Takuji Ikeda

Synthesis of AlPO4-5 powder by microwave heating : Influence of starting gel pH and reaction time

Abstract Extremely pure AlPO4-5 powder was synthesized using a microwave heating method. The yield as well as the size distribution of the AlPO4-5 crystals depended on the pH value of the starting gel. On reducing the pH value from 7.0 to 4.0, the average size of the product crystal was found to increase, the size distribution became wider, and the yield increased. The pH value of the supernatant liquid after the synthesis was always ca. 7.5, independent of the pH values of the starting gels. During the first step of crystallization, disk-like crystals are formed, and they grow in the c-axis direction resulting in the formation of a hexagonal rod-like shape. The channels of the crystals are completely open, and, therefore, the crystals obtained are suitable for use as a host to stabilize guest materials.

Neutron powder diffraction study of potassium clusters in zeolite K-LTA

Abstract The crystal structures of K-doped K-LTA below and above the Curie temperature of 8 K were analyzed by neutron powder diffraction on the basis of space group F23. We found alternate ordering of Si and Al in a distorted framework and different K clusters in two kinds of α-cages. Additional K + ions are distributed mainly in four- and eight-membered rings of the framework and partially occupy the center of an α-cage. K + ions near the center of a six-membered ring form two kinds of tetrahedral assemblies occupied statistically. Our structural data suggest that the K clusters in the adjacent α-cages have different magnetic moments. This is consistent with a spontaneous magnetic moment, which is observable even if the K clusters interact antiferromagnetically.

Structural study of the aluminophosphate AlPO4-5 by neutron powder diffraction

Abstract The structures of an aluminophosphate AlPO 4 -5 produced by a microwave-heating method were determined by neutron powder diffraction. An as-prepared sample incorporating triethylamine as a template crystallized in hexagonal form with space group P6cc. Template molecules were oriented in a head-to-tail manner along the c -axis. On the contrary, a sample calcined to remove triethylamine was orthorhombic with space group Pnn2 or Pcc2. The framework of AlPO 4 -5 proved to change easily by removing the guest material from its one-dimensional cavity.

Optical and X-ray diffraction study of AgI clusters incorporated into zeolite LTA

Abstract.AgI clusters are incorporated into cages of zeolite LTA. The loading density of AgI is systematically changed from 0.2 to 4.0 molecules per α-cage. The electronic states and structures of the AgI clusters, with regard to their optical absorption spectra and X-ray powder diffraction patterns, are discussed. At a dilute loading, a notable absorption band, assigned to the AgI molecule, appears at 5.6xa0eV. With increasing loading densities, new absorption bands appear at 4.7, 4.2, 4.1, and 3.6xa0eV. These bands show that (AgI)n (n=2-4) clusters with larger sizes are formed with increasing loading density. These clusters seem to exist in α-cages. The X-ray diffraction (XRD) patterns exhibit superlattice reflections at high loading densities. The appearance of the extra reflections has a correlation with the formation of (AgI)3 and (AgI)4 clusters. The Ag and I atoms of the cluster seem to be situated at the position of tetrahedral-based symmetry in the α-cage.

Incorporation of AgI clusters into the cages of zeolites LTA and FAU observed by optical spectra and X-ray diffraction patterns

Abstract The loading of AgI into the cages of zeolites LTA and FAU was performed by vapor-phase adsorption. The successful incorporation of AgI clusters into the cages was confirmed by optical absorption spectra and X-ray powder diffraction patterns. Large blue shifts of the absorption edges were observed in the spectra of adsorbed AgI to both zeolites, compared with the lowest excited state of AgI in the bulk. The present observation of the shift implies that a strong quantum confinement in the photoexcited state of AgI occurs, which leads to the conclusion that AgI clusters have been formed in the cages. In the X-ray powder diffraction pattern of AgI-loaded LTA, superlattice reflection peaks are observed which cannot be assigned either to the reflection of LTA or the AgI in the bulk.

Disordered Distribution of K+ Ions Interlayered in KxTi2−x/3Lix/3O4(x = 0.8)

Abstract An integrated software system, REMEDY, has been developed for MEMbased whole-pattern fitting. REMEDY was applied to the determination of the crystal structure and electron densities for KxTi2−x/3Lix/3O4 (x = 0.8). We have successfully revealed the positional disorder of K+ ions between Ti2−x/3Lix/3O4 layers consisting of edge-sharing (Ti, Li)O6 octahedra. Possible incorporation of residual water molecules is also suggested.

Optical Transitions of Agi and Agbr Clusters in Zeolite Fau

Abstract AgI and AgBr clusters are both incorporated into the cages of zeolite FAU. The absorption spectra of these clusters, whose loading densities varied from dilute to four AgI and AgBr molecules per supercage, are discussed. In both samples, the lowest absorption band shifts to the lower energy side with increasing loading density. The large blue shift in AgI clusters compared to the bulk can be qualitatively explained by the model of strong quantum confinement of the photoexcited electron-hole pair. In the sample of AgBr clusters with four molecules per cage, however, the energy of the lowest absorption band is close to that of the lowest direct transition of AgBr in bulk. This apparently weak quantum confinement in the photo-excited state seems to originate from the singular band structure in the bulk, i.e., AgBr is the indirect gap material.

Structural Study of a High Ionic Conductor Rb4Cu16I7.2Cl12.8

The structure of a high ionic conductor Rb4Cu16I7.2Cl12.8 at low temperature has been reinvestigated by use of angle-dispersive neutron powder diffraction. All the diffraction data have been analyzed with integrated software REMEDY consisting of RIETAN-2000 for whole-pattern fitting and MEED for calculating densities of electrons or atomic nuclei by the maximum-entropy method. The resulting nuclear-density maps have reconfirmed that Cu1-Cu2 chains are the main conduction pathways in this material, as previously suggested from the Rietveld analysis of neutron powder diffraction data.

Arrangement of K Clusters in the K-Doped Zeolite K-LTA

Abstract K clusters incorporated into zeolite K-LTA causes ferromagnetism below 8 K. Distribution of K+ ions in its α-cages was studied by X-ray powder diffraction. On the basis of space group F23, electron-density distribution was determined by Rietveld analysis followed by iteration of analysis by a maximum-entropy method and whole-pattern fitting. K+ ions were located in α-cages in such a way that K clusters between adjacent α-cages are not equivalent to each other, which will lead to the ferromagnetism with antiferromagnetic components.

Influence of gas flow conditions on cluster size distribution in the gas evaporation method

Cluster size distribution was found to be largely varied by altering the gas flow conditions in the gas evaporation method, where clusters are generated in one chamber and carried into the other chamber through a nozzle with a helium gas stream. Mean and mode diameters of clusters were found to be inversely proportional to the pressure difference between the two chambers, indicating that the gas flow condition influences the coalescence growth of the clusters in free space.