Na3Al2(PO4)3, a fast sodium conductor at high pressure: in-situ impedance spectroscopy characterisation and phase diagram up to 8 GPa

Solid-state syntheses on the Na3Al2(PO4)3 composition have been carried out from 850 to 1273 K at pressures up to 8 GPa. At 923 K and pressures above ca. 0.5 GPa, Na3Al2(PO4)3 crystallises in a NASICON-type structure and becomes an ionic conductor with a bulk conductivity of around 5.10−2 S/cm at 600 K, 1.5 GPa. Actually, the form recovered at room conditions is the monoclinic deformation of the rhombohedral NASICON cell. The two non-quenchable phase-transitions which lead to the true R-3c NASICON cell are met in high-pressure impedance spectroscopy (HPIS) experiments at 410 and 451 K, 0.4 GPa, and 454 and 508 K, 1.5 GPa, respectively. At 1073 K and pressures of 3.9, 6 and 8 GPa, another Na3Al2(PO4)3 form is obtained which is 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 is recovered in the lowest pressure syntheses, which displays a relatively low bulk conductivity comprised, at 0.3 GPa, between 5×10−4 S/cm at 900 K and 5×10−6 S/cm at 600 K. The effect of pressure is to stabilise 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.