Effect of addition of chromium(VI) oxide on the formation of the Tl–Sr–Ca–Cu–O 1212 phase

Abstract

It has been reported that the addition of chromium(VI) oxide to Tl–Sr–Ca–Cu–O precursor mixtures allows the formation of a phase of composition TlSr₂Ca_1–xCr_xCu₂O_{7 +δ}. This appears to form in significantly higher proportion than the parent, unsubstituted compound. Clearly, a greater understanding of the way in which this happens is of interest for effective processing and application of these materials. In this paper, we present preliminary findings of a study of the consequences of chromium oxide addition. This has been achieved by examining the products of reactions carried out at the same temperature, but with differing quench rates. These are studied primarily by powder X-ray diffraction and electron microscopy. We propose that the addition of chromium oxide acts in two ways: first, it alters the reaction mechanism by changing the composition of the molten precursor, and, secondly, it lowers the free energy of the superconducting phase.