Structural and Vibrational Properties of Crystals with a Rare-Earth Sublattice: Ab Initio Calculations

摘要

We investigated the crystal structure, vibrational, and elastic properties of crystals with a rare-earth sublattice related to different structural types at the ab initio level of modeling: elpasolite Cs_2NaRF_6 -> pyrochlore R_2Ti_2O_7 -> ferroborate RFe_3(BO_3)_4, where R is a rare-earth ion or yttrium. The calculations were performed in the framework of density functional theory using the hybrid functionals containing local and non-local contribution (i.e. Hartree-Fock exchange term) to the exchange energy. We used the CRYSTAL program for simulating periodic structures in the MO LCAO approximation. To describe the internal shell of a rare-earth ion of up to 4f, we used the nonrelativistic pseudopotential (《4f-in-core》) that describes the effect of internal electrons on the outer valence shells. The results of the calculations are in good agreement with the available experimental data of IR and Raman experiments and X-ray diffraction analysis for the rows of elpasolites, pyrochlores and ferroborates.