Polaronic centres in single crystals

The autolocalization of free electrons at regular lattice sites, namely at complex anions, has been demonstrated by ESR measurements. It has been shown that the centre is a shallow donor; its energy level is situated meV below the bottom of the conduction band. At temperatures of 50-60 K the captured electrons are detrapped and the formation of other paramagnetic centres occurs. In La-doped , an enhanced cross-section for electron trapping by impurity centres with respect to that for undoped samples is found. In contrast to the case for other tungstates, no intrinsic oxygen hole and/or centres were revealed in . The role of the centres in the processes of transport and recombination of nonequilibrium carriers in the lattice is discussed.