The modulated structure of the title compound (aNiNSal) was determined at 160 K by single-crystal X-ray methods. The refinements were performed in the superspace group otba2 (a00). Different models considering thermal fluctuations in the phase and the amplitude of the static modulation wave were calculated. The best refinement yielded the overall R factors R = 0.088, R w = 0.051 for 4322 independent reflexions including 2725 first-order and 118 secondorder reflexions. The positional parameters of the basic structure and of the average structure are nearly equal. The amplitudes of the one-dimensional displacive modulation waves decrease due to steric interactions between adjacent molecule stacks from 0.30 A at the central Ni u ion to 0.22 A at the peripheral C atoms. The modulation functions were found to be sinusoidal. Calculations with a phason factor yielded only slightly better R factors -no reliable proof for the existence of amplitude and phase fluctuations.
The temperature dependence of satellite reflections and diffuse scattering of a-CuNSal, Cu(CsHsNO)2 , was studied with X-ray film and counter techniques. At T t = 305 (2)K a continuous and reversible phase transition from a commensurate to an incommensurate orthorhombic structure was observed. At T C = 241 (2) K there occurs a first-order transition from the incommensurate to a commensurate monoclinic structure. In the incommensurate phase an intense diffuse scattering at the satellite reflections was observed, which is explained by phase fluctuations of the static displacement wave.
THREE-BEAM EFFECTS IN PENDELLOSUNG FRINGESor smaller than the corresponding two-beam gap. For a given sign of the deviation parameter, the direction of the fringe bending is found to depend on the sign of the product between the three structure factors involved in the three-beam calculations. This effect may thus be utilized to determine three-phase structure invariants experimentally. In structure work, however, the method may at the present stage of development only be applied in a limited number of cases as relatively large single crystals are needed.To determine the branches which contribute most strongly to the intensity the calculations of excitation coefficients have proved to be essential. Only the four branches, which correspond to the ones in the two-beam case, are, through such calculations, found to be of importance. These branches contribute to the intensity oscillations in pairs corresponding to the a and n components. The present calculations therefore show that polarization contributes in the usual way to the fading, but are not essential for the interpretation of the three-beam effects studied.
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