LOW SYMMETRY CRYSTAL FIELD IN LITHIUM SPINEL

Nous avons mesuré l'absorption optique et le spectre de luminescence du spinelle LiAl5O8 : Cr3+ à 293 °K, 77 °K et 4,2°K. Les valeurs obtenues ainsi que les résultats de mesures de RPE et RMN concordent avec une théorie du champ cristallin qui comprend le couplage spin-orbite et l'action d'un champ cristallin de faible symétrie sur un ion du site B dans les spinelles. Les formules des paramètres g, D et E de l'hamiltonien de spin et de la séparation du doublet ont été dérivées.The optical absorption and luminescence spectra of spinel LiAl5O8 : Cr3+ have been measured at 293 °K, 77 °K and 4.2 °K. The data together with the results of EPR and NMR measurements have been fitted to a crystal field theory which includes spin-orbit coupling and low symmetry crystal field acting on ion in the B site in spinels. Formulae for g, D, E parameters of spin-hamiltonian and 2E doublet splitting have been derived

Vanadium characterization in BTO: V sillenite crystals

Visible and Infrared Optical Absorption and Electron Paramagnetic Resonance (EPR) techniques have been used to characterize the intrinsic defects in sillenite type crystals: nominally pure Bi12TiO20 (BTO) and doped with vanadium (BTO:V). Optical quality crystals, with the composition Bi12.04±0.08Ti0.76±0.07V0.16±0.02O20, have been grown. Results obtained by these different techniques have shown unambiguously the 5+ valence state of the vanadium ion in BTO:V crystals. In pure BTO samples, the EPR and optical spectra show strong evidence of the presence of the intrinsic defect BiM3+ + ho+, which consists of a hole h+, mainly located on the oxygen neighbors of the tetrahedrally coordinated Bi3+ ion. After doping with vanadium, results have shown that the characteristic bands, associated to this hole defect center, disappear, suggesting its transformation in single Bi3+. Anisotropy of the EPR spectra , at 20 K, is related to Fe3+ impurities