1. V. Stadnyk and V. Gaba, Refractometry of Dielectric Crystals with Incommensurate Phases (Liga, Lviv, 2010) [in Russian].
2. V. I. Stadnyk, N. A. Romanyuk, and R. S. Brezvin, Electronic Polarizability of Ferroics (Ivan Franko National University of Lviv, Lviv, 2013) [in Russian].
3. V. I. Stadnyk, R. S. Brezvin, and P. V. Savchuk, Opt. Spectrosc. 113(3), 288 (2012).
4. A. Scacchi, Atti Accad. Napoli 3, 57 (1868).
5. M. G. Wyrouboff, Bull. Soc. Min. France 3, 198 (1880).