Role of rare earth sites and vacancies in the anomalous compression of modulated scheelite tungstates RE2 ( WO4)3

Abstract

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 \ensuremath{\alpha}-${RE}_{2}$(${\mathrm{WO}}_{4}{)}_{3}$ ($\mathit{RE}$ = La-Ho) family with modulated scheelite structure (\ensuremath{\alpha} 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 \ensuremath{\beta} 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 ${\mathrm{WO}}_{4}^{2\ensuremath{-}}$ tetrahedrons. These changes correlate with those in the alternating RE\dots{}RE\dots{}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.''