Ab initio calculation of the structure and optical properties of lead oxyhalides Pb$_{3}$O$_{2}X_{2}$ ($X$ = Cl, Br, I)

Using ab initio methods, we obtained information on the crystal structure, calculated the phonon spectra, and determined the optical properties of oxyhalides Pb$_{3}$O$_{2}$Br$_{2}$ and Pb$_{3}$O$_{2}$I$_{2}$. A compound of Pb$_{3}$O$_{2}$Br$_{2}$ is synthesized, and its phonon spectra are recorded. The experimentally observed vibrational bands are assigned. A comparative analysis of the crystal lattice parameters, the phonon spectra, and the anisotropy parameters of lead oxyhalides Pb$_{3}$O$_{2}X_{2}$ ($X$ = Cl, Br, I) is performed.