The lanthanide crystal field in cubic Cs₂NaLnCl₆elpasolites

Abstract

The octahedral lanthanide crystal field is calculated for trivalent lanthanide ions (Ln3+) in the cubic Cs2NaLnCl6 elpasolite structure. These calculations are based on a model which includes two types of Ln3+-ligand interactions: multipole(Ln3+)-point charge(ligand) and charge(Ln3+)-induced dipole(ligand). The latter interaction mechanism involving ligand dipolar polarization is shown to make the dominant contributions (∼80–85 per cent of the total) to the B 0 (4) and B 0 (6) octahedral crystal field coefficients. The calculated values of B 0 (4) are in good quantitative agreement with empirically determined values, while the calculated B 0 (6) values are found to be about 2–4 times too small in magnitude. The trends of both B 0 (4)(calc) and B 0 (6)(calc) across the lanthanide series are in excellent qualitative agreement with experiment. It is found that the six chloride ions in the first coordination sphere of a Ln3+ ion contribute >95 per cent of the calculated B 0 (4) and B 0 (6) values. The ligand parameters included in the crystal field model (charge, dipolar polarizability, and positional coordinates) are exactly the same as those used in the extant f-f intensity models for the Cs2NaLnCl6 elpasolite systems.