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 NdBa2Cu3Ox (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 Nd6Ba9.78Sr0.22Cu2Al6O30, 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.