ISSN 1063-7745, Crystallography Reports, 2007, Vol. 52, No. 6, pp. 1088–1093. © Pleiades Publishing, Inc., 2007. Original Russian Text © S.F. Burachas, A.A. Vasil’ev, M.S. Ippolitov, V.I. Man’ko, Yu.A. Savel’ev, G. Tamulaitis, 2007, published in Kristallografiya, 2007, Vol. 52, No. 6, pp. 1124–1130. 1088 INTRODUCTION Currently, high optical transparency and radiation hardness of oxide crystals are important for their prac- tical application. The nature of absorption centers in the transparency region of oxide crystals and changes in the color of crystals subjected to irradiation or annealing have not been properly studied. Deviation from stoichi- ometry in oxygen [1] is considered to be a characteristic defect of complex oxides. An oxygen deficit is inherent in oxides of heavy metals with a variable valence [2]. Analysis of the data in the literature on the specific fea- tures of the production of a number of crystals (PbWO 4 , LiNbO 3 , SrTiO 3 , and YAlO 3 ) showed that deviation from stoichiometry is a typical cause of the formation of defects in these crystals. In lead tungstate (PbWO 4 ) crystals, tungsten ions can have a variable valence, from 4 to 6. It is possible that, owing to the deviation from stoichiometry during crystal growth, inclusions of tungsten oxide are formed in the melt and then are incorporated into the regular lattice [3–8]. In [9–12], it was suggested that the main cause of the deviation from stoichiometry in lithium niobate (LiNbO 3 ) crystals is the oxygen deficit in nio- bium oxide (Nb 2 O 5 – x ) used for preparation of the start- ing melt. In [13–15], it was believed that, along with the deviation from stoichiometry in oxygen in strontium titanate (SrTiO 3 ), some part of regular Ti 4+ ions pass to the Ti 3+ state upon reductive annealing and form a char- acteristic “blue spot” defect. In [16], it was suggested that crystals of yttrium aluminate YAlO 3 :Cr subjected to reductive annealing contain aluminum in different valence states: Al 2 O 3 , AlO, and Al 2 O. The presence of such complexes in YAlO 3 was confirmed in [17]. In this study, by the example of PbWO 4 , LiNbO 3 , SrTiO 3 , and YAlO 3 crystals, we consider the causes of defect formation in oxide crystals with an oxygen defi- cit. INVESTIGATION OF THE TRANSPARENCY OF PbWO 4 AND LiNbO 3 CRYSTALS We studied the effect of irradiation and annealing on the optical characteristics of PbWO 4 and LiNbO 3 crys- tals. Two elements 22 × 22 × 180 mm 3 in size and cut from PbWO 4 crystals grown from a charge and doped with gadolinium (80 ppm) were analyzed. The crystals were colorless before irradiation. After irradiation (with a total irradiation dose up to 1 × 10 13 neutrons/cm 2 for 15 h), as well as after reductive annealing at a tem- perature of 850°C for 20 h in the atmosphere with an oxygen content of 0.001%, the crystals acquired a vio- let tint. Comparison of the transmittance distribution along the length of the elements (before and after annealing for the first element and before and after irra- diation for the second element (Figs. 1, 2)) shows that the absorption spectra recorded after irradiation and reductive annealing are almost identical. This fact indi- cates the same origin of the crystal coloring under these external actions. However, the color of the annealed crystals differs from the color of the irradiated ones. Influence of Cluster Defects of Variable Composition on the Optical and Radiative Characteristics of Oxide Crystals S. F. Burachas a , A. A. Vasil’ev a , M. S. Ippolitov a , V. I. Man’ko a , Yu. A. Savel’ev b , and G. Tamulaitis c a Russian Research Centre Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia e-mail: sburachas@inbox.ru b North Crystals Company, Apatity, Murmansk oblast, Russia c Vilnius University, Vilnius, Lithuania Received October 30, 2006 Abstract—It is shown that oxide crystals contain cluster defects of variable composition, which cause absorp- tion of light in the transparency region of crystals. The model based on the presence of cluster defects in oxide crystals explains well the experimental data on the thermal and radiative coloring of these crystals. It is noted that cluster defects accumulate oxygen in oxide crystals. These defects are responsible also for the photochro- mic effect in them. Application of the noted model made it possible to fabricate lead tungstate scintillators at North Crystals Company for the ALICE project (CERN) with almost 100% reproducibility of their operating characteristics. PACS numbers: 61.72.-y DOI: 10.1134/S1063774507060259 PHYSICAL PROPERTIES OF CRYSTALS