A simple method for the normal pressure synthesis of Cu 1− xTl xBa 2Ca 3Cu 4O 12− δ superconductor

Abstract

A very simple method for the direct synthesis of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− δ (Cu 1− x Tl x -1234) superconductor is reported. A predominant single phase Cu 1− x Tl x -1234 is achieved by the solid state reaction of Ba(NO 3) 2, CaCO 3 and Cu(CN) at first stage and with appropriate amount of Tl 2O 3 at the second stage. These materials were fired twice for 24 h at 880 °C followed by intermediate grinding. Thoroughly ground and fired material was mixed with Tl 2O 3 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 °C for 3–30 min. A predominantly single phase of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− δ 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 Cu 1− x Tl x -1234 were investigated by infrared absorption measurements. The absorption mode of O δ [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 Tl 3+ to Tl 1+; which resulted in an increase of [ T c( R = 0)] of the material.