Abstract:
The ab initio calculations of the crystal structure and the phonon spectrum of Cs$_2$NaYbF$_6$ and Cs$_2$NaYF$_6$ crystals with the elpasolite structure have been performed. The frequencies and types of fundamental vibrations have been determined. The calculations have been performed in the framework of the density functional theory using the molecular orbital method with hybrid functionals in the CRYSTAL09 program developed for the simulation of periodic structures. The outer 5$s$ and 5$p$ shells of the rare-earth ion have been described in Gaussian-type basis sets. The influence of inner shells, including 4$f$ electron shells, on the outer shells has been described using the pseudopotential. It has been shown that this approach allows the description of the phonon spectrum with the inclusion of the splitting of the longitudinal and transverse optical modes.
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