1. R. Zollweg, Phys. Rev.,111, 113, 1958.
2. H. Z. Harten, Zs. Phys., 126, 616, 1949.
3. N. Mott and R. Gurney, Electronic Processes in Ionic Crystals [Russian translation], IL, 1950.
4. A. A. Vorob'ev, Physical Properties of Ionic Compounds [in Russian], Ind. vo TGU, 1960.
5. K. Boer and H. Vogel, Ann. d. Phys., 17, 4, 1, 1955.