Stupka, A. A. “Frequencies of Long-Wave Phonon-Polaritons and Optical Phonons in Diatomic Ionic Crystals”. Ukrainian Journal of Physics 59, no. 8 (October 24, 2018): 793. Accessed August 11, 2022. https://ujp.bitp.kiev.ua/index.php/ujp/article/view/2018512.