A new aid to the determination of the point-group symmetry of transparent crystals

Abstract

The patterns formed by electrical breakdown paths provide a new way to investigate the pointgroup symmetry of transparent crystals because the paths lie along equivalent directions. A pointgroup is uniquely deternfined from the observation in a single-crystal slice of non-degenerate breakdown configurations. Such non-degenerate configurations are formed in most monoclinic and in all triclinic crystals, while most crystals of higher symmetry reveal degenerate configurations which are compatible with more than one point group. Degeneracy, however, is not an inherent property of the breakdown process and might be removed by suitable techniques. Since the paths are believed to be formed by electron avalanches, the symmetry shown by the breakdown pattern probably refers to the symmetry of the electrical fields within the crystal that influence the motion of electrons. The symmetry of the breakdown pattern is usually identical with the symmetry of the crystal determined by other methods. A new test for the lack of centrosymmetry in transparent crystals is described.

Vibrational spectra and the ferroelectric phase transition of colemanite

“…Colemanite 1, CaB304(OH)a' H20, is a hydrated borate mineral found in California, as well as in South America and the U.S.S.R. Aside from its commercial uses in borax products, colemanite was found to become ferroelectric, pyroelectric, and piezoelectric at approximately 270 K (Davisson 1956;Wieder 1959;Wieder and Clawson 1963), with the transition temperature dependent on the type and amount of impurities (Wieder 1959;Wieder et al 1962). While several infrared studies of colemanite have been reported on powdered samples (Weir 1966;Weir and Schroeder 1964;Plyusnina and Kharitonov 1963;Vlasova and Valyashko 1966;Valyashko and Wlassowa 1969) and one on single crystals (Burnel and Vierne 1970), we present the first polarized infrared and far infrared single crystal study of this mineral, including spectra of the low temperature ferroelectric phase.…”