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

Optical absorption and photoluminescence measurements at 9 K were performed on stoichiometric GaRGe2O7 and on R-doped LaGaGe2O7, R = Pr3+, Nd3+, polyc rystalline samples. In this monoclinic matrix, space group P2(1)/c (no. 14) , the lanthanide ion occupies a single crystallographic point site with sym metry C-1. From the crystal-held analysis of the optical spectra, energy le vel schemes and an expression of the associated wavefunctions for the 4f(2) and 4f(3) configurations were derived. In both cases, the fitting of exper imental Stark level energies and the phenomenological calculation of free-i on as well as crystal-field parameters (CFPs) were performed for the approx imate C-s (C-2) symmetry. Despite the low symmetry, very satisfactory corre lations between experimental and simulated energy level schemes were obtain ed, with root mean square deviations sigma = 10.1 and 14.8 cm(-1) for pr(3) and Nd3+, respectively. Energy levels and the composition of their wavefu nctions were checked for both cofigurations through a calculation of the th ermal evolution of the paramagnetic susceptibility chi, according to the Va n Vleck formula. The same calculation was accomplished with CFPs resulting from the semi-empirical simple overlap model (SOM), considering the real po int symmetry of R and C-1. Over the whole measured range, 1.7-300 K, a very good reproduction of the observed chi vs. T is yielded by each of the phen omenological and SOM calculated collections of CFPs. (C) 2000 Elsevier Scie nce B.V. All rights reserved.