SrMgF 4 has an extremely large bandgap E g of 12.50 eV as obtained from reflection dispersion. The symmetry of this crystal is monoclinic P2 1 at room temperature and transforms to the orthorhombic Cmc2 1 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 ( ![[similar]](http://www.rsc.org/images/entities/char_223c.gif) 0.017) and nonlinear susceptibility ( 0.044 pm V −1, an order lower than that in KDP), SrMgF 4 crystals are considered promising for nonlinear optics thanks to their transparency far in the vacuum ultraviolet spectral region. Phys. Chem. Chem. Phys., 2015 Single crystal SrMgF4 sample, 2 mm thick. The structure of SrMgF4: low-temperature (a) and high-temperature (b) phases. Green spheres are Sr ions located among the MgF6 polyhedra. Different colors of MgF6 octahedra in section (a) shows that they are turned each other and are crystallographically nonequivalent. The coordination of Sr ions in LT and HT SrMgF4 structures.
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