number in parentheses, the number in parentheses represents the standard deviation of the last digit of the number before the parentheses. ...
Interest in the deformation behavior and phase transformations of rare earth orthophosphates (REPO4s) spans several fields of science—from geological impact analysis to ceramic matrix composite engineering. In this study, the phase behavior of polycrystalline, xenotime DyPO4 is studied up to 21.5(16) GPa at ambient temperature using in situ diamond anvil cell synchrotron X-ray diffraction. This experiment reveals a large xenotime–monazite phase coexistence pressure range of 7.6(15) GPa and evidence for the onset of a post-monazite transformation at 13.9(10) GPa to scheelite. The identification of scheelite as the post-monazite phase of DyPO4, though not definitive, is consistent with REPO4 phase transformation pathways reported in both the experimental and the computational literature.
... This is because steady advancements in high performance computing and computational software, especially in ab initio methodsbased codes, allows nowadays for computation of complex systems containing hundreds of atoms from first principles (Jahn and Kowalski, 2014). Regarding ceramic compounds considered here, computational studies have been used to deliver information on: the structural (Rustad, 2012;Feng et al., 2013;Blanca-Romero et al., 2014;Beridze et al., 2016;Huittinen et al., 2017), the electronic structure (Tang and Holzwarth, 2003;Blanca-Romero et al., 2014;Kowalski et al., 2017a;Lee et al., 2017), the elastic (Wang et al., 2005;Feng et al., 2013;Ali et al., 2016;Ji et al., 2017a;Kowalski et al., 2017b), the thermodynamic (Mogilevsky, 2007;Feng et al., 2013;Li et al., 2014;Kowalski et al., 2015Ji et al., 2017b;Neumeier et al., 2017b;Eremin et al., 2019), the thermochemical (Rustad, 2012;Beridze et al., 2016;Kowalski, 2020), the electrochemical (Krishnamurthy et al., 2005b;Lee et al., 2017), and the radiation damage resistance Ji et al., 2017c;Jolley et al., 2017) parameters as well as materials at high-pressure (López-Solano et al., 2010;Stavrou et al., 2012;Ali et al., 2016;Shein and Shalaeva, 2016;Gomis et al., 2017). The relevant research activity increases steadily worldwide, with most of the papers published just recently. ...
Li x FePO 4 orthophosphates and fluorite- and pyrochlore-type zirconate materials are widely considered as functional compounds in energy storage devices, either as electrode or solid state electrolyte. These ceramic materials show enhanced cation exchange and anion conductivity properties that makes them attractive for various energy applications. In this contribution we discuss thermodynamic properties of Li x FePO 4 and yttria-stabilized zirconia compounds, including formation enthalpies, stability, and solubility limits. We found that at ambient conditions Li x FePO 4 has a large miscibility gap, which is consistent with existing experimental evidence. We show that cubic zirconia becomes stabilized with Y content of ~8%, which is in line with experimental observations. The computed activation energy of 0.92eV and ionic conductivity for oxygen diffusion in yttria-stabilized zirconia are also in line with the measured data, which shows that atomistic modeling can be applied for accurate prediction of key materials properties. We discuss these results with the existing simulation-based data on these materials produced by our group over the last decade. Last, but not least, we discuss similarities of the considered compounds in considering them as materials for energy storage and radiation damage resistant matrices for immobilization of radionuclides.
The synthesis methods, crystal structures, and properties of anhydrous monazite and xenotime (REPO4) crystalline materials are summarized within this review. For both monazite and xenotime, currently available Inorganic Crystal Structure Database data were used to study the effects of incorporating different RE cations on the unit cell parameters, cell volumes, densities, and bond lengths. Domains of monazite-type and xenotime-type structures and other AXO4 compounds (A = RE; X = P, As, V) are discussed with respect to cation sizes. Reported chemical and radiation durabilities are summarized. Different synthesis conditions and chemicals used for single crystals and polycrystalline powders, as well as first-principles calculations of the structures and thermophysical properties of these minerals are also provided.
