The classical polarizability theory of optical activity in crystals is elaborated and a method of calculating rotatory power from crystal structure data is described. By varying the polarizabilities of the atoms within reasonable limits, optical rotatory powers are computed for crystals of a-quartz, A1PO4, cinnabar, Bi12SiO2o, Bi12GeO2o, a~-LilO3, NaC103 and NaBrO3. In general, the agreement between theory and experiment is excellent. As expected, considerable variability in polarizabilities is required in different structures in order to obtain satisfactory fits to the observed refractive indices and rotatory powers. The orientations of anisotropic polarizability ellipsoids derived in the course of the calculation are compared with those suggested earlier by Glazer & Stadnicka [J. AbstractConditions for the intensity enhancement of a Bragg reflection are investigated through convergent-beam (CB) reflection electron diffraction experiments. This intensity enhancement is of great interest for reflection electron microscopy of surfaces. Comparison of CB reflection diffraction patterns with CB transmission diffraction patterns shows a very similar enhancement which can be explained by Bloch waves. The observed surface diffraction parabolae are closely related to Kikuchi envelopes. The intensity enhancement can be interpreted as a channelling phenomenon.0108-7673/86/060569-09501.50

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The phase transition in the relaxor ferroelectric lead scandium tantalate, Pb(Sc0.5Ta0.5)O3, is investigated in both ordered and partly disordered (Sc, Ta disorder) modifications through the technique of hard-mode Raman spectroscopy. The vibrational modes for ordered para phase, ordered ferroelectric phase, disordered para phase and disordered ferroelectric phase are assigned to different representations of the respective groups (Fm3m (Z=2), R3m (Z=2), Pm3m (Z=1) and R3m (Z=1)) using the usual group-theoretical methods. The temperature evolution of the strong Raman band at 61cm-1 (hard mode) is investigated from 102 K to 873 K, for both phase transitions, namely ordered para to ordered ferro and disordered para to disordered ferro phases. The temperature variations of integrated intensity, full width at half maximum and square of the frequency of the 61cm-1 band are discussed in the light of phase transition mechanisms involved in relaxor ferroelectrics for both cases.

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