Systématique simplifiée des composés ABX4 (X = O2−, F−) et evolution possible de leurs structures cristallines sous pression

Exploring Phase Transition and Structural Complexity in the Mixed Cation Uranium Oxide CaUNb2O8

The structures and high-temperature phase transition of CaUNb2O8 were studied in situ using synchrotron X-ray and neutron powder diffraction. Rietveld refinements provided an accurate description of the crystal structures of both the monoclinic fergusonite-type I2/b structure observed at room temperature and the tetragonal scheelite-type I41/a structure found at high temperatures. Bond valence sum analysis showed Nb5+ to be octahedrally coordinated in the monoclinic fergusonite-type structure, akin to other ANbO4 materials. Rietveld analysis of the variable temperature data allowed for the determination of accurate unit cell parameters and atomic coordinates, as well as revealing a reversible phase transition around ∼750 °C. The Nb-O bond distances display anomalous behavior, with a discontinuity in the longer Nb-O(1') distance coinciding with the phase transition suggestive of a reconstructive phase transition. Mode analysis identified the Γ2+ mode as the primary mode that drives the phase transition; this is linearly coupled to the induced spontaneous strain within the monoclinic fergusonite-type structure. Analysis of the temperature dependence of the Nb(z) positional parameter, as well as of the ϵ1-ϵ2 and ϵ6 strain parameters, showed that the phase transition is not strictly second order, with the critical exponent β ≠ 1/2. This study demonstrates the complex structural features of mixed cation metal oxides at elevated temperatures.

Synthesis and properties of anhydrous rare-earth phosphates, monazite and xenotime: a review

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.

Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO4

Scheelite-type metal oxides are a notable class of functional materials, with applications including ionic conductivity, photocatalysis, and the safe storage of radioactive waste. To further engineer these materials for specific applications, a detailed understanding of how their properties can change under different conditions is required─not just in the long-range average structure but also in the short-range local structure. This paper outlines a detailed investigation of the metal oxide CsReO4, which exhibits an uncommon orthorhombic Pnma pseudo-scheelite-type structure at room temperature. Using synchrotron X-ray diffraction, the average structure of CsReO4 is found to undergo a transformation from the orthorhombic Pnma pseudo-scheelite-type structure to the tetragonal I41/a scheelite-type structure at ∼440 K. In the X-ray pair distribution function analysis, lattice strain and rotations of the ReO4 tetrahedra are apparent above 440 K despite the increase in long-range average symmetry, revealing a disconnect between the structural models at different length scales. This study demonstrates how the bonding requirements and ionic radii of the A-site cation can induce disorder that is detectable at different length scales, affecting the physical properties of the material.

The identification of high-pressure phase transition sequence in selected tungstates and molybdates

Tungstates and molybdates possessing the scheelite- and wolframite-type (if present) structures hold a significant functional value. Their high-pressure phase diagrams are very complicated and controversial, and even some parts have not been characterized yet. In this study, we investigate the sequence of pressure driven structural phase transitions up to 100 GPa in these tungstate and molybdate families via first-principles structure predictions. Based on our structural predictions, it is possible for isostructural tungstates and molybdates to exhibit a phase transition sequence that is either similar or identical. Examples of these compounds are CaWO4, CaMoO4, and CdMoO4, in addition to EuWO4 and EuMoO4. However, the phase transition sequences of some tungstates and molybdates, especially those with different divalent cations, display noteworthy variations, revealing the intricate influence of ionic radii and electronic properties on crystal configurations. To obtain a deeper understanding of the high-pressure phase transition behavior of tungstates and molybdates, we analyze the high-pressure phase diagrams of MgWO4, SrWO4, and CaMoO4, representative examples of wolframite-type tungstate, scheelite-type tungstate, and scheelite-type molybdate, respectively, using x-ray powder diffraction. Our x-ray diffraction experiments and structure predictions consistently verify that the orthorhombic Cmca phase is a high-pressure phase of SrWO4. Structural configurations and mechanical properties of these predicted structures are discussed, and electronic properties are given. This study could have important implications for the fields of seismology and geophysics, as well as the utilization of these materials in various capacities, such as photocatalysts, photoanodes, and phosphors.

