Yoshiyuki Kowada

Glass Formation of High-Tc Compound BiCaSrCu2Ox by Rapid Quenching

A new high Tc compound, BiCaSrCu2Ox, was successfully prepared in glassy form by a twin-roller quenching method for the first time. The rapidly quenched glass has a glass transition temperature of 390°C and a crystallization temperature (Tx) of 444°C. Heat-treatment at higher temperatures above Tx increased the crystallinity and at 850°C produced X-ray patterns identical to those of the starting compound. The resistivity of the sample heat-treated at 850°C shows the onset Tc at 88 K. These findings are expected to facilitate fiber drawing for such high Tc compounds.

Raman spectra of TeO2-based glasses and glassy liquids : local structure change with temperature in relation to fragility of liquid

The short-range and intermediate-range structures of TeO2-based glasses and glassy liquids were studied on the basis of transformation-range viscosities and high-temperature Raman spectra. The values of Eη/Tg, one of the measures of fragility of a liquid, were higher in the Li2OTeO2 system than those in the conventional glass-forming system of Na2OSiO2. High-temperature Raman spectra revealed that the TeO4 trigonal bipyramidal units present in the 20Li2O · 80TeO2 glass at room temperature were converted into TeO3 trigonal pyramidal units with non-bridging oxygens as the temperature was increased to above the melting temperature, whereas the major structural units in conventional glass-forming systems like alkali silicates were not likely to change with increasing temperature. Such an unusually large change of local structure with temperature in TeO2-based glasses can be described by the structural change model in which the TeO(axial) bond cleavage occurs with heating. The large fragility of liquid in this system can also be explained by this model.

Electronic state of trivalent ionic conductors with Sc2(WO4)3-type structure

Abstract The electronic state of M 2 (WO 4 ) 3 trivalent ion conductors (M=Al, Sc, Lu, Yb, Tm, Er, Y) was calculated by the discrete-variational Xα molecular orbital (DV-Xα MO) method. The bond overlap populations between the M 3+ ion and the surrounding oxide ions and the net charge of M 3+ , W 6+ , and O 2− ions were compared with the experimental ionic conductivity of M 2 (WO 4 ) 3 crystals. The variation of the activation energy of ionic conductivity agreed qualitatively with the change of the bond overlap population between the migrating trivalent ion and the surrounding oxide ions. This result suggests that the bonding state of the trivalent ions plays an important role in trivalent ion conduction in M 2 (WO 4 ) 3 crystals.

Application of the DV-Xα cluster method to calculations of the electronic structure of silicate and phosphate glasses

The electronic structure of simple silicate and phosphate clusters, which are assumed to exist in rapidly quenched Li2OSiO2 and Li2OP2O5 glasses, has been calculated by the discrete variational (DV)-Xα molecular orbital method. The calculations have been carried out for several sizes of clusters, namely, tetrahedral units (SiO44− and PO43−), dimers (Si2O76− and P2O74−), and trimers (Si3O96− and P3O93−). For both systems the valence electronic state is characterized by an occupied valence band, with O 2p components, and an unoccupied band of Si or P orbitals, separated by an energy gap. The energy gap between the highest occupied molecular orbital (HOMO) and the loest unoccupied molecular orbital (LUMO) decreases with increasing cluster size. A mulliken population analysis show that the negative effective charge on the non-bridging oxygens is decreased with decreasing energy gap. The bond order of the SiO and PO bonds, and thus the strength of these bonds, is increased with increasing cluster size.

Variation of electronic state of AgI-based superionic conductors with movement of Ag ions

Abstract The electronic state of AgI-based superionic conductors was calculated by the DV-Xα cluster method. A model cluster, derived from the α-AgI crystal structure, was used. An Ag ion in the cluster was moved from a tetrahedral site to another tetrahedral site through a neighboring octahedral site. The electronic state of the same model cluster for Na ion was also calculated for comparison. The total bond order between the moving Ag ion and the other ions was decreased with the movement of the Ag ion and was slightly increased at the center of the octahedral site. The change of the total bond order of the Ag ion, however, was much smaller than that of the Na ion. This smaller change of the total bond order of the Ag ion should be one of the origins of the fast movement of Ag ions in AgI-based superionic conductors.

