THE INTERMEDIATE PHASE DURING (BI,PB)(2)SR2CACU2O8+DELTA TO (BI,PB)(2)SR2CA2CU3O10+DELTA PHASE-TRANSFORMATION

We report our observations on and identification of an intermediate phase, (Bi,Pb)(4)Sr4Ca3Cu5O18 (+) (delta) (denoted as Bi/4435), during the phase transformation of (Bi,Pb)(2)Sr2CaCu2O8+delta (Bi/2212) to (Bi,Pb)(2)Sr2Ca2Cu3O10+delta (Bi/2223) in powder-in-tube processed tapes. The unit cell of the phase consists of one half each of the Bi/2212 and the Bi/2223 cell, with lattice parameters of a approximate to b = 5.4 Angstrom and c = 33.9 Angstrom. The space group of the new phase was determined to be Pbmn using the extinction rule of the selected-area electron diffraction, as well as convergent beam electron diffraction and high-resolution electron microscopy. The presence of the Bi/4435 phase and the abundance of its local structure during the early stage of the transformation suggests that the Bi/2223 phase is formed by the rapid insertion of two extra Ca/CuO bi-layers, one at a time, into the two identical CuO/Ca/CuO building blocks of the Bi/2212 matrix. This finding may shed light on the mechanism of structural phase transformation in layered cuprate superconductors.