1. Optical Phonons and Phase Transitions in the Ammonium Halides
2. International Conference on Science and Technology of Non-Metallic Crystals;Hovi,1969
3. SLATER J. C., Quantum Mechanics of Molecules and Solids, Vol. II, pp. 57–60 McGraw-Hill, New
4. Electrons and Chemical Bonding;Gray,1964