Equation of state, structural behaviour and phase diagram of synthetic MgFe₂O₄, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch-Murnaghan equation of state result in K₀ = 181.5(± 1.3) GPa, K′ = 6.32(± 0.14) and K″ = -0.0638 GPa^-1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by α_a0 + α_a1 ^*(T-298) + α_a2/(T-298)², where α_a0 = 9.1(1) 10^-6 K^-1, α_a1 = 4.9(2) 10^-9 K^-2 and α_a2 = 5.1(5) 10^-2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the O'Neill model, whose parameters are α = 22.2(± 1.8) kJ mol^-1 and β = -17.6(± 1.2) kJ mol^-1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.