The usefulness of the application of the concept of Huang scattering to the understandings of the origin of diffuse scatterings near structural phase transitions are discussed. It is pointed out that in several phase transitions, the observed diffuse scatterings can not be interpreted in terms of critical fluctuations of the order parameters associated with the structural phase transitions, and that they are rather interpreted as Huang scattering due to random distribution of individual order parameter which is‘dressed’ by strain fields. Examples to show effective applications of this concept to analyze the experimental X-ray data and whence to understand microscopic mechanisms of structural phase transitions are presented.
In xonotlite, which has a structure built up of a column parallel to b, various polytypic structural variants may occur which have various periodicities along a and c. Since this phenomenon may be looked upon as a specific polytypism which occurs in two directions, a term‘compound polytypism’has been suggested. In order to represent the polytypic variants of xonotlite, the use has been proposed of a set of three symbols‘E, A, C’each specifying a stacking mode of columns. The number of the possible polytypes and the space groups were enumerated and listed based on a new version of the Gard's notation.
The macroscopic cleavage plane of TGS at the ferroelectric phase is slightly inclined to the b-plane when the cleavage propagating direction is roughly along the c-axis. The sense of the inclination is reversed with the direction of the cleavage propagation and with the polarity of domain. The microscopic structure of the cleavage surface was examined by electron microscopy utilizing the low angle shadow decoration technique. This technique revealed the detail of the surface on the crystallographic unit cell scale. The macroscopic inclination results from the formation of elementary steps of b0 in height, where b0 is the unit cell length along the b-axis. The cleavage steps are bent at domain boundaries to one particular side determined by the polarity of domain and the cleavage direction. The elementary step formation and the step bending at domain boundaries are interpreted by assuming that the bonding on the positive side of the cleavage crack front is more easily broken than that on the negative side. There is one particular cleavage direction in which steps of b0/2 in height are formed. The surface discontinuity of b0/4 or less were observed at domain boundaries. Such a characteristic surface structures are discussed in relation to the crystal symmetry of TGS. The cleavage plane in the unit cell is the positive side of the layer composed from SO4 and glycine I at y=1/4 or 3/4.