Zircon structure-types are ternary oxides with an ideal chemical formula of ATO4 (I41/amd), where usually A are lanthanides and actinides, with T = As, P, Si, V. This structure accommodates a diverse chemistry on both A- and T-sites, giving rise to more than seventeen mineral end-members of five different mineral groups, and forty-five synthetic end-members. Because of their diverse chemical and physical properties, the zircon structure-types are of interest to a wide variety of fields and may be used as ceramic nuclear waste forms and aeronautical environmental barrier coating, to name a couple. To support advancement of their applications, many studies have been dedicated to the understanding of their structural and thermodynamic properties. The emphasis in this review will be on recent advances in the structural and thermodynamic studies of zircon structure-type ceramics, including pure endmembers (i.e., zircon (ZrSiO4), xenotime (YPO4)), and solid solutions (i.e., ErxTh1-x(PO4)x(SiO4)1-x). Specifically, we offer an overview on the crystal structure, its variations and transformations in response to non-ambient stimuli (temperature, pressure and radiation), and correlation to thermophysical and thermochemical properties. We hope this review will promote further applications of these ceramic materials.
The behavior of natural monazite (La,Ce,Nd)PO4 at high pressure is investigated by Raman spectroscopy. Changes in the Raman spectra indicate a phase transition from monazite to a post-monazite phase. Single-crystal x-ray diffraction measurements of the decompressed sample reveal that the post-monazite phase has a distorted barite-type structure with space group P21/n. This phase transition is irreversible under quasihydrostatic conditions, and is reversible under nonhydrostatic conditions. Therefore, it is expected that the post-monazite phase cannot be found as a metastable phase at ambient conditions in impact craters due to nonhydrostatic conditions during shock metamorphism, supporting recent mineralogical observations in impact craters.
Pressure-induced phase transitions from the zircon structure-type (I4 1 /amd) to the scheelite structure type (I4 1 /a) are known for many ternary oxides systems (ABO 4). In this work, we present the first high-pressure study on synthetic stetindite (CeSiO 4) by a combination of in situ high-pressure synchrotron powder X-ray diffraction up to 36 GPa, implemented with and without dual sided laser heating, and in situ high-pressure Raman spectroscopy up to 43 GPa. Two phase transitions were identified: zircon to a high-pressure low-symmetry (HPLS) phase at 15 GPa and then to a scheelite at 18 GPa. The latter from HPLS scheelite phase was found irreversible; i.e., scheelite is fully quenchable at ambient conditions, as in other zircon-type phases. The bulk moduli (K 0) of stetindite, HPLS, and high-pressure scheelite phases were determined, respectively, as 171(5), 105(4), and 221(40) GPa by fitting to a second-order Birch−Murnaghan equation of state. The pressure derivatives of vibrational modes and Gruneisen parameters of the zircon-structured polymorph are similar to those of other orthosilicate minerals. Due to the larger ionic radii of Ce 4+ , with respect to Zr 4+ , stetindite was found to possess a softer bulk modulus and undergo the phase transitions at a lower pressure than zircon (ZrSiO 4), such observations are consistent with what were found in coffinite (USiO 4).
Rare earth elements (REEs), the 15 naturally occurring lanthanides plus yttrium and scandium, are ubiquitously used in modern life as they are critical components of many advanced devices and technologies. However, the demand for REEs is not equal, with the heavy rare earth elements (HREEs) having a higher demand. Xenotime (HREEPO 4) is an important HREE ore mineral and globally is an economical source of HREE. Most of the crystallographic and thermodynamic properties of xenotime endmembers have been elucidated by calorimetric, solubility, and high-pressure studies. Yet, in natural systems, endmembers are rarely encountered, and instead, REE solid solutions are more commonly observed. In this work, we characterize the crystal chemistry, thermodynamics of HREE mixing, and high-temperature material behaviors and thermochemistry of a synthetic erbium (Er)−ytterbium (Yb) binary xenotime solid solution (Er (x) Yb (1−x) PO 4) using a suite of experimental techniques, including X-ray fluorescence spectroscopy, synchrotron X-ray powder diffraction implemented with Rietveld analysis, Fourier transform infrared spectroscopy coupled with attenuated total reflectance, Raman spectroscopy, thermogravimetric analysis coupled with differential scanning calorimetry, and high-temperature oxide melt drop solution calorimetry. Our results shed light on the formation of natural xenotimes and lay the foundation for their industrial applications as thermal coating materials.