Effect of lithium doping on the structural, conduction mechanism and dielectric property of MnNbO4

The development of multifunctional materials is an exceptional research area, which is aimed at enhancing the versatility of materials according to their wide fields of application. Special interest was devoted here to lithium (Li)-doped orthoniobate ANbO₄ (A = Mn), in particular, the new material Li_0.08Mn_0.92NbO₄. This compound was successfully synthesized by a solid-state method and characterized using various techniques, including X-ray diffraction (XRD), which confirmed the successful formation of an ABO₄ oxide with an orthorhombic structure and the Pmmm space group. The morphology and elemental composition were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The vibrational study (Raman) at room temperature confirmed the existence of the NbO₄ functional group. The effects of frequency and temperature on the electrical and dielectric properties were studied using impedance spectroscopy. In addition, the diminishing of the radius of semicircular arcs in the Nyquist plots (-Z'' vs. Z') showed the semiconductor behavior of the material. The electrical conductivity followed Jonscher's power law and the conduction mechanisms were identified. The electrical investigations showed the dominant transport mechanisms in the different frequency and temperature ranges, proposing the correlated barrier hopping (CBH) model in the ferroelectric phase and the paraelectric phase. The temperature dependence in the dielectric study revealed the relaxor ferroelectric nature of Li_0.08Mn_0.92NbO₄, which correlated the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes. The results demonstrate that Li-doped Li_0.08Mn_0.92NbO₄ could be used both in dielectric and electrical applications.

Equations of State and Crystal Structures of KCaPO4, KSrPO4, and K2Ce(PO4)2 under High Pressure: Discovery of a New Polymorph of KCaPO4

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO₄, SrKPO₄, and K₂Ce(PO₄)₂ under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO₄, we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO₄ undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P3̅m1 and the high-pressure phase by space group Pnma. For KSrPO₄ and K₂Ce(PO₄)₂, no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.

Pressure-induced phase transitions in TmVO4 investigated by Raman spectroscopy

The structural behavior of TmVO₄ at pressures up to ∼ 55 GPa was investigated by Raman spectroscopy. The changes in Raman spectra suggest the existence of three phase transitions upon compression. The first phase transition appeared at ∼ 7.7 GPa, which was an irreversible phase transition from the ambient-pressure zircon phase to the scheelite phase, confirming previous X-ray measurements. Subsequently, the second reversible phase transition from the scheelite phase to the fergusonite phase occurred at ∼ 23 GPa. Additional changes in the Raman spectra were observed at ∼ 37 GPa, validating the third phase transition. Based on a comparison to related rare earth orthovanadates, we assumed that the post-fergusonite of TmVO₄ has an orthorhombic structure described by space group Cmca. The wavenumbers of the Raman modes and their pressure coefficients for all four phases of TmVO₄ are reported. Our study provides the vibrational difference in various polymorphs of TmVO₄, which will refine our understanding of the structural behavior of rare-earth orthovanadates.

Insights into the structural variations in SmNb1-xTaxO4 and HoNb1-xTaxO4 combined experimental and computational studies

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1-xTaxO4 (A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm-O distance and volume of the SmO8 polyhedra were invariant of the Ta content across the SmNb1-xTaxO4 solid solution whereas the average M-O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1-xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M'-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb-O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M-O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm-1 due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra.

Borates or phosphates? That is the question

Chemical nomenclature is perceived to be a closed topic. However, this work shows that the identification of polyanionic groups is still ambiguous and so is the nomenclature for some ternary compounds. Two examples, boron phosphate (BPO₄) and boron arsenate (BAsO₄), which were assigned to the large phosphate and arsenate families, respectively, nearly a century ago, are explored. The analyses show that these two compounds should be renamed phosphorus borate (PBO₄) and arsenic borate (AsBO₄). Beyond epistemology, this has pleasing consequences at several levels for the predictive character of chemistry. It paves the way for future work on the possible syntheses of SbBO₄ and BiBO₄, and it also renders previous structure field maps completely predictive, allowing us to foresee the structure and phase transitions of NbBO₄ and TaBO₄. Overall, this work demonstrates that quantum mechanics calculations can contribute to the improvement of current chemical nomenclature. Such revisitation is necessary to classify compounds and understand their properties, leading to the main final aim of a chemist: predicting new compounds, their structures and their transformations.

