The nature of nonequivalent Nd3+ centers in CNGG and CLNGG

Abstract

Abstract Results on the spectroscopic investigations and modeling of Nd3+ nonequivalent center structure in calcium niobium gallium garnet (CNGG) and calcium lithium niobium gallium garnet (CLNGG) are presented. The low temperature absorption, emission under selective excitation and lifetimes on a large variety of samples with Nd3+ varying between 0.1 and 5 at.% are analyzed. At least 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 CLNGG disordered crystals is proposed. The relative intensities for all the centers can be explained consistently by the structural possibilities of occupation of the first cationic octahedral sphere around the Nd3+ site. Starting from an unique average composition of this sphere our modeling suggests a correlated placement of the cations in the first octahedral sphere around Nd3+: the larger charge of Nd3+ as compared to the substituted Ca2+ leads to a local depletion of Nb5+ ions, accompanied by a corresponding enhancement of the lower-charge cations ( Ga 3+ , Li + ) or vacancies as compared with the undoped crystals.