Amirulla M. Mamedov, V. I. Shil'nikov, T. R. Mekhtiev, I. S. Ibragimova, “Energy spectrum and optical properties of $\mathrm{Ba}_{x}\mathrm{Sr}_{1-x}\mathrm{Nb}_{2}\mathrm{O}_{6}$ and $\mathrm{Ba}_{2}\mathrm{NaNb}_{5}\mathrm{O}_{15}$ in the energy region $0.1$–$35.0$ eV”, Fizika Tverdogo Tela, 25:6 (1983), 1620–1626
