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The Sc(2)(WO(4))(3)-type phase (Pbcn) of Y(2)(MoO(4))(3), Er(2)(MoO(4))(3) and Lu(2)(MoO(4))(3) has been prepared by the conventional solid-state synthesis with preheated oxides and the negative thermal expansion (NTE) has been investigated along with an exhaustive structural study, after water loss. Their crystal structures have been refined using the neutron and x-ray powder diffraction data of dehydrated samples from 150 to 400 K. The multi-pattern Rietveld method, using atomic displacements with respect to a known structure as parameters to refine, has been applied to facilitate the interpretation of the NTE behavior. Polyhedral distortions, transverse vibrations of A· · ·O-Mo (A = Y and rare earths) binding oxygen atoms, non-bonded distances A· · ·Mo and atomic displacements from the high temperature structure, have been evaluated as a function of the temperature and the ionic radii.

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X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire α-RE 2 (WO 4 ) 3 (RE = La-Ho) family with modulated scheelite structure (α phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters a, c, and β was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter a decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the a axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the WO 2− 4 tetrahedrons. These changes correlate with those in the alternating RE …RE …RE chains and blocks of cationic vacancies arranged along the a axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having "hinge frames."

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We have studied the effect of pressure on the structural and vibrational properties of lanthanum tritungstate La 2 (WO 4 ) 3 . This compound crystallizes under ambient conditions in the modulated scheelite-type structure known as the α phase. We have performed x-ray diffraction and Raman scattering measurements up to a pressure of 20 GPa, as well as ab initio calculations within the framework of the density functional theory. Up to 5 GPa, the three methods provide a similar picture of the evolution under pressure of α-La 2 (WO 4 ) 3 . At 5 GPa, we begin to observe some structural changes, and above 6 GPa we find that the x-ray patterns cannot be indexed as a single phase. However, we find that a mixture of two phases with C2/c symmetry accounts for all diffraction peaks. Our ab initio study confirms the existence of several C2/c structures, which are very close in energy in this compression range. According to our measurements, a state with medium-range order appears at pressures above 9 and 11 GPa, from x-ray diffraction and Raman experiments, respectively. Based upon our theoretical calculations we propose several high-pressure candidates with high cationic coordinations at these pressures. The compound evolves into a partially amorphous phase at pressures above 20 GPa.

A large number of known AB compounds adopt the B1 (NaCl) structure. Under pressure, most of them transform to the B2 (CsCl) structure directly or via intermediate phase(s). The computational investigations suggest that the dense B2 phase is favored at high pressure for silver halides with the B1-structure (AgX, X=Cl, Br and I). The experimental studies have shown that AgCl transforms from B1 to B2 at 17 GPa, 200 ℃ [1]. In-situ X-ray work [1] showed two intermediate phases existing between the B1-B2 phase transition in AgCl, with the structure of KOH and TlI, respectively. As for AgBr and AgI, both are known to undergo B1-KOH-type structural transitions [2] in the pressure range between 13-16 GPa. In this study, we extended the experimental conditions to higher pressure to investigate the phase transitions in AgBr and AgI and to find out if they share the same topological transition as AgCl under high pressure. The silver halides were studied in the diamond anvil cell (DAC) by angular dispersive X-ray powder diffraction technique. The highpressure phase transformations of AgCl, AgBr and AgI were investigated up to 28, 41 and 56 GPa, respectively. In AgCl, a phase transition sequence B1 → KOH → TlI → B2 was observed at high pressure, room temperature. The pathway of transformations in AgBr was found to be B1→ (unknown phase) → KOH → TlI at high pressure. The "unknown phase" observed in AgBr was found for the first time and has been detected in three consecutive runs carried out in this study. At 35.5 GPa, we applied the laser heating (estimated temperature ~ 500 ℃) and found that TlI-type AgBr maintained the same structure type (TlI). For AgI, the KOH-type high-pressure phase was observed up to 27 GPa. Upon further compression to the pressure of 56 GPa, and after annealing by laser heating (estimated temperature ~ 500 ℃), AgI did not transform to the B2 phase. Based on the current experimental results, we conclude that the topological transition of B1-KOH-TlI-B2 type AgCl can be achieved by applying pressure along. The occurrence of the TlI phase in AgBr at high pressures, shows that the structural transition sequence of B1-B2 in AgBr is similar to the one found in AgCl. Yet, an additional new low-pressure phase is found to exist in AgBr between the B1 and KOH phase. Computational studies confirm the same transition sequence for the B1-B2 transition of AgI [3]. Within the experimental data resolution, KOH-type AgI is identified to be stable up to 27 GPa, at least. Compared with AgCl and AgBr, the stability range of the KOHtype AgI is surprisingly wide, an effect which has also been noted in the computational study [3]. However, our x-ray diffraction data of AgI above 40 GPa do not confirm the TlI structure type, which was predicted to be the stable polymorph in the computational studies [3]. In general, the transition pressures of this multiple series of solidsolid phase transformation are increasing with decreasing ionicity of this series of AgX compounds. The ionicities for AgCl, AgBr and AgI are 0.869, 0.847 and...

In order to clarify the polymorphism in the lithium sulfate family, LiRE x (NH 4 ) 1 À x SO 4 (0.5 x 4.0 mol%, nominal value; RE = Er 3+, Yb 3+and Dy 3+ ) crystals were grown from aqueous solution by slow evaporation between 298 and 313 K. The doping of the samples allowed us to obtain two polymorphic forms, and , of LiNH 4 SO 4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two newpolytypes, which we have named 1 -and 2 -LAS. They present the same space group P2 1 /c and the following relation among their lattice parameters: a 2 = Àc 1 , b 2 = Àb 1 , c 2 = À2a 1 À c 1 . In order to evaluate the stability of the newpolytypes, we performed thermal analysis, X-ray diffraction and dielectric spectroscopy on single crystals and polycrystalline samples over the cyclic temperature range: 190 ! 575 ! 190 K. The results obtained by all the techniques used in this study demonstrate that -polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric -phase above 500 K. In addition, a comparative study of -andpolytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive ! transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an -habit.