Structure and Dynamic and Nonlinear Optical Properties of Hg2F2 Crystals

The structure of the Hg2F2 compound was studied experimentally and theoretically within the density functional theory. X-ray diffraction studies enabled us to determine the space group of the crystal (I4/mmm), and the comparison of the experimentally obtained Raman spectra of these crystals with the theoretical ones enabled us to find and uniquely identify all bands in the spectra. The stability of the I4/mmm phase was analyzed in a wide range of uniaxial and hydrostatic pressures. It is shown that the uniaxial compression of a crystal along a tetrad axis results in a phase transition to the Cmcm structure at P-c = 8 GPa, whereas the hydrostatic compression results in a previously unknown phase transition to the Cmca phase at P-c = 9 GPa. The calculations of the cubic nonlinear susceptibility of Hg2F2 predict a giant value of this quantity.