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 Nd 4 Ba 2 Cu 2 O 10 (Nd-422) bar was supported on two NdBa 2 Cu 3 O 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 Nd 6 Ba 9.78 Sr 0.22 Cu 2 Al 6 O 30, has been determined by single-crystal X-ray diffraction. The space group is P 6 3 mc; 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.