Energy Transfer Between Two Sites in Nd3+:KLiYF5

High resolution spectroscopy of Nd3+ in Potassium Lithium Yttrium Fluoride (KLiYF5 ) at ≤80 K reveals two crystal-field sites which are equally populated independent of concentration. Site selective excitation of photoluminescence distinguishes each site's contribution to the spectra. The analysis of integrated line intensities reveals a bidirectional energy transfer between Nd3+ ions in different sites. A rapid increase in transfer rate with increasing Nd concentration is explained only if non-radiative transfer via exchange interaction contributes strongly to the effect. The observed relative decrease with increasing concentration of those emission lines which overlap strongly with opposite-site absorption lines implies that radiative transfer is also important. The energy transfer strongly depends on temperature (2 K – 80 K) which indicates the participation of phonons.