First-principles study of cadmium vacancy in CdWO4 crystal

The structural relaxation, electronic structures and optical properties of CdWO4 crystal containing cadmium vacancy (CWO:V-Cd) are studied by the CASTEP code. The cadmium vacancy (V-Cd) would trap two holes to maintain the local electrical neutrality. The calculated structural relaxation and electronic density of states reveals that the existent form of the hole in CWO:V-Cd should be oxygen molecular ion (O-2(3-)). Therefore an associated color center [O-2(3-)-V-Cd-O-2(3-)] named as V-F center would form in the crystal. The calculated optical absorption spectrum for CWO:V-Cd indicates that this V-F center would cause an absorption band peaking at about 650 nm with a shoulder at around 400 rim, which is in agreement with the experimental optical absorptions of the blue-grey colored CWO crystal. (C) 2009 Elsevier Masson SAS. All rights reserved.