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.