The optical spectroscopic properties of Cr3+ were investigated by means of optical absorption, fluorescence, fluorescence line-narrowing, selective excitation and radiative lifetime measurements. These results were then compared with data which had been obtained by using techniques which included electron spin resonance and electron nuclear double resonance, and were shown to be consistent with the existence of five Cr3+ centers. The predominant center was due to substitution at the Li+ site, which provided a weak crystal field in which Cr3+ emitted into a broad 4T2  4A2 band. By using fluorescence line-narrowing methods, 2 centers which experienced a stronger crystal field were shown to be due to substitution at this same site; probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest neighbor cation shell. The addition of Mg2+ ions did not appear to create new Cr3+ complexes. Instead, it modified the nature of the disorder and thereby changed the relative concentrations of the various centers. It thus became possible to identify 2 other lightly occupied Cr3+ centers, where substitution occurred at the Nb5+ site. The total of five Cr3+ centers provided a consistent interpretation of all of the available spectroscopic evidence; including electron spin resonance and electron nuclear double resonance. This also highlighted the difficulty of theoretically studying sites where the differences in the energy levels of Cr3+ ions were too small to be distinguished by available models.

P.I.Macfarlane, K.Holliday, J.F.H.Nicholls, B.Henderson: Journal of Physics - Condensed Matter, 1995, 7[49], 9643-56