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

¹ Dipartimento Scienze Mineralogiche e Petrologiche, Università degli Studi di Torino, Via Valperga Caluso 35, 10025 Torino, Italy
² National Research Council, IDPA, Milan section, Via Botticelli 23-20133 Milano, Italy
³ Dipartimento Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23-20133 Milano, Italy
⁴ European Synchrotron Radiation Facility, ESRF-F 38043 Grenoble Cedex, France

Correspondence: ^* E-mail: alessandro.pavese{at}unimi.it

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 _V = ₀ + ₁T + ₂/T^–2, where only the first coefficient was found to be significant [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 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.

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