Optical spectroscopy and crystal-field effects on the paramagnetic susceptibility of rare-earth germanates GaRGe2O7, R=Pr, Nd Original Research Article

Optical absorption and photoluminescence measurements at 9 K were performed on stoichiometric GaRGe2O7 and on R-doped LaGaGe2O7, R=Pr3+, Nd3+, polycrystalline samples. In this monoclinic matrix, space group P21/c (no. 14), the lanthanide ion occupies a single crystallographic point site with symmetry C1. From the crystal-field analysis of the optical spectra, energy level schemes and an expression of the associated wavefunctions for the 4f2 and 4f3 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 Cs (C2) 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 Pr3+ and Nd3+, 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 C1. 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.