No abstract

Type II clathrates are interesting due to their potential thermoelectric applications. Powdered X-ray diffraction (XRD) data and density functional calculations for NaxSi136 found a lattice contraction as x increases for 0 < x < 8 and an expansion as x increases for x > 8. This is explained by XRD data that shows that as x increases, the Si28 cages are filled first for x < 8 and the Si20 cages are then filled for x > 8. Motivated by this work, here we report the results of first-principles calculations of the structural and vibrational properties of the Type II clathrate compounds AxSi136, AxGe136, and AxSn136. We present results for the variation of the lattice constants, bulk moduli, and other structural parameters with x. These are contrasted for the Si, Ge, and Sn compounds and for guests A = Na, K, Rb, and Cs. We also present calculated results of phonon dispersion relations for Na4Si136, Na4Ge136, and Na4Sn136 and we compare these for the three materials. Finally, we present calculated results for the elastic constants in NaxSi136, NaxGe136, and NaxSn136 for x = 4 and 8. These are compared for the three hosts, as well as for the two compositions.

Structural and elemental analysis of meltprocessible hightemperature superconductors by surface science and x ray diffraction measurements

Superconducting 123 films can be fabricated using the green 211 phase as a substrate. The superconducting characteristics of these films are better than the characteristics found when other oxide compounds are used as substrates.Using high temperature processing, 211 phase oxide can be partially converted to 123 phase. Using the same process, a new high T copper oxide compound with non -rare earth elements was prepared. High temperature processing presents an alternative synthetic route in the search for new high Tc superconductors.