Structural phase transitions and optical absorption ofLiInSe2under pressure

We have investigated high-pressure optical and structural properties of the ternary chalcogenide ${\mathrm{LiInSe}}_2$ by optical absorption and reflection and by powder x-ray diffraction. The fundamental absorption edge of ${\mathrm{LiInSe}}_2$ in the ambient-pressure β-${\mathrm{NaFeO}}_2$ phase is observed at 2.03 eV. The corresponding band gap is either indirect or pseudodirect. A higher-lying direct absorption edge is observed near 2.9 eV. The variation of these gaps with pressure has been measured up to the first phase transition (4.2 GPa). We propose a tentative assignment of interband transitions based on the band structure of related chalcopyrite compounds and by taking into account the measured pressure coefficients. At 4.2 GPa ${\mathrm{LiInSe}}_2$ transforms from the orthorhombic β-${\mathrm{NaFeO}}_2$-type to the cubic NaCl-type structure. At room temperature the NaCl phase is metastable from ambient pressure up to at least 25 GPa. Above 1.5 GPa thermal annealing of the NaCl phase at 210 °C–230°C causes ordering of the cations and a transition to the α-${\mathrm{NaFeO}}_2$-type structure (rhombohedral; R3¯m). Annealing at 210°C–230°C below 1 GPa results in a phase transformation from the metastable α-${\mathrm{NaFeO}}_2$ or NaCl phases to the chalcopyrite-type phase (tetragonal; I4¯2d). Reflectivity spectra show the high-pressure phases to be nonmetallic.