Electronic states of binary tellurite glasses

Abstract Electronic states of binary tellurite glasses have been calculated by the DV-Xα cluster method. The calculation was achieved for several model clusters with various polymerization degrees and with various second component ions, such as glass-forming ions, intermediate ions, and modifier ions. The bonding nature of each bond in the clusters of binary tellurite glasses is discussed and compared with the silicate glasses. In the TeO 4 trigonal bipyramid (tbp) unit, bond order between the Te ion and the equatorial oxygen ion (O eq ) was much larger than that between the Te ion and the axial oxygen ion (O ax ). In the cluster for the binary tellurite glasses, the bond order of the clusters with the modifier ions was almost zero. The bond order with the glass-forming ions had values larger than those with the modifier ions. In the case of the intermediate ions, the bond order ranged from −0.3 to 0.1. This change of the bond order of M-O Te-O-M with second component ions had a similar tendency to that in the binary silicate glasses.

Electronic states of transition metal ions in silicate glasses

Abstract The electronic states of transition metal ions in silicate glasses have been calculated by the DV-Xα cluster method. The molecular orbitals, mainly composed of the 3d atomic orbitals of the transition metal ions, are located in the energy gap resulting from the SiO 4 units. Although the net charge on the transition metal ions increases with an increase in the initial charge, the charge saturates at a certain value. The bond order between the transition metal ions and the oxygen ions is smaller than that between the silicon atoms and the oxygen ions but is generally larger than that between alkali and alkaline earth ions.

High-temperature structure and crystallization kinetics of Li2OTeO2 glasses

Abstract A unique crystallization behavior was observed in the Li 2 OTeO 2 system; the crystallization occurred easily on heating the glasses but hardly occurred at all when the corresponding liquids were cooled. The local structure and crystallization kinetics of the rapidly quenched glasses and liquids in this system have been studied by high-temperature Raman spectroscopy and differential scanning calorimetry. The Li 2 OTeO 2 liquids exhibited much higher fractions of TeO 3 trigonal pyramidal units with non-bridging oxygens than the corresponding glasses. Whether or not the crystallization from the liquids occurred easily depended on the similarity in the local structure between the liquids and the main crystallization products. The crystallization kinetics revealed that the number of nuclei was possibly saturated during the quenching of the liquids in the glass preparation, which explains the unique crystallization behavior of the Li 2 OTeO 2 system.

Electronic states of Ag ions in AgI-based superionic conducting glasses

Electronic states of AgI-based superionic conducting glasses have been calculated by the DV-Xα cluster method. The calculation was carried out for the AgI 4 tetrahedral unit. The NaI 4 tetrahedral unit was also considered for comparison with the AgI 4 unit. The net charge of the Ag ion in the AgI 4 cluster was found to be about +0.08, which was smaller than that of the Na ion, +0.39, in the NaI 4 cluster. The bond order between the Ag and the I ions, however, was almost the same as that between the Na and the I ions in the NaI 4 cluster. This result suggests that the total interaction between the Ag and the I ions, including both ionic and covalent bonding in the AgI 4 cluster, is smaller than that between the Na and the I ions in the NaI 4 cluster. This characteristic electronic state of the Ag ion is one of the causes of the fast movement of Ag ions in Agl-based glasses.

Electronic states of modifier ions in silicate glasses

Abstract The electronic states of modifier ions in a variety of alkali and alkaline earth silicate glasses have been calculated by the DV-Xα cluster method. The energy level structures in the clusters with alkaline earth ions are similar to those with the alkali ions. In both cases of alkali and alkaline earth ions, the net charge of the modifier ions is increased and the bond order between the modifier ions and the non-bridging oxygen ions is decreased with an increase in the atomic number of the modifier ions. The bond order in the case of alkaline earth ions is larger than that in the case of alkali ions.