STRUCTURAL PHASE-TRANSITIONS AND OPTICAL-ABSORPTION OF LIINSE2 UNDER PRESSURE

We have investigated high-pressure optical and structural properties of the ternary chalcogenide LiInSe2 by optical absorption and reflection and by powder x-ray diffraction. The fundamental absorption edge of LiInSe2 in the ambient-pressure beta-NaFeO2 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 LiInSe2 transforms from the orthorhombic beta-NaFeO2-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-degrees-C-230-degrees-C causes ordering of the cations and a transition to the alpha-NaFeO2-type structure (rhombohedral; R 3mBAR). Annealing at 210-degrees-C-230-degrees-C below 1 GPa results in a phase transformation from the metastable alpha-NaFeO2 or NaCl phases to the chalcopyrite-type phase (tetragonal; I42dBAR). Reflectivity spectra show the high-pressure phases to be nonmetallic.