Perovskite at high P‐T conditions: An in situ synchrotron X ray diffraction study of NaMgF3 perovskite

Abstract

The lattice distortion and structural phase transition of NaMgFa perovskite (Neighborite) have been studied using synchrotron X ray powder diffraction at high pressure and temperature. Changes in the unit cell dimensions of the perovskite are determined by conventional peak indexing and least squares routines. The stress field within the high‐pressure cell assembly is analyzed, and the yield strength of the NaMgF3 perovskite is determined at high P and T. The pressure‐ and temperature‐induced dimensional changes of the NaMgF3 perovskite structure are expressed empirically as a combination of compression/expansion of the [Mg‐F] bond length and tilting of the MgF6 octahedral framework. The linear thermal expansions of the NaMgF3 perovskite observed at different pressures show significant anisotropy with αa > αc > αb which reflects the decrease of structural distortion and the development of a phase transition in the perovskite with increasing temperature. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero, in a manner expected for a ferroelastic phase transition, as the temperature approaches the transition point Tc. The apparent [Mg‐F] bond lengths of the MgF6 octahedra experience drastic shrinkage with increasing temperature just prior to the transition. Despite a 12% change in volume due to compression, the experimental results on NaMgF3 perovskite show that the thermal expansivity is independent of pressure, i.e., dα/dP ≈ 0, and, compatibly, that the compressibility is independent of temperature, i.e., dβ/dT 0. However, the dominant compression mechanism is the compression of the octahedral bond length, whereas the dominant mechanism for thermal expansion is the diminishing of octahedral tilting. The Earth's mantle may be isochemical if the thermal expansion of MgSiO3 perovskite at high pressure behaves like NaMgF3, of which the Anderson‐Grüneisen parameter is near zero, i.e., δs ≈ 0. It is observed that crystal structure of the NaMgF3 perovskite transforms directly from the orthorhombic Pbnm phase to the cubic phase at all pressures. The transition temperature is observed to increase with increasing pressure with a positive slope of 45 K/GPa.