Thermal expansion and structural distortion of perovskite — data for NaMgF3 perovskite. Part I

The crystal structure of NaMgF3 perovskite (Neighborite) has been studied at high temperature by X-ray powder diffraction. Data were collected using a position sensitive detector with a monochromatic synchroton radiation source. Changes in unit cell and atomic positions of the perovskite structure were defined using the Rietveld refinement technique. The linear and volumetric thermal expansion coefficients are observed to be αa = 4.04 × 10−5 K−1, αb = 1.53 × 10−5 K−1, αc = 3.06 × 10−5 K−1, αv = 8.80 × 10−5 K−1 for the orthorhombic Pbnm phase, and αa0 = 3.16 × 10−5 K−1, αv0 = 9.49 × 10−5 K−1 for the cubic Pm3m phase of NaMgF3 perovskite, respectively. The temperature-induced linear and volumetric changes of the centrosymmetrically distorted ABX3 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 BX6 octahedral framework. The considerable anisotropy of linear thermal expansion, αa > αc > αb, for the orthorhombic Pbnm phase reflects the progressive decrease of structural distortion and the development of the phase transition of the NaMgF3 perovskite. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero as the temperature approaches Tc = 765°C in the manner expected for a ferroelastic phase transition. More interestingly, the apparent (MgF) bond lengths of the MgF6 octahedra shrink dramatically throughout a temperature interval of about 100°C before the phase transition. The volumetric thermal expansion increases drastically in a critical manner as the temperature approaches Tc.