Solid-state syntheses on the Na3Al2(PO4)3 composition were carried out from 850 to 1273K at pressures up to 8GPa. At 923K and pressures above about 0.5GPa, Na3Al2(PO4)3 crystallized in a NASICON-type structure and became an ionic conductor with a bulk conductivity of around 5 x 10-2S/cm at 600K, 1.5GPa. Actually, the form recovered at room conditions was the monoclinic deformation of the rhombohedral NASICON cell. The two non-quenchable phase-transitions which lead to the true R-3c NASICON cell were met in high-pressure impedance spectroscopy experiments at 410 and 451K, 0.4GPa, and 454 and 508K, 1.5GPa, respectively. At 1073K and pressures of 3.9, 6 and 8GPa, another Na3Al2(PO4)3 form was obtained which was isostructural to rhombohedral Na3Fe2(AsO4)3 (II-NaFeAs), the high-temperature modification of Na3Fe2(AsO4)3 garnet (I-NaFeAs). A third Na3Al2(PO4)3 form of unknown structure was recovered in the lowest pressure syntheses, which displayed a relatively low bulk conductivity comprised, at 0.3 GPa, between 5 x 10-4S/cm at 900K and 5 x 10-6S/cm at 600K. The effect of pressure was to stabilize the NASICON structure for a compound with relatively small cations (Na, Al, P). In addition, high pressures allow a transition from the NASICON structure to the II-NaFeAs structure, which had never been obtained under the sole effect of temperature at ambient pressure.

Na3Al2(PO4)3, a Fast Sodium Conductor at High Pressure – in situ Impedance Spectroscopy Characterisation and Phase Diagram up to 8GPa. F.Brunet, N.Bagdassarov, R.Miletich: Solid State Ionics, 2003, 159[1-2], 35-47