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 (ZnCr 2 O 4 ) 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: K 0 = 183.1(±3.5) GPa, K’ = 7.9(±0.6), K’’ = .0.1278 GPa -1 (implied value), at V 0 = 577.8221 Å 3 (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 = α 0 + α 1 T + α 2 /T -2, where only the first coefficient was found to be significant [α 0 = 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 ZnCr 2 O 4 with respect to its oxide constituents (Cr 2 O 3 and rocksalt-like ZnO). Spinel is expected to decompose into oxides at about 18 GPa and room temperature, in absence of sluggish kinetics.