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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Solid-state synthesis and phase transitions of RE2(MoO4)3 (RE ≡ Nd, Sm, Eu, and Gd) samples have been monitored by X-ray thermodiffraction with synchrotron radiation. The experiment was divided in two stages. In the first heating, different non-stoichiometric molybdates (Eu4Mo7O27, Eu2Mo4O15, and Pr2Mo4O15 structure types) emerged from the RE2O3 and MoO3 oxides before the expected phases (with α-Eu2(WO4)3 and La2(MoO4)3 structure types and the β-Gd2(MoO4)3 phase). The formation and coexistence of intermediate phases have been explained by common structural motifs with unit cell volumes per atom among those with the formula RE2(MoO4)3. Subsequent heating–cooling cycles showed the occurrence of the reversible and reconstructive α [La2(MoO4)3] ↔ β phase transition, including the less common transition β → α [La2(MoO4)3] obtained by heating the β′-Gd2(MoO4)3 phase from room temperature and clarifying much of the controversy in the literature. The transition mechanisms were studied by proposing a common supercell and comparing the RE and vacancy ordering within similar layers of MoO4 2– tetrahedra. The possible formation of stacking faults in Nd2(MoO4)3 was explained as a mixture of modulated scheelite phases. This research supports the importance of a directed and rational synthesis analyzing the intermediate products and their phase transitions for the enrichment of materials with new or improved properties.

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