Structural transformations in ZnS:Cu in the course of thermal annealing

The influence of annealing at 800$^\circ$C on the photoluminescence, electron spin resonance, and X-ray diffraction spectra of powder-like ZnS:Cu, obtained by the self-propagating high-temperature synthesis of a charge, consisting of Zn, S, and CuCl, is studied. It is shown that variation in the material’s heating rate up to the annealing temperature leads to a nonmonotonic variation in the spectral location and full-width at half-maximum of the photoluminescence band in the blue-green spectral region, as well as in the Mn$^{2+}$ paramagnetic center concentration. It is established that the cubic and hexagonal ZnS phases, as well as the ZnO and CuZn phases, are present in the powder after synthesis. It is shown that annealing of the obtained powder at 800$^\circ$C leads to three processes: the transformation of the hexagonal ZnS phase into the cubic phase, the oxidation of ZnS and CuZn, and the diffusion of Cu into the bulk of the ZnS microcrystals from the CuZn phase. A model attributing the observed variations in luminescence and electron spin resonance spectra to the diffusion of Cu and Mn impurities into the microcrystal bulk, particularly from the CuZn phase, and to their accumulation at extended defects is suggested.