The nature of nonequivalent Nd3+ centers in CNGG and CLNGG

Results on the spectroscopic investigations and modeling of Nd3+ nonequival ent center structure in calcium niobium gallium garnet (CNGG) and calcium l ithium niobium gallium garnet (CLNGG) are presented. The low temperature ab sorption, emission under selective excitation and lifetimes on a large vari ety of samples with Nd3+ varying between 0.1 and 5 at.% are analyzed. At le ast five different types of centers have been separated. Based on spectral data along with previous reports on the composition of the crystals a model of Nd3+ five-center structure in CNGG and CLNCG disordered crystals is pro posed. The relative intensities for all the centers can be explained consis tently by the structural possibilities of occupation of the first cationic octahedral sphere around the Nd3+ site. Starting from an unique average com position of this sphere our modeling suggests a correlated placement of the cations in the first octahedral sphere around Nd3+: the larger charge of N d3+ as compared to the substituted Ca2+ leads to a local depletion of Nb5ions, accompanied by a corresponding enhancement of the lower-charge cation s (Ga3+, Li+) or vacancies as compared with the undoped crystals. (C) 2001 Elsevier Science B.V. All rights reserved.