Growth and structural characterization of needlelike metastable crystals in the Nd-Ba-Cu-(Al)-O system

An unconventional technique was used to grow crystals in the Nd-Ba-Cu-O system. A Nd₄Ba₂-Cu₂O₁₀ (Nd-422) bar was supported on two NdBa₂Cu₃O_x (Nd-123) pellets in an alumina crucible and was heated in a horizontal furnace at a higher temperature than the peritectic decomposition point of Nd-422. Brown Al-doped Nd-Ba-Cu-O needlelike crystals belonging to a new phase were grown directly on the crucible. The structure of the new phase, whose composition is Nd₆Ba_9.78Sr_0.22Cu₂Al ₆O₃₀, has been determined by single-crystal X-ray diffraction. The space group is P6₃mc; the unit cell parameters are a = 11.526(2) Å, c = 6.963(1) Å, and Z = 1. A full matrix least-squares refinement yielded R(F) = 0.0405 for 3131 independent reflections. This new phase is a metastable polymorphic modification of Nd-422 solid solution. The preferential directional growth of the needles lies along the c-axis. Many needles are affected by twinning by merohedry, with the twinning plane parallel to the (001) crystallographic plane.