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 YBa₂Cu₃O₁ (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.