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 $B_{S0}=173.2(5)\mathrm{GPa}$, $G_0=55.5(2)\mathrm{GPa}$, $\mathrm{\partial }{B}_S/\mathrm{\partial }P=2.99(9)$, $\mathrm{\partial }G/\mathrm{\partial }P=-0.23(3)$, $\mathrm{\partial }{B}_S/\mathrm{\partial }T=-0.0209(10)\mathrm{GPa}/\mathrm{K}$, $\mathrm{\partial }G/\mathrm{\partial }T=0.0042(4)\mathrm{GPa}/\mathrm{K}$, $({\mathrm{\partial }}^2{B}_S/\mathrm{\partial }{T}^2{)}_P=-1.7(1)\times {10}^{-5}{\mathrm{GPa}}^2/{\mathrm{K}}^2$, and $({\mathrm{\partial }}^{2}G/\mathrm{\partial }{T}^2{)}_P=-2.5(1)\times {10}^{-5}{\mathrm{GPa}}^2/{\mathrm{K}}^2$. The origin of the thermally induced anomaly in the shear modulus for ${\mathrm{Fe}}_3{\mathrm{O}}_4$ magnetite is ascribed to the coupling of local atomic distortions and short-range charge ordering of sixfold-coordinated ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ 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.

• Received 23 September 2017
• Revised 4 April 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.024009