Thermally Induced Anomaly in the Shear Behavior of Magnetite at High Pressure

Thermoelasticity and acoustic velocities of polycrystalline magnetite are studied at simultaneously high pressures and temperatures up to 8.6 GPa and 1123 K using ultrasonic interferometry in conjunction with in situ x-ray techniques. Here, we report temperature-driven anomalies in the shear behavior at temperatures up to ∼450 K, together with pressure-induced softening in the shear properties. Fitting the current data to finite strain equations, we obtain the bulk and shear moduli, as well as their pressure and temperature dependences, namely BS0=173.2(5)GPa, G0=55.5(2)GPa, ∂BS/∂P=2.99(9), ∂G/∂P=-0.23(3), ∂BS/∂T=-0.0209(10)GPa/K, ∂G/∂T=0.0042(4)GPa/K, (∂2BS/∂T2)P=-1.7(1)×10-5GPa2/K2, and (∂2G/∂T2)P=-2.5(1)×10-5GPa2/K2. The origin of the thermally induced anomaly in the shear modulus for Fe3O4 magnetite is ascribed to the coupling of local atomic distortions and short-range charge ordering of sixfold-coordinated Fe2+ and Fe3+ ions at the octahedral sites in the inverse-spinel structure. These findings or results provide high-P thermoelasticity data of magnetite and present an opportunity to gain a good understanding of the underlying mechanism of temperature-driven anomalies in magnetite-based solid solutions and spinel-structured materials for their applications in extreme conditions.