Sodium magnesium fluorine(3) perovskite: Physical properties, crystal chemistry and ferroelastic phase transition

Crystal structure and phase transition of NaMgF{dollar}sb3{dollar} perovskite (Neighborite) has been studied at in-situ high pressure and high temperature by synchrotron X-ray powder diffraction. Changes in unit cell dimensions, atomic positions, and thermal vibrational parameters of the perovskite structure were defined using the Rietveld refinement technique.; Compression and thermal expansion data of the NaMgF{dollar}sb3{dollar} perovskite agree well with the proposed model that the pressure and temperature induced linear and volumetric dimensional changes of the centrosymmetrically distorted ABX{dollar}sb3{dollar} perovskite structure can be empirically expressed as a combination of the change of the (B-X) bond length and the change of tilting of the BX{dollar}sb6{dollar} octahedral framework.; The observation of the peak splitting associated with dimensional differences of the unit cell and the superlattice diffractions related to octahedral tiltings indicates that the crystal structure of NaMgF{dollar}sb3{dollar} perovskite transforms directly from orthorhombic Pbnm to cubic Pm3m phase with increasing temperature. The result suggest that the order parameters, namely the octahedral tiltings {dollar}phi{dollar} = {dollar}phisb{lcub}rm z{rcub}sp+{dollar} and {dollar}Theta{dollar} = {dollar}phisb{lcub}x+y{rcub}sp-,{dollar} are strongly coupled together. The tilting angle of the MgF{dollar}sb6{dollar} octahedral framework rapidly decreases toward zero, in a Landau-type of manner expected for ferroelastic phase transition, as the temperature approaches T{dollar}sb{lcub}rm c{rcub}.{dollar} So does the spontaneous strain of the crystal. The (Mg-F) bond lengths of the MgF{dollar}sb6{dollar} octahedra apparently shrink throughout a temperature interval close to the transition point. The transition temperature of NaMgF{dollar}sb3{dollar} perovskite is observed to increase with increasing pressure and having a positive slope of about 45 K/GPa. Despite a 12% volume change, the experimental results on NaMgF{dollar}sb3{dollar} perovskite shows that the compressibility has no significant temperature dependence, i.e. d{dollar}beta{dollar}/dT {dollar}approx{dollar} 0, and compatibility, that the thermal expansivity is independent of pressure, i.e. d{dollar}alpha{dollar}/dP {dollar}approx{dollar} 0.; The experimental data of MgSiO{dollar}sb3{dollar} perovskite show that thermal expansion is mostly accommodated by the octahedral tilting, while, the dominant compressional mechanism is the octahedral bond compression. It is considered that the significant temperature dependence of shear modulus may be a plausible explanation to the lateral variation of the seismic tomography. Alternatively, the temperature insensitivity of bulk modulus, dK/dT {dollar}approx{dollar} 0, may also well explain the seismically observed lateral variation of dlnV{dollar}sb{lcub}rm s{rcub}{dollar}/dlnV{dollar}sb{lcub}rm p{rcub}ge2{dollar} in the lower mantle. (Abstract shortened by UMI.)...