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3. J. Heijboer, Ann N.Y. Acad. Sci. 279 (1976) 104.
4. G.P. Mihailov, in Physics of non-crystalline solids, ed. J.A. Prins (North-Holland, Amsterdam 1965) p. 270.
5. N.J. Trappenier, ibid.Physics of non-crystalline solids, ed. J.A. Prins (North-Holland, Amsterdam, 1965), p. 197.