STRUCTURAL AND TRANSPORT-PROPERTIES OF (BI,PB)(4)SR3CA3CU4-MFEMOX (M=0-0.06) GLASSES - PRECURSORS FOR HIGH T-C SUPERCONDUCTORS

Microstructure and transport properties (between 80-450 K) of the Fe d oped as-quenched Bi3Pb1Sr3Ca3Cu4-mFemOx (m=0.0, 0.02, 0.04, 0.05, and 0.06) type multicomponent glassy precursors for high-temperature super conductors were first reported in this paper. It has been shown from x -ray diffraction, scanning electron microscopy, thermal analysis, dens ity, and oxygen molar volume measurements that single phase homogeneou s glasses are formed. Even with small addition of Fe, a large change o f glass transition temperature (T-g) indicates appreciable change of g lass network structure of the undoped (Bi,Pt)(4)Sr3Ca3Cu4Ox glass whic h 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 brea k up some of the original Cu-O-Cu bonds forming some new Cu-O-Fe and F e-O-Fe type bonds. A nonlinearity (increase) in the de conductivity si gma(dc) of the as-quenched glass sample is observed around m = 0.02. T his increase of de conductivity is ascribed to be due to the additiona l hopping between Fe2+ and Fe3+ which finally destroys superconductivi ty for m greater than or equal to 0.06. This is also responsible for t he 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 theta(D)/2, theta(D) being the Debye tempe rature) sigma(dc), data of the glasses were explained by considering s mall polaron hopping conduction mechanism. The de conductivity of thes e glasses are found to follow Greaves' variable range hopping model in the intermediate range of temperature. The polaron hopping model of S chnakenberg and Emin can predict the conductivity data in the high-tem perature regime. All the Fe-doped glass ceramics showed superconductin g transitions with T-c between 100 and 110 K. The corresponding zero r esistance temperature, T-oo which is maximum (79 K) for m = 0, decreas es consistently with increase of Fe-concentration and finally supercon ducting behavior is destroyed for m greater than or equal to 0.06. Thi s behavior is consistent with-pair breaking mechanism. (C) 1998 Americ an Institute of Physics.