The coefficient of thermal expansion and elastic stiffnesses of spinel structure MgGa₂O₄ were determined from 103 K to 1673 K using dilatometry and resonant ultrasound spectroscopy. The state of cation order was investigated on specimens quenched from temperatures up to 1473 K via single-crystal X-ray diffraction. Even at room-temperature, the material is stiffer than what was expected from DFT simulations at 0 K, however, the stiffness falls within the predicted range based on the stiffness of the constituent oxides of MgGa₂O₄. The anisotropy of its longitudinal elastic stiffness is low, whereas there is a high anisotropy of the shear resistance compared to other cubic materials. At about 820 K–860 K, the temperature dependences of both thermal expansion and elastic properties change rapidly. Cation reordering also starts in this temperature range; the state of order is static at lower temperatures. Thus, MgGa₂O₄ undergoes a glass-like transition when heated above 820 K–860 K, where the state of cation order starts relaxing towards equilibrium in laboratory timescales. Landau-theory for nonconvergent cation ordering can describe the observed cation order at elevated temperatures well.

The ultrasound attenuation in langasite crystals increases rapidly at about 800 K with increasing temperature for reasons that are not well understood. In this paper, the attenuation quantified as Q ^-1 of the langasite-type materials La ₃ Ga ₅ SiO ₁₄ (LGS) and La ₃ Ta _0.5 Ga _5.5 O ₁₄ (LGT) was studied from room temperature to 1653 and 1608 K, respectively, using resonant ultrasound spectroscopy. Two to three attenuation peaks can be seen. A change of the magnitudes of the largest two attenuation peaks in LGT was correlated with the changing color of an LGT sample, which is related to its oxygen vacancy concentration. Thus, the attenuation likely involves oxygen vacancies. The observed Q ^-1 can be explained well by a model based on the anelastic relaxation of two to three noninteracting point defects causing Debye peak-like attenuation maxima. The activation energies of the largest two relaxation peaks match the activation energies for different conductivity mechanisms in LGS and LGT. Thus, the oxygen movement-based conductivity and the relaxation processes seem to involve the exchange of ions and vacancies on the same positions. The largest two attenuation peaks are probably caused by the movement of ions induced by two different phenomena, the deformation of the lattice (pointdefect relaxation) on the one hand and the electric field via the piezoelectric effect (piezoelectric/carrier relaxation) on the other hand.

A comprehensive study of sillenite Bi₁₂SiO₂₀ single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c ₁₁ and c ₁₂ are found to increase continuously with pressure while c ₄₄ increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi₁₂ MO₂₀ crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c ₄₄ and the piezoelectric stress coefficient e ₁₂₃ are almost unaffected by this dissipation.

The elastic properties of rare-earth scandates were only reported at room temperature based on simulations and experimental measurements with poor agreement thus far. Using resonant ultrasound spectroscopy and inductive gauge dilatometry, we determined the elastic stiffnesses, their temperature dependence, and thermal expansion coefficients of SmScO${}_3$ , TbScO${}_3$ , and DyScO${}_3$ between 103 K and 1673 K. Our set of elastic stiffnesses shows high internal consistency, e.g., the relations $c_{11}>c_{33}>c_{22}$ , $c_{66}>c_{44}>c_{55}$ , and $c_{13}\ge c_{12}>c_{23}$ hold for all crystal species at room temperature. The structures become overall stiffer with decreasing $RE$ -radius and increased charge density. The behavior of $c_{44}$ at low temperatures indicates in all $RE$ ScO${}_3$ a structural instability that might lead to an orthorhombic$\to$ monoclinic transition involving shear of the (100)-plane upon increasing pressure. The transition seems to be promoted by a decreasing $RE$ -radius. Anomalies in two mixed resistances of TbScO${}_3$ below room temperature are indicative of at least one more structural instability that may also cause a phase transition where the structure is sheared. So far, only magnetic phase transitions at about 3 K have been observed in $RE$ ScO${}_3$ in literature. The thermoelastic properties in [100] and [001] directions of all materials become increasingly isotropic at high temperatures, suggesting decreased structural tilt. (100) or (010) crystal cuts should be chosen for applications of a $RE$ ScO${}_3$ as a substrate material, when mostly isotropic thermal expansion or longitudinal stiffness in-plane is desired, respectively.