Defect simulation and crystal field studies of Ln 3+:LiCaAlF 6 and LiSrAlF 6

Abstract

LiCaAlF 6 (LiCAF) and LiSrAlF 6 (LiSAF) crystal hosts doped with trivalent positive lanthanide ions (Ln 3+) were studied via defect simulation techniques and crystal field parameter calculations. Depending on the ratio of ionic radius of the dopant to the host divalent cation, the luminescent ions can occupy the Sr 2+ (or Ca 2+) site or the Al 3+. When the dopant enters at the divalent cation site, charge compensation is needed that changes drastically the starting symmetry of this site. However, when the Al 3+ site is preferred, no charge compensation schemes are involved, but nevertheless the lattice is distorted since the sizes of the Ln 3+ ions are bigger than the Al 3+ ion. The actual positions of the first neighbours of the Ln 3+ ion were input in the crystal field parameters equations, through which the conclusions on site symmetry of the optically active ion have been drawn and these results allows the prediction of possible optical activity based on the possibility of the 4f–4f transitions.