Absorption, luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO4 Original Research Article

A single crystal of cadmium tungstate (CdWO4) containing approximately 200 ppm of molybdenum was grown by the Czochralski method and then characterized in a series of optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), and electron paramagnetic resonance (EPR) experiments. The Mo6+ ions substitute for W6+ ions and serve as recombination sites for electrons and holes when the crystal is exposed to ionizing radiation. A charge-transfer absorption band for the Mo6+ ions was observed near 320 nm at 10 K. The PL experiments, performed at low temperature with 325 nm excitation, showed a Mo-associated emission peaking near 680 nm. A direct correlation of the 680 nm emission and the 320 nm absorption band was established by the PLE data. When these doped CdWO4 crystals are exposed at low temperature either to light that is near or above the band gap or to X-rays, the Mo6+ ions can trap an electron and form stable Mo5+ ions. The EPR spectrum of the Mo5+ ions was observed at temperatures near 15 K, and a complete set of parameters describing the g matrix was obtained from an angular dependence study.