Diffraction superstructure

It was established by X-ray [9.15], neutron [9.16], and electron diffraction, and by high resolution electron microscopy (HREM) [9.17] that the deviation from the average structure of Bi/2212 is reasonably well described by the introduction of a displacement field. However, in electron diffraction superstructure reflections are seen also near the forward direction, Fig. 9.3(a), and they should not be there in a kinematieal diffraction pattern unless the displacements are extremely large. Unfortunately electron diffraction is prone to multiple scattering. Thus, we cannot rule out the possibility that the superstrueture reflections near the center of these diffraction patterns are caused by the other reflections further out in reciproeal space these may aet as new incident beams bringing superstructure spots caused by displacement into the central region of the diffraction pattern. 

In the paramagnetic regime, the evolution of the EPR line width and g value show the presence of two transitions, observed at 142 and 61 K in the Mo salt, and at 222 and 46 K in the W salt. Based on detailed X-ray diffraction experiments performed on the Mo salt, the high temperature transition has been attributed to a structural second-order phase transition to a triclinic unit cell with apparition of a superstructure with a modulation vector q = (0,1/2, 1/2). Because of a twinning of the crystals at this transition, it has not been possible to determine the microscopic features of the transition, which is probably associated to an ordering of the anions, which are disordered at room temperature, an original feature for such centrosymmetric anions. This superstructure remains present down to the Neel... 

More recently, the reconstruction of the clean W(100) surface has also been studied by He diffraction. These studies reveal a complex behavior during the transition. Only at temperatures below 240 K sharp diffraction spots centered at the (1/2,1/2) positions are observed. In the temperature range between 400 K and 240 K broad superstructure spots are observed which progressively shift to the (1/2,1/2) position upon cooling. Lapujoulade and Salanon explain this behavior in the framework of a domain wall model reconstructed domains of various sizes are separated by dense domain walls, which disappear continuously upon cooling. 

The precise details of the structure of LLNio.5-Mno.s02 are complex, with a superstructure being observed in the X-ray diffraction pattern. Long-range order has been detected by transmission electron microscopy, and the domain size of this... 

The existence of a superstructure was revealed by satellite spots in the XRD single crystal diffraction pattern of partly dehydrated goethite. The superstructure was considered to be an in-... 

The crystal chemistry of BajRC C has been systematically studied by single-crystal and powder diffraction methods with R = La, Pr,... Yb, in addition to the conventional yttrium compound [(52)(53) (54) and references therein]. With the exception of La, Pr, and Tb, the substitution of Y with rare-earth metals has little or no effect on the superconductivity, with the values of Tc ranging from 87 to 95K. Also, a relatively small change is observed in the cell constants of these compounds. The La, Pr, and Tb-substituted materials are not superconductors. A detailed structural analysis of the Pr case (52) did not show any evidence of a superstructure or the presence of other differences with the atomic configuration of the yttrium prototype. 

By means of HREM and electron diffraction a variety of intergrowth defects have been characterized in both the single and double layer Tl-systems. In some (Tl,Ba) systems weak superstructure modulations have been reported (47)(48)(58). 

Bond valences can be used in conjunction with other techniques, particularly powder diffraction where, for example, light atoms are difficult to refine in the presence of heavy atoms. Adding the chemical constraints of the bond valence model can stabilize the refinement, particularly in the case of superstructures that have high pseudo-symmetry (Thompson et al. 1999). 

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