1. H. Kronmüller, A. Seeger, M. Wilkens: Z. Physik171, 291 (1963)
2. W. Schmatz: Riv. Nuovo Cimento5, 398 (1975)
3. H. Scheuer: Dissertation, Universität Bochum (1977)
4. F.V. Lawrence, P. Vilaseca, R.C. Jones: Research Report R66-11, Department of Civil Engineering, MIT, Cambridge, Massachusetts (1966)
5. S. Kadecková, B. Sesták: Growth Cryst.7, 255 (1969)