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 Na 4.5 FeP 2 O 8.5 F 0.5 (NFP) and Na 4 TiP 2 O 9 (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 (520 K-540 K), are essentially different. For NTP crystals, the phase transition in the range 520 K-540 K is reduced to the disordering of the Na sublattice and gives rise to a symmetry from monoclinic (sp. gr. P 1s P2/c , a = 8.631(1) A, b = 7.541(1) A, c = 7.049(1) A, β = 116.10(2)°, with wave vector q = 0.2a* + 0.2c*) to orthorhombic (sp. gr. Ibam, a = 15.647(8) A, b = 14.989(8) A, c = 7.081(5) A). Moreover, the geometrical configuration of the conductivity channels is considerably changed