Identification of trapping sites for OH− molecular ions in CdWO4 Original Research Article

Infrared absorption bands due to OH− molecular ions have been investigated in single crystals of CdWO4. The peak positions of these OH− absorptions depend strongly on the nature of the adjacent entity (either an impurity or vacancy) responsible for trapping the proton. Six OH− spectra were observed in the undoped, and intentionally doped CdWO4 crystals included in our study. The respective trapping entities are the monovalent impurity ions Li+, Na+ and Cu+ substituting for the Cd2+ ions, the pentavalent impurities Nb5+ and Ta5+ substituting for the W6+ ions, and the Cd vacancies. Peak positions and widths of the individual absorption bands were monitored as a function of temperature. Electron paramagnetic resonance (EPR) helped assign specific absorption bands to the Cu+–OH− and VCd–OH− centers. A 900°C thermal anneal in air converted the Cu+–OH− centers to Cu2+ ions (observable with EPR). Illuminating a crystal with 325 nm light at temperatures near 90 K caused a hole to be trapped adjacent to a VCd–OH− center, thus forming an h+–VCd–OH− center, which was observable with EPR also.