L. C. Ting, H. H. Hsieh, H. H. Kang, D.C.ling, H. L. liu, W. F. Pong, F. Z.Chien, and P.H. Hor

1. Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes RbxWOy L. C. Ting, H. H. Hsieh, H. H. Kang, D.C.ling, H. L. liu, W. F. Pong, F. Z.Chien, and P.H. Hor

2. Introduction 1.TTB phase of Na0.23WO3 and Pb0.33WO3 are found to have tetragonal and pentagonal tunnels. 2.The hexagonal tunnels are in the HTB phase of RbxWO3 with 0.19 ≦ x ≦ 0.33. 3.It was pointed out by Brusetti et al. that the screening mechanism is probably not very effective in the HTB phase of RbxWO3, since the variation of Tc with rubidium concentration x in the range of 0.23 ≦x≦ 0.28 doesn’t follow the commonly observed trend of increasing Tc with decreasing x.

3. EXPERIMENTAL Samples investigated were prepared by the solid-state reaction method. The proper stoichiometric amounts of high purity powers of Rb2WO4, WO3, and W were thoroughly mixed and ground, then pressed into pellets. Subsequently, the pellets were sealed in quartz tubes in vacuum with pressure of 10^(-2) Torr. Samples were fired at 650℃for 5 h followed by heating at 850 ℃ for 15 h. Space group- P63/mcm 3A1g+3A2g+B1g+2B2g+3E1g+6E2g+2A1u+4A2u+6B1u+4B2u+9E1u+7E2u=75 IR Raman

4. 1.The amount of impurity phase decreases with increasing oxygen concentration for all samples studied. 2.It appears that the diffraction peaks broaden significantly as the oxygen concentration reaches 3.07, indicative of a poorly crystalline phase present.

6. 1.y < 3.00 have a lower Tc around 3K and a sharper superconducting transition width; y >3.00 have a higher Tc around 5K and a wider super conducting width. 2.The superconducting transition temperature and the transition width obtained from resistivity measurements are remarkably in good agreement with hose determined from magnetic measurements

7. Metallic behavior observed in the normal-state(y≤3.00); while the semiconducting-like feature observed (y>3.00) • The observed results indicate that the normal-state transport properties are not intimately related with superconductivity in RbxWOy

8. Tc < 2K Superconductivity suppressed region Tc ~ 3K Superconductivity uniform region Tc > 3K Superconductivity enhanced region

9. Discussions 1.The lower superconducting transition temperature accompanied by the sharper superconducting transition width is consistently observed in RbxWOy with lower oxygen concentrations regardless of the value of x, suggesting that superconductivity observed in these compounds might have two different origins. 2. The electron-phonon interaction weakened by the screening effect of the carriers is certainly not able to explain the observed phenomenon. 3.The ordering of the rubidium atoms induced by the oxygen vacancy site in the WO6 matrix may play an important role in the superconductivity present in the low oxygen concentration regime, we speculate that the ordering of the rubidium atoms would be greatly influenced by the oxygen atoms sliding into the tunnels. This in turn would induced a local structure distortion and give rise to distinct superconducting properties.