Structural and transport properties of (Bi,Pb)4Sr3Ca3Cu4−mFemOx (m=0–0.06) glasses: Precursors for high Tc superconductors

Abstract

Microstructure and transport properties (between 80–450 K) of the Fe doped as-quenched Bi3Pb1Sr3Ca3Cu4−mFemOx (m=0.0, 0.02, 0.04, 0.05, and 0.06) type multicomponent glassy precursors for high-temperature superconductors were first reported in this paper. It has been shown from x-ray diffraction, scanning electron microscopy, thermal analysis, density, and oxygen molar volume measurements that single phase homogeneous glasses are formed. Even with small addition of Fe, a large change of glass transition temperature (Tg) indicates appreciable change of glass network structure of the undoped (Bi,Pb)4Sr3Ca3Cu4Ox glass which is reflected in the properties of the corresponding high-temperature annealed glass termed as glass-ceramic (GC) samples. These GC samples are found to be superconductors. Addition of Fe is considered to break up some of the original Cu–O–Cu bonds forming some new Cu–O–Fe and Fe–O–Fe type bonds. A nonlinearity (increase) in the dc conductivity (σdc) of the as-quenched glass sample is observed around m=0.02. This increase of dc conductivity is ascribed to be due to the additional hopping between Fe2+ and Fe3+ which finally destroys superconductivity for m⩾0.06. This is also responsible for the nonlinear variation of activation energy of the Fe doped glass and hence change in the superconducting properties of the glass ceramics. The high temperature (above θD/2, θD being the Debye temperature) σdc data of the glasses were explained by considering small polaron hopping conduction mechanism. The dc conductivity of these glasses are found to follow Greaves’ variable range hopping model in the intermediate range of temperature. The polaron hopping model of Schnakenberg and Emin can predict the conductivity data in the high-temperature regime. All the Fe-doped glass ceramics showed superconducting transitions with Tc between 100 and 110 K. The corresponding zero resistance temperature, Tco which is maximum (79 K) for m=0, decreases consistently with increase of Fe-concentration and finally superconducting behavior is destroyed for m⩾0.06. This behavior is consistent with-pair breaking mechanism.