The sites of Gd3+ in the luminescent matrix La1−xGdxMgAl11O19: Single crystal structure determination and site-selective excitation of Gd3+

Abstract

Single crystals of La1−xGdxMgAl11O19 for x=0.02 to 1 have been grown from the melt by the Verneuil (flame fusion) method. The localization of Gd3+ ions in the matrix has been obtained using x-ray diffraction and Gd3+ fluorescence techniques giving the average and the local structure of the material. The resolution of the crystal structure, of La0.4Gd0.6MgAl11O19 homolog, indicates that this compound is of the distorted magnetoplumbite (MP) type (hexagonal P63/ mmc). Lanthanide ions lie in the mirror plane in two kinds of sites: the (2d) regular MP one (D3h symmetry) occupied by La3+ ions, the distorted (12j) one (Cs symmetry) partially filled up and containing only Gd3+. Some oxygen ions of the Ln coordination polyhedron may be missing leading to a lowering of the true symmetry of the sites. Site selective excitation of the fluorescence of Gd3+ and emission spectra have been carried out on crystals with different x values. It indicates that Gd3+ ions are distributed mainly among two sites, A and B. Crystal field analysis of the splitting of the 6P terms of Gd3+ determined on the excitation spectra show that site A is close to the ideal D3h symmetry while site B is a strongly distorted site. This leads to the identification A=(2d), B=(12j). The occupancy is larger for site B than for site A in agreement with the refinement of the structure. Selective laser excitation into the A site induces emission of the B sites as a result of energy transfer which is demonstrated by the fluorescence decay. No evidence of energy migration is found for gadolinium content up to 100%. Fluorescence spectroscopy and crystal structure determination appear complementary to obtain a detailed description of the sites of Gd3+ in La1−xGdxMgAl11O19.