E.M. Roginskii - Dr. Sc. (Phys.-Math.), Senior Researcher, Ioffe Institute RAS, Saint-Petersburg. E-mail: e.roginskii@mail.ioffe.ru А.С. Крылов - Dr. Sc. (Phys.-Math.), Professor, Senior Researcher, L.V. Kirensky Institute of Physics SB RAS, Krasnoyarsk. E-mail: shusy@iph.krasn.ru Yu.F. Markov - Dr. Sc. (Phys.-Math.), Professor, Senior Researcher, Ioffe Institute RAS, Saint-Petersburg. E-mail: Yu.Markov@mail.ioffe.ru

The paper presents the study of molecular crystals within density functional theory (DFT) and by Raman spectroscopy. Among the variety of crystalline systems, the so-called molecular crystals formed by molecules, which are associated with each other by long-range forces of various nature (for example, by forces appearing during the polarization of one molecules by other ones; hydrogen bonds; van der Waals forces; etc.), can be isolated into a separate series. The atoms inside the molecules are bound by substantially stronger (covalent or ionic) bonds. The most clearly pronounced representative of this series are molecular organic crystals (MOCs). Among inorganic molecular crystals, which undergo structural phase transitions (PT), one able to emphasize the family of halogenides of univalent mercury Hg2Hal2 (Hal = F, Cl, Br, I). These crystals are used as model objects when studying effects of phase tran-sitions.These crystals exhibit phase transition, being cooled to Tc = 186 (Hg2Cl2) from the tetragonal phase into the orthorhombic one. The PTs are induced by the condensation of the "slowest" lowest_frequency TA branch in the X point of the BZ boundary of the te-tragonal paraphrase and are accompanied by doubling the unit cell. Ab initio calculations in the framework of the charge-density functional theory (DFT) with the representation of electron wave functions in the basis of the linear combination of atomic orbitals (LCAO) resulted in the calculation of the electronic structure of calomel crystals and investigation of the phonon spectrum in various points of the reciprocal space. It was found a good agreement with experimental results wile studying the abovementioned PT by DFT method with long-range corrections applied. The phonon dynamics in low-temperature phase have shown the softening of phonons at the set of high symmetry points in the Brillouin Zone (BZ). The experimental study by raman spectroscopy method have proved the softening of phonon in the BZ center (Г-point) and the PT under high hydrostatic pressure (Pc ~ 9 GPa) was observed. The phonon spectra at Г-point of benzyl crystals (C6H5CO)2 was also studied with DFT approach including long-range corrections. Benzil crystals undergo PT at ~ 83 K. The PT is accompanied by doubling the unit cell, M-Г "flip" of the boundary M point of the BZ into the center (Γ-point), and appearance of spontaneous deformation . It is assumed that this PT is a trigger one. There two kind of instabilities the first one in the Gamma point instability which induces the instability in the BZ boundary (M-point) which leads to the doubling of unit cell. The calculations have no shown the instability in the center of BZ (Г-point) thus the assumed earlier model of PT have to be upgraded.

 

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