First-principles study of mechanical, electronic structure, and optical properties for cubic fluoroperovskite XMgF3 (X=Al, Ga, In, Tl) under high pressure*

The effects of pressure on the mechanical, electrical structure, and optical characteristics of cubic fluoroperovskite XMgF3 (X = Al, Ga, In, Tl) are investigated based on the first principles methods. The calculated lattice constants are in good agreement with those in the literature. According to the mechanical properties, all the compounds exhibit ductility and anisotropy, and the inter-atomic binding bonds of these materials are ionic. According to Kleinman's parameter, bond stretching is dominant in XMgF3 (X = Al, Ga, In, Tl). Among all studied compounds, AlMgF3 carries the highest machinable index, indicating that it is the most machinable and suitable material for industrial applications. The electronic structures show that AlMgF3 and InMgF3 are indirect bandgap semiconductors, while GaMgF3 and TlMgF3 are direct bandgap insulators at 0 GPa. The values of band gap for all the compounds decrease with the increasing pressure. X-s and F-p orbital states contribute primarily to the top of the valence band, whereas X-p orbital states dominate the bottom of the conduction band. The overall shape of electron state density under pressure does not change significantly, but its peak widens. The optical properties of XMgF3 (X = Al, Ga, In, Tl) crystal at zero pressure and high pressures are further investigated. The results indicate that XMgF3 compounds have outstanding ultraviolet (UV) absorption properties and could be used in UV optical devices. Moreover, the low reflectivity values of XMgF3 crystals make the compounds potential anti-reflective coating materials.