Thermoelastic properties of magnesiowüstite, (Mg_1-xFe _x )O: Determination of the Anderson-Grüneisen parameter by time-of-flight neutron powder diffraction at simultaneous high pressures and temperatures

The ability to perform neutron diffraction studies at simultaneous high pressures and high temperatures is a relatively recent development. The suitability of this technique for determining P -V-T equations of state has been investigated by measuring the lattice parameters of Mg_1-x Fe _x O (x = 0.2, 0.3, 0.4), in the range P < 10.3 GPa and 300 < T < 986 K, by time-of-flight neutron powder diffraction. Pressures were determined using metallic Fe as a marker and temperatures were measured by neutron absorption resonance radiography. Within the resolution of the experiment, no evidence was found for any change in the temperature derivative of the isothermal incompressibility, ∂K_T/∂T, with composition. By assuming that the equation-of-state parameters either varied linearly or were invariant with composition, the 60 measured state points were fitted simultaneously to a P -V-T-x equation of state, leading to values of ∂K_T/∂T = -0.024 (9) GPa K^-1 and of the isothermal Anderson-Grüneisen parameter δ_T = 4.0 (16) at 300 K. Two designs of simultaneous high-P/T cell were employed during this study. It appears that, by virtue of its extended pressure range, a design using toroidal gaskets is more suitable for equation-of-state studies than is the system described by Le Godec, Dove, Francis, Kohn, Marshall, Pawley, Price, Redfern, Rhodes, Ross, Schofield, Schooneveld, Syfosse, Tucker & Welch [Mineral. Mag. (2001), 65, 737-748].