High-pressure electronic structure and phase transitions in monoclinic InSe:: X-ray diffraction, Raman spectroscopy, and density functional theory

We have studied the crystal and electronic structure of monoclinic (MC) InSe under pressure finding a reversible phase transition to a Hg2Cl2-like tetragonal phase. The pressure evolution of the crystal structure was investigated by angle-dispersive x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to 30 GPa. From the diffraction experiments, we deduced that MC InSe becomes gradually more symmetric under pressure, transforming the crystal structure into a tetragonal one at 19.4 +/- 0.5 GPa. This phase transition occurs without any volume change. Raman measurements under pressure confirmed the occurrence of a monoclinic-to-tetragonal transformation. The nondegenerate modes in the MC phase, especially the A(g)(4) modes, exhibit a negative pressure coefficient, converging with the B-g(1) modes, and becoming an Eg mode in the tetragonal phase. The experimental results are interpreted through density-functional theory (DFT) electronic-structure and total-energy calculations, which showed that beyond 18 GPa the tetragonal phase is the most stable phase. It is also shown that along the continuous change from monoclinic to tetragonal InSe, there is a progressive decrease of the band gap and eventually, in the tetragonal phase, there occurs a small band overlap. However, the Raman-effect and optical-absorption measurements suggest that this overlap is probably due to the usual DFT band-gap underestimation. Tetragonal InSe is most likely a low-gap semiconductor. The bonding in the monoclinic phase and that in the tetragonal InSe phase are compared.