Ab initio investigation into structural, mechanical and electronic properties of low pressure, high pressure and high pressure-high temperature phases of Indium Selenide

• β- and ε-polytypes are ground state phases of InSe. B1-type is a high pressure phase.
• Monoclinic P2/m and C2/m structures are high pressure-high temperature layered phases.
• β and ε InSe behave in brittle manner. HP and HP-HT phases behave in ductile manner.
• Elastic anisotropy of InSe is most pronounced in its low symmetry phases.
• P2/m and C2/m InSe are indirect and direct gap semiconductors respectively.

All-electron self-consistent full potential augmented plane waves+local orbital method (FP-APW+lo) within generalized gradient density approximation (GGA) is used to study crystalline Indium Selenide (InSe) compound at low pressure (hexagonal), high pressure (cubic) and high pressure – high temperature (monoclinic) phases. Bulk properties such as equilibrium lattice constants, bulk modulus and its pressure derivative of these phases are determined and compared to available experimental and theoretical data. Our calculations show that InSe exhibits a slight preference to crystallize at zero pressure in β  D3h1 phase rather than in ε  D6h4 polytype, and under appropriate pressures, phase transitions occur toward B1 (rock-salt) phase. Stiffness constants are calculated and other elastic properties such as sound velocity, Debye temperature and elastic anisotropy are then determined for all the studied structures and compared with available literature results. Our results show that monoclinic InSe phases are both lamellar structures and present strong elastic anisotropies compared to the other phases. Moreover, we predict a possible phase transition under pressure from P2/m to C2/m phase. Our electronic investigation, with and without neglecting spin-orbit effect and using both modified Becke-Johnson (mBJ) and Engel-Vosko (EV) corrections, shows that a substantial improvement of the band gap value has been achieved for hexagonal InSe over previous theoretical results in literature, besides the indirect gap value of monoclinic InSe is reported for the first time.