Structural phase transitions in the superionic conductors Na9{Fe2[PO4]4(O, F)2} and Na8{Ti2[PO4]4O2} in the temperature range 520–540 K

The Na4.5FeP2O8.5F0.5(NFP) and Na4TiP2O9 (NTP) crystals are superionic conductors with Na+ ion conductivity. These compounds are isostructural, but the symmetry changes taking place during the phase transition in the same temperature range (520K– 540K), are essentially different. For NTP crystals, the phase transition in the range 520K-540K is reduced to the disordering of the Na sublattice and gives rise to a symmetry from monoclinic (sp. gr. P[unk], a = 8.631(1) Å, b = 7.541 (1) Å, c = 7.049(1) Å, = 116.10(2)°, with wave vector [unk] = 0.2[unk]+0.2[unk]*) to orthorhombic (sp. gr. Ibam, a = 15.647(8) Å, b = 14.989(8) Å, c = 7.081(5) Å). Moreover, the geometrical configuration of the conductivity channels is considerably changed. Also, the site occupancy coefficients of Na atoms are changed at the phase transition. For NFP crystals, the phase transition in the same temperature range leads to a symmetry change from Ibam (a = 15.520(1) Å, b = 14.820(1) Å, c = 7.119(1) Å) to sp. gr.I[unk] (a = 15.645(1) Å, b = 14.866(1) Å, c = 7.144(1) Å, [unk] = 4/15[unk]*). Above and below this phase transition, the Na sites are only statistically occupied. The site occupancy coefficients of Na atoms and the geometrical configurations of the main conductivity channels are not essentially changed at the phase transition.