We have studied the crystal and electronic structure of monoclinic (MC) InSe under pressure finding a reversible phase transition to a ${\mathrm{Hg}}_2{\mathrm{Cl}}_2$-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 $30\mathrm{GPa}$. From the diffraction experiments, we deduced that MC InSe becomes gradually more symmetric under pressure, transforming the crystal structure into a tetragonal one at $19.4\pm 0.5\mathrm{GPa}$. 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 $18\mathrm{GPa}$ 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.

• Received 22 October 2007

DOI:https://doi.org/10.1103/PhysRevB.77.045208