P-V equation of State, thermal expansion, and P-T stability of synthetic zincochromite (ZnCr2O4 spinel)

Abstract

The elastic properties and thermal behavior of synthetic zincochromite (ZnCr₂O₄) have been studied by combining room-temperature high-pressure (0.0001–21 GPa) synchrotron radiation powder diffraction data with high-temperature (298–1240 K) powder diffraction data. Elastic properties were obtained by fitting two Equations of State (EoS) to the P-V data. A third-order Birch-Murnaghan model, which provides results consistent with those from the Vinet EoS, yields: K₀ = 183.1(±3.5) GPa, K’ = 7.9(±0.6), K” = −0.1278 GPa⁻¹ (implied value), at V₀ = 577.8221 ų (fixed). Zincochromite does not exhibit order-disorder reactions at high temperature in the thermal range explored, in agreement with previous studies. The volume thermal expansion was modeled with α_V = α₀ + α₁T + α₂/T⁻², where only the first coefficient was found to be significant [α₀ = 23.0(4) 10⁻⁶ K⁻¹]. Above 23 GPa diffraction patterns hint at the onset of a phase transition; the high pressure phase is observed at approximately 30 GPa and exhibits orthorhombic symmetry. The elastic and thermal properties of zincochromite were then used to model by thermodynamic calculations the P-T stability field of ZnCr₂O₄ with respect to its oxide constituents (Cr₂O₃ and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.