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

A system formed by a bar of Y2BaCuO5 (Y211) phase supported on two YBa2Cu3Ox (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 BaCuO2. 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.