Electronic excitations in ZnWO₄ and Zn_xNi_1-xWO₄ (x = 0.1 - 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO₄ as well as the Zn_xNi_1-xWO₄ solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2. 5 eV, being due to radiative electron transitions within the [WO₆]^6- anions, becomes modulated by the optical absorption of Ni²⁺ ions in the Zn_xNi_1-xWO₄ solid solutions; and (ii) no significant change in the excitation spectra of Zn_{0. 9}Ni_{0. 1}WO₄ is observed compared to pure ZnWO₄. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO₄.