Silica occurs as minerals involving more than 20 different phases. Most silica mineral structures can persist up to high temperatures, where thermal expansion coefficients are nearly null or rather negative. Quartz, well known by its unparalleled applications of piezoelectricity and, of course, as the most common silica mineral, shows the α-β structure transition, which was found by Le Chatelier in the end of the 19th century and has continuously drawn attention of a wide variety of researchers. However, the structural problems such as high temperature structures and structure transformations in many silica minerals have been yet established. Tridymite, for example, shows successive displacive-structure transitions with varying temperature among disordered states, but the theoretical aspect of its mechanism has remained unknown so far. In this article, we review the recent structural problems in quartz, tridymite and cristobalite, all low-pressure polymorphs, and then introduce an example of successful application of molecular dynamics simulation to quartz, stressing the importance of unified application of diffraction crystallography and computer simulation to the complicated problems in silicas.
Three dimensional crystal structures of the oxidized (as isolated) and H2-reduced forms of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F were determined. The active site of the enzyme in the oxidized form is a hetero binuclear Ni-Fe complex with four cysteinyl ligands and unusual four non-protein ligands (SO, CO, CN, S) . Among non-protein ligands, the monatomic S ligand, which makes a bridge between the Ni and Fe atoms has been confirmed to be liberated as a H2S upon reduction with H2. The possible mechanism of the activation process of the active site is discussed.
Measuring techniques have been described for the polarized reflection spectra on single crystals by means of a microscope-spectrophotometer. Key technical issues as well as basics for the spectrum evaluation have also been presented.
The colossal dielectric properties of relaxors are related to the appearance of polar nanoregions in cubic nonpolar matrix. In this hetero-phase structure, the long-range order is locally clamped and the average symmetry of the specimen remains to be cubic down to the low temperature. However, the application of the electric field yields the evolution of longrange order and the ferroelectric state prevails on the specimen. However, it develops with quite long incubation time. We report here this peculiar phenomenon which was observed by optical, X-ray and TEM techniques, and discuss the origin.
Potassium clusters stabilized in regular cages of aluminosilicate zeolite LTA are investigated in detail in term of structure as well as magnetic properties. A ferromagnetic properties have been observed when more than two potassium atoms are adsorbed into each α cage of LTA. When the average number of adsorbed K metal exceeds two atoms per α cage, new diffraction lines are observed at the forbidden indices of original framework structure of LTA. The crystal structure is well analyzed by the space group F23 with a larger unit cell. The origin of the forbidden diffraction lines is assigned to the superstructure of two different potassium clusters stabilized periodically in adjacent α cages. Most of adsorbed K atoms occupy the side positions of 4-membered rings in the α cage. The periodic difference of their occupancy is main origin of the superstructure reflection. At higher densities of K adsorption, an atom occupies one of the centers of clusters. A discussion is given for the relation between the structure and the magnetic properties.
Low-temperature heat capacity of pure zirconia and yttria stabilized zirconia (YSZ) was measured by adiabatic calorimetry. YSZ showed excess heat capacity compared with pure zirconia. The excess heat capacity, which should be attributed to the stabilization of cubic structure caused by yttria doping of several mol %, decreases with increasing yttria content. The vibrational property and defect structure are discussed in detail. The molecular dynamics simulation performed on the YSZ showed that the lattice relaxation around oxygen vacancy played an important role in vibrational property of YSZ.
The structure of grain boundaries in rocks, which are composed mainly of fine-grained quartz and feldspar, was studied by high-resolution electron microscopy (HREM) . At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in connected grains meet each other at interfaces with no other appreciable phases. In these boundaries, it has been found that some of the straight segments of the boundaries correspond to a low-index plane of one of the connected grains. Boundaries containing voids, with a spheroidal shape elongated along the boundaries, were observed. It is suggested that these boundaries were formed by healing of microcracks.
A model peak profile function synthesized by multiple convolutions of the intrinsic peak profile with the instrumental functions of a conventional powder X-ray diffractometer has been developed. The general recipe for deriving accurate instrumental functions as well as an efficient numerical method for evaluating the convolutions are presented. The validity of the model function has been examined by comparing the experimental peak profiles measured in different conditions. The use of the convoluted model profile function provides a convenient and reliable way to estimate the integrated intensities, intrinsic peak positions and broadenings.