First-principles study of cadmium vacancy in CdWO4 crystal

The structural relaxation, electronic structures and optical properties of CdWO crystal containing cadmium vacancy (CWO:V) are studied by the CASTEP code. The cadmium vacancy (V) 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 should be oxygen molecular ion (O). Therefore an associated color center [O-V-O] named as V center would form in the crystal. The calculated optical absorption spectrum for CWO:V indicates that this V center would cause an absorption band peaking at about 650 nm with a shoulder at around 400 nm, which is in agreement with the experimental optical absorptions of the blue-grey colored CWO crystal