Compound formation in the system (\mathrm{Li}-\mathrm{P}-\mathrm{Se}) is investigated by differential thermal analysis, differential scanning calorimetry (DSC), X-ray powder diffraction, ({ }^{7} \mathrm{Li},{ }^{17} \mathrm{Se}), and ({ }^{31} \mathrm{P}) magic-angle spinning NMR, and high-temperature ({ }^{31} \mathrm{P}) static NMR. No glasses can be formed by melt-quenching techniques. The two new crystalline phases found possess stoichiometries (\mathrm{Li}{4} \mathrm{P}{2} \mathrm{Se}{6}) and (\mathrm{Li}{7} \mathrm{PSe}{6}) and are not isostructural with their known sulfide analogs. (\mathrm{Li}{4} \mathrm{P}{2} \mathrm{Se}{6}) crystallizes in the orthorhombic system, (a=11.239(5) \AA), (b=11.811(5) \AA, c=13.528(5) \AA, V=1796 \AA^{3}, d() calc ()=4.151 \mathrm{~g} \cdot \mathrm{cm}^{-3}(Z=8)). It contains dimeric (\left[\mathrm{P}{2} \mathrm{Se}{6}\right]^{4-}) units, the two (\mathrm{P}) atoms of which are structurally inequivalent. (\mathrm{Li}{7} \mathrm{PSe}{6}) is indexed in the cubic system, (a=20.73 \AA), and has two crystallographically inequivalent (\left[\mathrm{PSe}{4}\right]^{3-}) groups. In addition, the two types of selenium atoms, belonging to (\mathrm{PSe}{4}^{3-}) and (\mathrm{Se}^{2-}) units, respectively, are distinguished by a large chemical shift difference. The thermal behavici of (\mathrm{Li}{7} \mathrm{PSe}{6}) is investigated by muttiple DSC studies and in situ high-temperature NMR, suggesting that this phase is metastable at room temperature. 1993 Academic Press, Inc.