A STUDY OF THE SYSTEM LI-P-SE

Compound formation in the system Li-P-Se is investigated by differential thermal analysis, differential scanning calorimetry (DSC), X-ray powder diffraction, 7Li, 77Se, and 31P magic-angle spinning NMR, and high-temperature 31P static NMR. No glasses can be formed by melt-quenching techniques. The two new crystalline phases found possess stoichiometries Li4P2Se6 and Li7PSe6 and are not isostructural with their known sulfide analogs. Li4P2Se6 crystallizes in the orthorhombic system, a = 11.239(5) Å, b = 11.811(5) Å, c = 13.528(5) Å, V = 1796 Å3, d (calc) = 4.151 g · cm-3 (Z = 8). It contains dimeric [P2Se6]4- units, the two P atoms of which are structurally inequivalent. Li7PSe6 is indexed in the cubic system, a = 20.73 Å, and has two crystallographically inequivalent [PSe4]3- groups. In addition, the two types of selenium atoms, belonging to PSe3-4 and Se2- units, respectively, are distinguished by a large chemical shift difference. The thermal behavior of Li7PSe6 is investigated by multiple DSC studies and in situ high-temperature NMR, suggesting that this phase is metastable at room temperature. © 1993 Academic Press, Inc.