Tailoring structural properties of lanthanum orthoniobates through an isovalent substitution on the Nb-site

The structure and thermomechanical properties of As-substituted lanthanum orthoniobates are presented and an in-depth analysis of a broad range of other substituents is performed.

New pressure-induced polymorphic transitions of anhydrous magnesium sulfate

Discovery of phase transitions in MgSO₄ under compression. The high-pressure phases involve a coordination number increase for magnesium.

High-Pressure Phase Transformations in TiPO4 : A Route to Pentacoordinated Phosphorus

Titanium(III) phosphate, TiPO₄  , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO₄ reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO₄ -V is the first inorganic phosphorus-containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO₄ -V coexists with TiPO₄ -IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO₄ -II is found to be an incommensurately modulated phase whereas a lock-in transition at about 7 GPa leads to TiPO₄ -III with a fourfold superstructure compared to the structure of TiPO₄ -I at ambient conditions. TiPO₄ -II and TiPO₄ -III are similar to the corresponding low-temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin-Peierls interactions.

Phase transformations and indications for acoustic mode softening in Tb-Gd orthophosphate

At ambient conditions the anhydrous rare earth orthophosphates assume either the xenotime (zircon) or the monazite structure, with the latter favored for the heavier rare earths and by increasing pressure. Tb0.5Gd0.5PO4 assumes the xenotime structure at ambient conditions but is at the border between the xenotime and monazite structures. Here we show that, at high pressure, Tb0.5Gd0.5PO4 does not transform directly to monazite but through an intermediate anhydrite-type structure. Axial deformation of the unit cell near the anhydrite- to monazite-type transition indicates softening of the (c1133  +  c1313) combined elastic moduli. Stress response of rare-earth orthophosphate ceramics can be affected by both formation of the anhydrite-type phase and the elastic softening in the vicinity of the monazite-phase. We report the first structural data for an anhydrite-type rare earth orthophosphate.

Nanoscale stabilization of the scheelite-type structure in La(0.99)Ca(0.01)NbO4 thin films

In this paper we report on the deposition of La0.99Ca0.01NbO4 thin films with scheelite-type crystal structure. Thanks to the film's nanostructure, we were able to stabilize the tetragonal scheelite-type structure phase at room temperature, which involves a full removal of the fergusonite-scheelite phase transition.

Structures of two new high-pressure forms of AlPO4 by X-ray powder diffraction and NMR spectroscopy

The crystal structures of two new high-pressure AlPO4 phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic (P\bar 1) whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (P2_1/c). 31P MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. 27Al 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO4 tetrahedra. The P\bar{1} phase is isostructural with FeVO4 and AlVO4. The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.

The effects of pressure, temperature and composition on the crystal structures of α-quartz homeotypes

α-Quartz and its homeotypes are of great importance for both materials and Earth sciences. The properties of these materials depend strongly on their crystal structures and particularly the intertetrahedral bridging angle and the tetrahedral tilt angle. These angles are highly dependent on composition and the external parameters pressure and temperature. The behavior of the eleven known α-quartz homeotypes, along with examples of α-quartz-type solid solutions, are compared. The distortion in α-quartz-type structures decreases as a function of temperature and increases as a function of pressure. Thermal stability depends on initial structural distortion and on the electronic configuration of the cation. Pressure stability also depends on the former and on cation size. Transitions to new crystalline and/or amorphous forms, often with increased cation coordination number, are commonly observed at high-pressure. The combined use of high-pressure and high-temperature can be used to synthesize novel α-quartz homeotypes in compounds with small cations.