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Li(H2O)2-x[Zr2(PO4)3]: A Li-Filled Langbeinite Variant (x = 0) as a Precursor for a Metastable Dehydrated Phase (x = 2)

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journal contribution
posted on 22.03.2011, 00:00 by Shuang Chen, Stefan Hoffmann, Katja Weichert, Joachim Maier, Yurii Prots, Jing-Tai Zhao, Rüdiger Kniep
Li(H2O)2−x[Zr2(PO4)3] (x = 0) was synthesized under mild hydrothermal conditions. The crystal structure (single-crystal X-ray diffraction (XRD) data: cubic, space group P213 (No. 198), a = 10.2417(1) Å, V = 1074.28(2) Å3, Z = 4) contains a langbeinite-framework consisting of ZrO6 octahedra and PO4 tetrahedra sharing common corners. H2O molecules (crystal water) occupy the large cages extending along the 3-fold axes, thereby completing the langbeinite-type structural arrangement: {(H2O)2[Zr2(PO4)3]} vs K2[Mg2(SO4)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 H2O···Li+···OH2. The thermochemical properties of Li(H2O)2−x[Zr2(PO4)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 °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 α-phase; NASICON-type structure) during heating (dynamic, 10 °C/min) at ∼970 °C. Depending on the maximum temperatures chosen for long-time annealing procedures (1180 and 800 °C, respectively) the α and β high-temperature phases (rhombohedral and orthorhombic, respectively) are formed, which undergo reversible phase transitions to the α′ (∼60 °C) and the β′ low-temperature phases (∼300 °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.