Radiation Measurements 42 (2007) 767 – 770 www.elsevier.com/locate/radmeas Excited states of molybdenum oxyanion in scheelite and wolframite structures A. Kotlov a , L. Jönsson b , H. Kraus c , V. Mikhailik c , V. Nagirnyi a , ∗ , G. Svensson d , B.I. Zadneprovski e a Institute of Physics, University of Tartu, 142 Riia Str., 51014 Tartu, Estonia b Department of Physics, Lund University, Professorsgatan 1, 22100 Lund, Sweden c Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK d Department of Inorganic Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden e All-Russia Research Institute of Mineral Materials Synthesis, Irkutskaya Str. 1, 601600 Alexandrov, Vladimir Region, Russia Received 19 December 2006; accepted 1 February 2007 Abstract Scheelite CdMoO 4 and wolframite CdWO 4 :Mo (0.04–0.4 wt%) single crystals were studied using the time-resolved spectroscopy under UV excitation over a temperature range of 1.85–300 K. The threshold energies for the creation of free charge carriers were measured using the method of photostimulated luminescence. The decay kinetics of the main emission in CdMoO 4 and the molybdenum-related emission in CdWO 4 :Mo was studied and the parameters of the triplet excited states of molybdenum-related oxyanions were calculated. © 2007 Elsevier Ltd. All rights reserved. Keywords: CdMoO 4 ; Luminescence; Decay kinetics; Triplet states 1. Introduction Tungstate crystals are prone to be contaminated by molybde- num. In ZnWO 4 and CdWO 4 scintillators, the presence of this impurity has been shown to cause additional bands of red lu- minescence peaking at 1.78 and 1.82eV, respectively (Földvári et al., 1990; Nagirnyi et al., 2004; Garces et al., 2003). The rel- atively slow decay of these bands deteriorates the performance of the scintillator. CdWO 4 :Mo has wolframite structure with MoO 6 impurity complexes replacing regular WO 6 oxyanions. CdMoO 4 crystals have scheelite structure, and MoO 4 com- plexes should be responsible for the excitonic emission. The latter system is not much studied. Some preliminary results on emission and excitation spectra of CdMoO 4 have been reported by Mikhailik et al. (2005). By comparing the luminescence properties of molybdenum centres in wolframites and scheel- ites one could estimate how much they are affected by the ∗ Corresponding author. Tel.: +372 7428946; fax: +372 7383033. E-mail address: vetal@fi.tartu.ee (V. Nagirnyi). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.02.009 symmetry of the luminescence centre. The present work is also aimed to investigate the effect of the crystal symmetry on the structure of the excited states of molybdenum oxyanion centres by studying the decay kinetics of molybdenum-related emissions. 2. Experimental methods The CdWO 4 :Mo crystals studied here were grown at Chalmers University of Technology, Göteborg from a high- temperature solution using the slow cooling technique. The starting material was prepared from a mixture of CdO, MoO 3 and WO 3 in a Na 2 WO 4 solvent. The microprobe analysis showed Mo contents of 0.04, 0.1 and 0.4 wt% in the samples chosen for the study. The concentration of Na in the crystals grown did not exceed 1.3 at%. The CdMoO 4 single crystal was grown using the Czochralski technique. The methods for measuring excitation spectra of photostim- ulated recombination luminescence, steady-state emission and excitation spectra as well as the emission decay kinetics have been described elsewhere (Nagirnyi et al., 2004).