A simple method for the normal pressure synthesis of Cu1-xTlxBa2Ca3Cu4O12-delta superconductor

摘要

A very simple method for the direct synthesis of Cu1-xTlxBa2Ca3Cu4O12-delta (Cu1-xTlx-1234) superconductor is reported. A predominant single phase Cu1-xTlx-1234 is achieved by the solid state reaction of Ba(NO3)(2), CaCO3 and Cu(CN) at first stage and with appropriate amount of Tl2O3 at the second stage. These materials were fired twice for 24 h at 880 degrees C followed by intermediate grinding. Thoroughly ground and fired material was mixed with Tl2O3 in appropriate amount and pellets were made by applying a pressure of 5 tons. Pellets were wrapped in an aluminum foil and sintered at 880 degrees C for 3-30 min. A predominantly single phase of Cu1-xTlxBa2Ca3Cu4O12-delta was achieved with an inclusion of unknown impurity. The resistivity measurements established the onset temperature of superconductivity [T-c (onset)] at 120 K and zero resistivity critical temperature [T-c(R = 0)] at 106 K. Bulk superconductivity was also confirmed by ac-magnetic susceptibility measurements. The surface of the samples was analyzed by electron microscopy and their composition by energy dispersive X-ray spectroscopy (EDX) measurements. For optimizing the carriers in the final compound, post-annealing experiments were carried out in the flowing nitrogen atmosphere. For the study of the doping mechanisms, in the post-annealed samples, the phonon modes of Cu1-xTlx-1234 were investigated by infrared absorption measurements. The absorption mode of O-delta [O3] atoms decreased in intensity after the post-annealing and the apical oxygen modes were softened to lower wave numbers. The softening of these phonon modes suggested the reduction of thallium from Tl3+ to Tl1+; which resulted in an increase of [T-c(R = 0)] of the material. (c) 2005 Elsevier B.V. All rights reserved.