Growth and Structural Characterization of Needlelike Crystals in the Y−Ba−Cu−O System

Abstract

A system formed by a bar of Y 2 BaCuO 5 (Y211) phase supported on two YBa 2 Cu 3 O x (Y123) pellets was processed by thermal treatment at high temperature. The first effect was a change in chemical composition inside the bar, yielding new phases: mostly Y123 and BaCuO 2 . Their concentrations were higher near the bar ends. A second, very interesting result was the growth of two kinds of acicular crystals underneath the whole bar: green, transparent Y211 needles and needles with a green core of Y211, covered by a Y123 layer. Micro Raman and SEM analyses clearly identified the two different phases. XRD inspection has confirmed that Y211 crystals are high-quality single crystals. The preferential growth direction for Y211 needles is along the crystallographic b axis. A possible explanation, based on the VLS mechanism, for their growth is suggested.

Transition of Y2BaCuO5 phase in powder melting processed YBa2Cu3O7−x superconductors

“…Mele et al think that the preferential growth direction for acicular Y211 phase is along the crystallographic b axis [11,12] . In the melted texture YBCO samples, the formation of Y211 phases is through two ways: preexisting and recrystallization.…”