Optical spectroscopy and crystal-field effects on the paramagnetic susceptibility of rare-earth germanates GaRGe 2O 7, R=Pr, Nd

Abstract

Optical absorption and photoluminescence measurements at 9 K were performed on stoichiometric GaRGe 2O 7 and on R-doped LaGaGe 2O 7, R=Pr 3+, Nd 3+, polycrystalline samples. In this monoclinic matrix, space group P2 1/c (no. 14), the lanthanide ion occupies a single crystallographic point site with symmetry C 1. From the crystal-field analysis of the optical spectra, energy level schemes and an expression of the associated wavefunctions for the 4f 2 and 4f 3 configurations were derived. In both cases, the fitting of experimental Stark level energies and the phenomenological calculation of free-ion as well as crystal-field parameters (CFPs) were performed for the approximate C s (C 2) symmetry. Despite the low symmetry, very satisfactory correlations between experimental and simulated energy level schemes were obtained, with root mean square deviations σ=10.1 and 14.8 cm −1 for Pr 3+ and Nd 3+, respectively. Energy levels and the composition of their wavefunctions were checked for both configurations through a calculation of the thermal evolution of the paramagnetic susceptibility χ, according to the Van Vleck formula. The same calculation was accomplished with CFPs resulting from the semi-empirical simple overlap model (SOM), considering the real point symmetry of R and C 1. Over the whole measured range, 1.7–300 K, a very good reproduction of the observed χ vs. T is yielded by each of the phenomenological and SOM calculated collections of CFPs.