LATTICE DYNAMICS AND ELASTIC PROPERTIES OF LIMS2 (M = B, AL, GA, IN) WITH A CHALCOPYRITE STRUCTURE
The hypothetical structure of chalcopyrite is considered using the methods of the theory of the functional density theory DFT, embedded in the program code CRYSTAL14, for the first time for LiMS2 (M = B, Al, Ga, In) compounds, which usually crystallize in the orthorhombic phase, and the equilibrium structural parameters a and c (in Å): 5.4632 and 8.1464 (LiBS2), 5.6770 and 10.1316 (LiAlS2), 5.6902 and 10.3749 (LiGaS2), 5.8090 and 11.2801 (LiInS2). For the center of the Brillouin zone of chalcopyrite of simulated crystals, the frequencies of optical vibrations active in the infrared spectrum and in the spectrum of Raman scattering of light are calculated. The group-theoretical analysis is carried out and the contributions of the vibrations of the Li, M, and S atoms forming LiMS2 crystals to the vibrational modes with symmetry A1, A2, B1, B2, and E are established. The obtained frequency spectra uniquely identify the LiMS2 crystals in the chalcopyrite structure. They can serve as a reference point and be useful to experimenters when trying to synthesize model samples, as well as when investigating the possibilities of phase transitions in the crystals considered. Elastic constants, phase velocities of elastic waves, fundamental elastic moduli, Poisson’s coefficients, microhardness and Grüneisen parameters are calculated. The valuesof the elastic constants satisfy the Born conditions. The isoelectronic series LiBS2→ LiAlS2→ LiGaS2 → LiInS2 is considered, which reflects the effect of a change in the chemical composition on the regular change in physical and physico-chemical properties in the group of LiMS2 crystals. It has been established that with increasing atomic number of cations M, strength, Young’s modulus, the modulus of all-round compression, microhardness and phase velocities of elastic waves decrease.