Structure and Dynamic and Nonlinear Optical Properties of Hg 2 F 2 Crystals
- PDF / 726,875 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 202 Views
AND LIQUIDS
Structure and Dynamic and Nonlinear Optical Properties of Hg2F2 Crystals E. M. Roginskiia, Yu. F. Markova, and A. I. Lebedevb,* a Ioffe
Physical–Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia b Moscow State University, Moscow, 119991 Russia *e-mail: [email protected] Received October 29, 2018; revised November 22, 2018; accepted November 28, 2018
Abstract—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 Pc = 8 GPa, whereas the hydrostatic compression results in a previously unknown phase transition to the Cmca phase at Pc = 9 GPa. The calculations of the cubic nonlinear susceptibility of Hg2F2 predict a giant value of this quantity. DOI: 10.1134/S1063776119030208
1. INTRODUCTION Crystals of mercury(I) halides Hg2Hal2 (Hal = F, Cl, Br, I) are typical quasimolecular crystals. These are of considerable interest due to their applications. Thus, for example, mercury(I) iodide is characterized by a record low transverse sound velocity among other 1 10 solids (V110 = 254 m/s), record high birefringence Δn = +1.5 and acoustooptic interaction constant (M2 = 4284 × 10−18 CGS units for transverse acoustic (TA) wave) [1]. These crystals are used in practice as active elements of acoustooptic filters, acoustic delay lines, polarizers, modulators, deflectors, etc. Mercury(I) halides have a simple crystal structure built of linear Hal–Hg–Hg–Hal molecules that are weakly bound to each other, and their linear chains form a crystal with the space group I4/mmm. In these crystals, pronounced effects of phase transitions are observed, and this is why they are used as model objects in studies of general problems of structural phase transitions. In particular, at Tc = 186 K in Hg2Cl2 and at phasetransition temperature Tc = 144 K in Hg2Br2, improper ferroelastic phase transitions induced by the condensation of the soft TA phonon at the X point on the boundary of the Brillouin zone (BZ) of the tetragonal paraphase occur from the tetragonal to the orthorhombic Cmcm phase. At T ≤ Tc, the transitions are accompanied by the doubling of the unit cell, the X → Γ remapping in the BZ, and the appearance of
spontaneous strain and ferroelastic domains [2, 3]. In Hg2I2, the phase transition was observed only under high hydrostatic compression (Pc = 9 kbar at T = 293 К) [4]. However, no one has observed the phase transition in the Hg2F2 crystals so far. Moreover, even the question of its lattice symmetry (cubic, tetragonal, monoclinic) is
Data Loading...
