Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 259-265 [ doi:10.1107/S0108768198011239 ]

Crystal lattice and phase transitions in Na₄TiP₂O₉ (NTP) and Na_4.5FeP₂O₈(O,F) (NFP) superionic conductors as a function of high pressures and temperatures

B. Maximov, M. Sirota, S. Werner and H. Schulz

Abstract: The lattice dynamics of Na₄TiP₂O₉ (tetrasodium titanium diphosphorus nonaoxide, NTP) and Na_4.5FeP₂O₈(O,F) (nonasodium diiron tetraphosphorus difluoride octadecaoxide, NFP) crystals, which are superionic conductors with Na⁺-ion conductivity, were studied under high pressures. Lattice constants as a function of hydrostatic pressure were measured on a four-circle diffractometer using a high-pressure cell with diamond anvils. At 1.78 $\pm$ 0.15 GPa NTP undergoes a reversible phase transition from the modulated monoclinic (pseudo-orthorhombic) modification which is stable under atmospheric conditions. A similar phase transition in NTP is observed at 523 K. For NFP, it may be assumed that at least three phase transitions occur when the pressure increases from atmospheric to 12 GPa, at $1.39\pm0.08$ , $4.52 \pm0.32$ , and $6.02\pm0.02$ GPa, as concluded from the change in the unit-cell parameters and in the color of the crystals: the color changes from ginger (dark orange) to pink at $\sim$ 1.5-2.0 GPa pressure and to violet at $\sim$ 6.0 GPa.