Effect of isoelectronic impurities K+ and I− on luminescence of CsBr:Eu2+ crystals

Abstract

We have investigated the photoluminescence (PL) and photostimulated luminescence (PSL) spectra at 300 K to study the effect of isoelectronic impurities K + and I -on the formation and energy structure of Eu 2+ -V Cs isolated dipole centers and aggregate centers in the form of single crystals of CsEuBr 3 in CsBr:Eu 2+ single crystals. We have shown that K + and I -impurities in a concentration of 5 mol% do not have a substantial effect on the energy spectrum of isolated dipole centers in CsBr:Eu 2+ single crystals and the processes for the formation of such centers during growth of CsBr:Eu single crystals from the melt by the Bridgman method. We have established that in Cs 0.95 K 0.05 Br:Eu 2+ , more favorable conditions are realized for the formation of aggregate centers than in CsBr:Eu 2+ and CsBr 0.95 K 0.05 Br:Eu 2+ single crystals. So in order to improve the storage properties of phosphors based on CsBr:Eu 2+ , in particular for increasing the efficiency of PSL excitation, it is expedient to dope them with K + impurity in a concentration up to 5 mol%.Introduction. The phosphor CsBr:Eu 2+ has better storage properties than the traditional phosphor BaFBr:Eu 2+ for use in digital luminescence x-ray radiography systems [1]. At the same time, the storage properties of solid solutions based on CsBr:Eu 2+ have not been studied in detail. In [2], the effect of isoelectronic impurities K + and I -on the x-ray luminescence (XRL) spectra and on the energy structure of trapping centers was studied in CsBr single crystals activated by europium oxide EuO. It has been shown that when CsBr:Eu single crystals are doped with KBr and CsI impurities in concentrations of 5 mol%, close to the maximum possible from the standpoint of obtaining structurally homogeneous single crystals of solid solutions of these compounds [3,4], charge carrier trapping centers are formed with energy depth smaller than the energy depth of F centers. The hypothesis has been discussed that the presence of shallower electron traps may be the reason for the long-wavelength shift of the PSL excitation spectrum of these single crystals (λ max = 670 nm) and the decrease in the effective contour for overlap of this band with the emission band of an He-Ne laser (λ = 633 nm). This is undesirable from the standpoint of using these materials as storage phosphors. At the same time, we may hope that K + impurity in the above-indicated concentration leads to appreciable broadening of the PSL excitation spectrum toward shorter wavelengths, since the maximum of the F absorption band for KBr single crystals, which coincides with the maximum in the PSL spectrum, is located at 630 nm [5]. However, the PSL excitation spectra of these single crystals have not been studied, and this question has remained unexplained. The effect of K + and I -impurities on the conditions for the formation of Eu 2+ -containing isolated diple centers Eu 2+ -V Cs and more complex aggregate centers during growth of these single crystals from the melt by the Bridgman method has also not been stu...