Refractive index, dispersion, and birefringence of silicon carbide polytypes.
The refractive index of each of the four common silicon carbide polytypes has been measured over the visible range. The data were analyzed in an attempt to relate the birefringence to the relative hexagonal character of the polytype. A general relationship exists, namely, that the birefringence increases with increasing hexagonal character of the polytype. This relationship is not sufficiently precise to use for the identification of polytypes.
Study of the bands of uniform birefringence in faulted ZnS
Abstract Several vapor phase grown hexagonal ZnS crystals, faulted as synthesized and also by heating perfect specimens, have been scanned with a 10 μ beam X-ray oscillation camera. Structure and birefringence of many individual bands have been compared; some bands were wide enough to permit several adjoining exposures. Structural changes within bands were as abrupt as those occurring across band boundaries; there was other evidence of heterogeneity in bands which were too narrow for direct exploration. Heterogeneity within bands directly refutes simple models for the birefringence. Additional refutation derives from the establishment (in another report) of a twoparameter model for the disorder in ZnS; even if there were any homogeneously scattering regions they could not be characterized by a single faulting parameter. Thus the value of the birefringence of a band is not a simple measure of the degree of disorder, nor of relative amounts of hexagonal and cubic sequences, as has sometimes been assumed.
Eindimensionale Fehlordnung in Kristallen und ihr Einfluss auf die Röntgeninterferenzen. II. Berechnung der fehlgeordneten dichtesten Kugelpackungen mit Wechselwirkungen der Reichweite 3
The general theory of X-ray diffraction by a one-dimensionally disordered structure, developed in a previous communication, is here applied to the special case of close-packed structures having a `three-layer influence' on the arrangement of the neighbouring planes. The intensity of diffraction is expressed in terms of two quantities ℵν and Kν which are themselves functions of α and β, where (1 − α) is the probability that a fourth layer continues a hexagonal arrangement and β is the probability that a fourth layer continues a cubic arrangement of the three preceding layers. The line widths and displacements of the positions of certain reflexion-maxima depend on the degree of order and so enable this quantity to be determined experimentally.