We have grown cubic centimetre-size crystals of YBa2Cu3Ox suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y2BaCuO5 and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y2BaCuO5 concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x=6.5, the crystals were detwinned and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaics volume was in the majority orthorhomic domain. The neutron (0,0,6) and (1,1,0) rocking curve widths were ~1 degree per crystal and ~2.2 degrees for the mosaic, and the oxygen chain correlation lengths were >100 A in the a- and b-directions and ~50 A in the c-direction.
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