New phases in the Na3PO4-Mg3(PO4)2 binary quenched from 1000 °C

Abstract

Abstract A reinvestigation of the Na3PO4-Mg3(PO4)2 binary at 1000 °C has identified two new ternary oxide phases and confirmed the existence of an orthorhombic solid solution phase in the vicinity of NaMg4(PO4)3. Cubic γ-Na3PO4 shows a solid solution up to ~20 mol. % Mg3(PO4)2. Between ~24 and ~29 mol. % a new γ-Na3PO4-related rhombohedral superstructure phase is observed. At ~44 mol. % Mg3(PO4)2 a new phase with a cristobalite-related structure is observed, which on cooling rapidly to room-temperature transforms to KGaO2-type (Pbca: a = 10.409(2) Å, b = 14.492(2) Å, c = 5.211(1) Å). The NaMg4(PO4)3 solid-solution phase extends between ~72 and 80 mol. % Mg3(PO4)2. Surprisingly, and contrary to previous investigations, there is no phase at 50 mol. %, i.e. with stoichiometry NaMgPO4. All high-temperature annealing was performed in sealed Pt tubes to prevent loss of components to the atmosphere, which is suspected as the reason for the quite different results reported by some previous workers.