In situ synchrotron x-ray diffraction was conducted on polycrystalline DyPO4 to elucidate the details of the pressure-induced transition from the xenotime polymorph to the monazite polymorph. We used three different pressure-transmitting media (neon, a 16:3:1 methanol-ethanol-water mixture, and potassium chloride) to investigate the effect of hydrostaticity on the phase behavior. Specifically, our data clearly show a hydrostatic onset pressure of the xenotime-monazite transition of 9.1 GPa, considerably lower than the 15.3 GPa previously determined by Raman spectroscopy. Based on (quasi)hydrostatic data taken in a neon environment, third-order Birch-Murnaghan equation-of-state fits give a xenotime bulk modulus of 144 GPa and a monazite bulk modulus of 180 GPa (both with pressure derivatives of 4.0). Structural data and axial compressibilities show that DyPO4 is sensitive to shear and has an anisotropic response to pressure. More highly deviatoric conditions cause the onset of the transition to shift to pressures at least as low as 7.0 GPa. We attribute early transition to shear-induced distortion of the PO4 tetrahedra. Our characterization of the high-pressure behavior of DyPO4 under variable hydrostaticity is critical for advancing rare earth orthophosphate fiber coating applications in ceramic matrix composites and may inform future tailoring of phase composition for controlled shear and pressure applications.
Monazite and xenotime, the RE(P04) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(P04) phases for all individual stable rare earth elements. Monazite is monoclinic, P2/n, and xenotime is isostructural with zircon (space group 14/amd). Both atomic arrangements are based on (001) chains of intervening phosphate tetrahedra and RE polyhedra, with a REOgpolyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a RE09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the struc- ture "transforms" from xenotime to monazite, the crystallographic properties are com- parable along the (00I) chains, with structural adjustments to the different sizes of RE atoms occurring principally in (00I). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down (001). In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along (010), relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional 02' atom to complete the RE09 polyhedron.
Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type YPO4 and ErPO4, and monazite-type GdPO4, EuPO4, NdPO4, and LaPO4 were performed in a diamond-anvil cell up to 30 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is at 19.7 GPa in YPO4 and 17.3 GPa in ErPO4. In LaPO4 a nonreversible transition is found at 26.1 GPa and a barite-type structure is proposed for the high-pressure phase. For the other three monazites studied, their structures were found to be stable up to 30 GPa. Evidence for additional phase transitions or chemical decomposition of the materials was not found in the experiments. The equations of state and axial compressibility for the different phases are also determined. In particular, we found that in a given compound the monazite structure is less compressible than the zircon structure. This fact is attributed to the higher packing efficiency of monazite versus zircon. The differential bond compressibility of different polyhedra is also reported and related to the anisotropic compressibility of both structures. Finally, the sequence of structural transitions and compressibilities are discussed in comparison with other orthophosphates.
Using laser-heated diamond-anvil cells, we have observed CaSO4 undergoing phase transitions from its ambient anhydrite structure to the monazite type, and at highest pressure and temperature to crystallize in the barite-type structure. On cooling, the barite structure distorts from an orthorhombic to a monoclinic lattice to produce the AgMnO4-type structure. The barite-structured form of CaSO4 that we encounter at high pressure and temperature has been, in particular, long expected as a high- pressure phase of CaSO4-anhydrite from systematic trends of similar AIIBVIO4-type sulfates, selenates, and tellurates, but has not been observed before. Similarly, the monoclinic distortion of the barite structure has itself been proposed as an intermediate phase between anhydrite and barite types through comparison with the phase diagrams of NaBF4 and NaClO4. This result has important consequences for identifying structural trends between different ABO4-type phases of Group II sulfates, selenates, tellurates, chromates, molybdates and tungstates that crystallize in anhydrite, zircon, monazite, barite and scheelite-type structures at ambient and high pressures.
