1. R. Loudon, Adv. Phys. 13, 423 (1964). ibid. 17, 243 (1968)
2. A.S. Barker, R. Loudon, Rev. Mod. Phys. 44, 18 (1972)
3. O. Madelung, U. Rössler (ed.), Landolt–Börnstein. New Series, Group III, vol. 17 a to i, 22 a and b, 41 A to D (Springer, Berlin, 1982–2001)
4. J. Geurts, Prog. Cryst. Growth Charact. 32, 185 (1996)