Effect of bivalent impurity of defects creation efficiency in alkaline metal sulfates

Full text: An efficiency of stable radiating defects creation in ionic crystals by subthreshold mechanisms always depends on probability of the return recombination of created defects - interstitial atom or an ion and vacancy. In pure ionic crystals from hundred Frenkel pairs of defects single defect survives on the average. The survival of defects always defines a geometrical site of interstitial anion. In alkali halide crystals with bivalent impurity (Ca, Sr) creation efficiency of stable defects increases for the order. It is known, that increase in defects creation efficiency is connected to formation of cation vacancies near bivalent impurity for indemnification of a lattice charge. Cation vacancy provides a location of interstitial atoms. We assume that the similar situation should be realized in sulfates alkaline and alkali earth metals. For example, in crystals CaS04 with rare-earth impurity defects creation efficiency increases, at least, for one order in comparison with pure crystals CaSO4. In literatures, the mechanism of increase in defects creation efficiency is not discussed and defect form speaks with localization of electron and hole by trivalent impurities. We investigated defect formation in crystals K2SO4, LiNaSO4 with impurity Cu+, Tl+ and Pb+ irradiated with X-rays at temperature of liquid nitrogen. Defect formation efficiency was estimated on light sum under a thermoluminescence (TL) band responsible for return defect recombination. Under our assumptions, in sulfates at a room temperature the universal mechanism of defects creation is realized at disintegration excited anionic complex SO2- on reaction: SO42-→ SO3-va+e-+O0 defects creation efficiency on this mechanism depends on stabilization of interstitial oxygen. In a crystal K2SO4-Pb defects creation efficiency on the order is more than in pure K2SO4, and on 3-5 times more than in comparison of crystals K2SO4-Cu, LiNaSO4-Cu, K2SO4-Tl. At return recombination hole O0- centers with the electronic centers SO4-va+e- should appear radiation of auto located exciton near oxygen vacancy. In crystal K2SO4-Pb radiation 580 nm is revealed by us. A radiation band 580 nm arises at direct excitation of a crystal by X-rays at 300 K, and also in spectral structure of TL at 420-430 K, responsible for recombination annealing of defects SO4-va+e- and O0. We assume, that the centers SO4-va+e- and O0 is effectively created near to impurity Pb+vc- at disintegration anionic complex SO42-, i.e. interstitial oxygen O02 is stabilized on cation vacancies. It is discussed defect creation mechanisms in sulfates in wide area of temperatures