1. Migoni, R., Bilz, H., B�uerle, D.: Phys. Rev. Lett.37, 1155 (1976)
2. Migoni, R., Bilz, H., B�uerle, D.: Proc. Internat. Conf. Lattice Dynamics. Balkanski, M. (ed.), p. 650. Paris: Flamarion Press 1978 and Ferroelectrics20, 157 (1978)
3. Wagner, D., B�uerle, D., Schwabl, F., Dorner, B., Kraxenberger, H.: Proc. 4th Europ. Meeting on Ferroelectricity, Portoroz 1979. Ferroelectrics26, 725 (1980)
4. Wagner, D., B�uerle, D., Schwabl, F., Dorner, B., Kraxenberger, H.: Z. Physik B (1980)
5. Yacoby, Y., Kr�hler, W.W., Just, S.: Phys. Rev. B13, 4132 (1976)