We report an experimental and theoretical lattice-dynamics study of yttrium orthovanadate (YVO4) up to 33 GPa together with a theoretical study of its structural stability under pressure. Raman-active modes of the zircon phase are observed up to 7.5 GPa, where the onset of an irreversible zircon-to-scheelite phase transition is detected, and Raman-active modes in the scheelite structure are observed up to 20 GPa, where a reversible second-order phase transition occurs. Our ab initio total-energy calculations support that the second-order phase transition in YVO4 is from the scheelite to the monoclinic M-fergusonite structure. The M-fergusonite structure remains up to 33 GPa and on pressure release the sample reverts back to the metastable scheelite phase. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the zircon, scheelite, and M-fergusonite phases are discussed.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
ScPO4 and YPO4 with the tetragonal zircon-structure were studied at room temperature and pressures up to ~50 GPa. Pressure-induced phase transitions to the sheelite structure occur at 30 GPa for ScPO4 and 16.3 GPa for YPO4, respectively. In addition to the scheelite-type high-pressure phase, an intermediate phase with the monoclinic monazite-type structure formed during the phase transition process of YPO4. The high-pressure phases of ScPO4 and YPO4 are not quenchable on pressure release. The pressure dependence of the total energy of the different phases was calculated using density functional method, and the results confirm the experimentally observed phase relations under pressure. Structural parameters and compressibility of each phase were determined by refinement of the x-ray diffraction patterns. The high-pressure phase of ScPO4 has a very large bulk modulus (376(8) GPa).
A comprehensive critical review of the phase fields, metastable modifications, solid solution ranges and phase transitions of monazite- and zircon-type REEXO4 (X = P, As, V) compounds is given. Monazite-type REEPO4 compounds are stable for REE = La to Gd and metastable for Th to Ho; zircon-type members exist for REE = Gd to Lu, and Y, Sc. REEAsO4 compounds with monazite-type structure exist for REE = La to Nd, while zircon-type compounds are known for REE = Pni to Lu, and Y, Sc; no metastable arsenate members are known. The only stable monazite-type REEVO4 is LaVO4, but metastable members are known for REE = Ce to Nd. Zircon-type REEVO4 compounds are stable for REE = Ce to Lu, and Y, Sc, and metastable for REE = La. Solid solution series are complete only if minor size differences exist between REE3+ or X5+ cations in respective end-members. Phase transitions occur under pressure (zircon --> (monazite -->) scheelite) and at very low temperatures. The evaluation of the metastable phase fields and of naturally occurring members suggests that metastable modifications of REEXO4 compounds can occur in nature under certain conditions (formation at temperatures < similar to200-300degreesC; formation via hydrated precursor phases; stabilisation by various impurity cations).
Une systématique classique des composés ABX4 (X = O2−, F−) consiste à rattacher leurs structures cristallines à des structures-types dans le systéme de coordonnées [] avec rA, rB, rX rayons ioniques respectifs des cations, A, B et de l'anion X. Ce type de classification a été révisé sur les bases suivantes: utilisation d'un ensemble cohérent de valeurs de rayons ioniques tenant compte des coordinences et choix conventionnel de l'ordre formulaire ABX4 tel que rA > rB. Il en résulte une unification de la représentation, qui permet de prévoir empiriquement les tendances du polymorphisme sous pression de ces composés.
Murnaghan's theory of finite strain is developed for a medium of cubic symmetry subjected to finite hydrostatic compression, plus an arbitrary homogeneous infinitesimal strain. The free energy is developed for cubic symmetry to include terms of the third order in the strain components. The effect of pressure upon the second-order elastic constants is found and compared with experiment, with particular reference to the compressibility; the pressure-volume relation in several approximations is compared with the measurements to 100,000 kg/cm2. The simplest approximation is shown to give a satisfactory account of most of the experimental data. The results are also compared with some of the calculations based on Born's lattice theory.
Experimental work has shown that YLiF4 has a first phase transition from the ambient pressure scheelite to a fergusonite structure at 10GPa , and a second one at 17GPa to an unresolved structure. Theoretical work has proposed either M - or M' -fergusonite as the second structural phase, and wolframite or a P21/c -symmetry structure as the third. In this ab initio study, we find that the M' -fergusonite structure is a better candidate for the second phase of YLiF4 , and propose a new structure with Cmca symmetry as the third.