Oxygen isotope effect in YBa/sub 2/Cu/sub 3/O/sub 7/ prepared by burning YBa/sub 2/Cu/sub 3/ in /sup 16/O and /sup 18/O

We prepared YBa/sub 2/Cu/sub 3/ powder by ball milling a 2:1 molar mixture of the intermetallics BaCu and CuY. We synthesized YBa/sub 2/Cu/sub 3/(/sup 16/O)/sub 7-//sub x/ and YBa/sub 2/Cu/sub 3/(/sup 18/O)/sub 7-//sub x/ by oxidizing the YBa/sub 2/Cu/sub 3/ powder in /sup 16/O and /sup 18/O. The /sup 16/O//sup 18/O ratios were determined by laser-ionization and sputtering-ionization mass spectroscopy. The YBa/sub 2/Cu/sub 3/(/sup 16/0)/sub 7-//sub x/ sample had 99.8 at. %/sup 16/O, and the YBa/sub 2/Cu/sub 3/(/sup 18/O)/sub 7-//sub x/ sample had 96.5 at. %/sup 18/O. Susceptibility measurements of the superconducting transition temperature (T/sub c/) = 91.7 K for /sup 16/O; half-point transition at 84 K show an isotope effect of 0.4 +- 0.1 K.