EPR OF TUNGSTEN IMPURITIES IN KTIOPO4 SINGLE-CRYSTALS GROWN FROM MOLTEN TUNGSTATE SOLUTIONS

The presence of tungsten impurities in KTiOPO4 (KTP) crystals grown from molten tungstate solutions has been observed by means of electron paramagnetic resonance (EPR) at 90 K. The angular dependence of the EPR signals and their behavior under thermal treatments allow us to distinguish two tungsten centers. The analysis of the measured g and hyperfine matrices for both centers indicates that the signals are due to W5+ located at the two titanium sites of the KTP structure. Moreover, it is shown that pentavalent tungsten is incorporated as the tungstenyl WO3+ ion, whose formation is favored by the existence of one short ''titanyl'' Ti-O bond for each site. The results for the tungstenyl ion are compared to those for other oxycations previously studied in KTP. On the other hand, the intensity changes induced on the EPR signals by the thermal treatments indicate that most tungsten impurities enter the Til site in the hexavalent state for the as-grown crystals. Besides, the presence of some tetravalent tungsten at the Ti2 site created by the reducing treatment is inferred.