PHASE-TRANSITIONS AND THE STRUCTURE OF NAMNF3 PEROVSKITE CRYSTALS AS A FUNCTION OF TEMPERATURE AND PRESSURE

Sodium manganese fluoride crystals, NaMnF3, orthorhombic, Pnma, have been studied at high pressures up to 5.60 GPa by X-ray single-crystal diffraction and at temperatures from 5 to 980 K by X-ray powder diffraction. At T(c) = 969 K (0.1 MPa) a phase transition to a cubic symmetry has been observed and has been confirmed by DTA measurements. At high pressures the distortion of the NaMnF3 structure from the ideal cubic perovskite structure becomes larger than at ambient pressure. At low temperatures no phase transition changing the crystal symmetry has been detected, but thermal contraction of the crystal is non-linear and becomes anomalous near the transition temperature of NaMnF3 to antiferromagnetic phase, T(N) = 66 K. The crystal structure has been determined at 2.00 and 3.80 GPa.