High-Pressure High-Temperature Stability and Thermal Equation of State of Zircon-Type Erbium Vanadate

The zircon to scheelite phase boundary of ErVO$_4$ has been studied by high-pressure and high-temperature powder and single-crystal X-ray diffraction. This study has allowed us to delimit the best synthesis conditions of its scheelite-type phase, determine the ambient-temperature equation of state of the zircon and scheelite-type structures, and obtain the thermal equation of state of the zircon-type polymorph. The results obtained with powder samples indicate that zircon-type ErVO$_4$ transforms to scheelite at 8.2 GPa and 293 K and at 7.5 GPa and 693 K. The analyses yield bulk moduli K$_0$ of 158(13) GPa for the zircon phase and 158(17) GPa for the scheelite phase, with a temperature derivative of dK$_0$/dT = −[3.8(2)] × 10$^{–3}$ GPa K$^{–1}$ and a volumetric thermal expansion of $α_0$ = [0.9(2)] × 10$^{–5}$ K$^{–1}$ for the zircon phase according to the Berman model. The results are compared with those of other zircon-type vanadates, raising the need for careful experiments with highly crystalline scheelite to obtain reliable bulk moduli of this phase. Finally, we have performed single-crystal diffraction experiments from 110 to 395 K, and the obtained volumetric thermal expansion $(α_0 )$ for zircon-type ErVO$_4$ in the 300–395 K range is [1.4(2)] × 10$^{–5}$ K$^{–1}$, in good agreement with previous data and with our experimental value given from the thermal equation of state fit within the limits of uncertainty