Since the first observation of the Si (111) 7×7 structure by Schlier and Farnsworth in 1959, many structure models have been already proposed. They are, for instance, vacancy model by Lander-Morrison, chain model by Seiwatz, dislocation model by Takeishi-Hirabayashi, buckling models by Levine et al. Miller-Haneman and Chadi et al, and their modified ones. However, LEED intensity analysis was not tested for the majority of the models. Only for few models, the intensity analysis were actually carried out, but the agreement between the LEED observation and the calculated intensity was not necessarily enough. Recently, Ino proposed a new structure model which contains some scattering matter arranged in a regular triangular form with side 2a, a being the size of the substrate unit mesh. The new model was deductively obtained as a structure which satisfies a Patterson function calculated by using a mean intensity distribution of various RHEED patterns. The intensity distributions calculated from the model agreed well with that obtained not only from RHEED but also LEED patterns. A comparison and discussion among the all proposed models are given on the basis of the kinematical intensity analysis of LEED and RHEED.
A gem opal consists of monodisperse colloidal spheres of silica being arranged in orderly arrays. This must have been formed in the geological past from a colloidal suspension of silica and then be desicated. The formation process from the suspension would be a sort of the phase transition from a disordered to an ordered state which is seen in a monodisperse latex. This transition is characterized by a repulsive interaction between the particles, and recently identified as Kirkwood-Alder transition that is considered to be an essence of the liquid-solid transition. The opal structure is the same as that in monodisperse latexes which can be seen under a light microscope. It is interesting that such structures reflect some aspects of the atomic structure in crystals. There are sometimes found, in multicomponent opals, superstructures such as AlB2-and CaZn13-type and these are also found in binary mixtures of monodisperse latexes. Now colloid science is opening a new aspect in the investigation of the structure of alloys and some compounds.
The molecular dynamics (MD) calculations on silicate melts and glasses, assuming purely ionic potential functions of the Born-Mayer-Huggins type, have been successful so far in obtaining three-dimensional structures which are in harmony with observed data such as RDF and Raman spectra under ambient pressures. The increase in coordination number of silicon (from 4 to 6) was observed without change in potential parameters on compaction, accompanied by a 3-5 percent increase in the mean Si-O distance. The silicate systems, however, have their own difficulties : The mobility of ions is so small that the duration required to attain thermal equilibrium appears to be much longer than that for alkali halides (presumably by a factor of 100 to 1000), and the number of ions in the basic cell (usually 216 for alkali halides) might have to be much greater to permit full development of silicate framework without local strain concentration. The difficulties, which should render the calculation to be practically impossible in terms of available machine time and budget, make it inevitable to find suitable conditions (initial configuration, cooling rate, number of particles and so on) to give desired properties with reasonable accuracies within a reasonable machine time.