Change of uranium lattice site in LiNbO3 induced by thermal reduction treatment

Abstract

The doping of lithium niobate (LiNbO3) with uranium is of high interest due to the possible infrared laser action of U3+. On the other hand LiNbO3:U crystals grown from melt contain uranium only as U6+. It has been recently shown that by thermal reduction the valence state can be altered to U3+/U4+. Studies on the lattice site of uranium show that U occupies exclusively the Nb site in as-grown material, while after thermal reduction at 1000°C for 16 h under high vacuum ca. 20% of the U atoms have been transferred to the Li site. These findings confirm the assumption that the dopant valence state is the determining factor for the impurity lattice site: dopants with valence states + 1 to + 4 are located on the Li site, those with valence states + 5 or + 6 on Nb sites.