The introduction of a Monte Carlo technique into the multisolution phase-determining method has made it possible to use no less than 10-100 reflections as the starting set. By means of this Monte Carlo direct method, eight centrosymmetric and thirty-four noncentrosymmetric unknown structures have been elucidated up to now ; all the structure elucidations have been accomplished in moderate computing time. These excellent results have established that the new method is a powerful means of phase determination. The present article further discusses the automatic application of the Monte Carlo direct method to unknown structures.
During the transmission electron microscope observations of β''- and β''''-alumina with rhombohedral symmetry, electron damage due to the loss of the conduction planes takes place to form the defect blocks ; whereas the hexagonal counterparts, β- and β'''-alumina are relatively stable. Based on the structure images taken by a high-resolution 1 MeV electron microscope, new structure models of the defect blocks are proposed in place of the previous two models which were mutually incompatible. The results are discussed in terms of the differences in the crystal-chemical characteristics between the rhombohedral and the hexagonal groups of compounds.
Sensitized luminescence in doped crystals illustrates most directly the occurrence of energy migration. This process can be described with rate equations, just as chemical reactions in fluid solutions. Time-resolved measurements are now possible and they disclose some differences between fluid and crystalline systems. What is unique in crystalline systems is the anisotropy. Anisotropic migration of energy, illustrated by three organic systems (naphthalene, 1, 4-dibromonaphthalene and a clathrate compound thiourea/2, 3-dimethylnaphthalene) is discussed in terms of random walks in a lattice. The idea of exciton percolation is briefly introduced.