The solid solutions Na3(1−x)Alx▭2xPO4: room temperature powder neutron diffraction studies of the two compositions x=0.025 and x=0.2

The system Na3(1−x)Alx▭2xPO4, where ▭ represents a cation vacancy, has been examined under ambient conditions at the two compositions x=0.025 and x=0.2 by powder neutron diffraction. The presence of Al3+ ions stabilises the high-temperature face-centred cubic phase of sodium orthophosphate, II-Na3PO4, with a0=7.424(2) Å (x=0.025) and a0=7.421(2) Å (x=0.2) at 25°C. At the lower dopant concentration, the structure corresponds closely to that of II-Na3PO4. The PO3-4 groups are orientationally disordered about the origin with sodium ions occupying the 141414 and 121212 sites in space group Fm3m. For x=0.2, vacancies are found solely at the 121212 position. This is consistent with the interpretation of these phases in terms of a cation-excess anti-fluorite structure. The influence of the structural evolution on the sodium-ion conductivity is discussed and a basis for the formation of low melting-point glasses for x > 0.5 is proposed.