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Abstract

Li(H(2)O)(2-x)[Zr(2)(PO(4))(3)] (x = 0) was synthesized under mild hydrothermal conditions. The crystal structure (single-crystal X-ray diffraction (XRD) data: cubic, space group P2(1)3 (No. 198), a = 10.2417(1) angstrom, V = 1074.28(2) angstrom(3), Z = 4) contains a langbeinite-framework consisting of ZrO(6) octahedra and PO(4) tetrahedra sharing common corners. H(2)O molecules (crystal water) occupy the large cages extending along the 3-fold axes, thereby completing the langbeinite-type structural arrangement: {(H(2)O)(2)[Zr(2)(PO(4))(3)]}(-) vs K(2)[Mg(2)(SO(4))(3)]. The filled langbeinite variant is completed by additional Li(+) ions taking positions between two neighboring water molecules and via the formation of linear arrangements H(2)O center dot center dot center dot Li(+)center dot center dot center dot OH(2). The thermochemical properties of Li(H(2)O)(2-x)[Zr(2)-(PO(4))(3)] (0 <= x <= 2) were studied by thermogravimetry-differential thermal analysis (TG-DTA), as well as by isothermal annealing combined with powder XRD investigations. Above 200 degrees C, the crystal water of the cubic hydrate is irreversibly released and the dehydrated phase keeps the cubic host structure. The dehydrated phase is metastable and transforms exothermally to a stable phase (probably the alpha-phase; NASICON-type structure) during heating (dynamic, 10 degrees C/min) at similar to 970 degrees C. Depending on the maximum temperatures chosen for long-time annealing procedures (1180 and 800 degrees C, respectively) the alpha and beta high-temperature phases (rhombohedral and orthorhombic, respectively) are formed, which undergo reversible phase transitions to the alpha' (similar to 60 degrees C) and the beta' low-temperature phases (similar to 300 degrees C), respectively. Although the dehydrated cubic phase can be expected to show a high Li-ion conductivity, the metastable character of this phase will prevent any application without further stabilization of the crystal structure.