Structures and optical properties of two phases of SrMgF4

Abstract

SrMgF₄ has an extremely large bandgap E_g of 12.50 eV as obtained from reflection dispersion. The symmetry of this crystal is monoclinic P2₁ at room temperature and transforms to the orthorhombic Cmc2₁ phase near 478 K as the temperature increases. The acentric character of the low-temperature (LT) phase is confirmed by pyroelectric luminescence at T < 440 K. The fundamental absorption edge of the LT phase is located at 122 nm (10.15 eV). A considerable difference between the absorption edge and bandgap E_g is due to the strong exciton absorption. The first-principles electronic structure, refractive indices, nonlinear susceptibility and polarizability were calculated for both LT and high-temperature (HT) phases. Band-to-band transitions are direct for the LT phase but indirect for HT. In spite of relatively low birefringence (∼0.017) and nonlinear susceptibility (∼0.044 pm V⁻¹, an order lower than that in KDP), SrMgF₄ crystals are considered promising for nonlinear optics thanks to their transparency far in the vacuum ultraviolet spectral region.