UNUSUAL STRUCTURAL RELAXATION FOR RARE-EARTH IMPURITIES IN SAPPHIRE -AB-INITIO STUDY OF LANTHANUM

A first-principles density-functional local density approximation stud y of substitutional La, using 120-atom supercells of alpha-Al2 O3, sho ws that La assumes a highly displaced position, moving over 0.5 Angstr om from the original Al site. The La-O nearest neighbor distance is mu ch smaller than the sum of standard La+3 and O-2 ionic radii. This is due to La being more ionic than in its own oxide, and to the role of t he 5d shell, as the electron density within the displaced La is quite anisotropic. We discuss the implications for a systematic description of rare-earth ions implanted in sapphire.