1. Landolt-Börnstein, New Series, Vol. 3, Ferroelectric and Antiferro-electric Substances, Springer Verlag, (1969) and (1975)
2. C.W. Fairall and W. Reese, Phys.Rev. B 10 (1974) 882
3. T. Matsuo, M. Tatsumi, H. Suga and S. Seki, Solid State Comm. 13 (1973) 1829
4. K. Gesi, Phys. Stat. Sol. (a) 33 (1976) 479
5. P. Fischer, Z.Krist. 54 (1914) 528