Partial substitution of /sup 18/O in YBa/sub 2/Cu/sub 3/O/sub 7/: Investigations of inhomogeneities and their effect on /ital T//sub c/

The possibility of site-selective substitution of /sup 18/O into YBa/sub 2/Cu/sub 3/O/sub 7/ was explored. Samples containing various quantities of /sup 18/O were prepared by processing in /sup 18/O/sub 2/ both at 950/degree/C and at 400/degree/C. The samples were characterized by secondary ion mass spectroscopy (SIMS), temperature programmed desorption (TPD) and reduction (TPR), Raman spectroscopy, and magnetization measurements. Measurements of the shifts in the Raman active modes with /sup 18/O substitution and of the ratios of /sup 18/O to /sup 16/O by TPD, TPR, and SIMS show that even for temperatures as low as 400 /degree/C and times as short as 2 h, /sup 18/O is not substituted exclusively into the chain site (O1) in YBa/sub 2/Cu/sub 3/O/sub 7/. In addition, there is not consistent variation in the shifts in /ital T//sub c/ with the degree of substitution; therefore, the isotope effect for a sample with 100% /sup 18/O cannot be predicted by a linear extrapolation of data obtained for samples with partial /sup 18/O substitution. The mechanism of oxygen substitution, the difficulities of measuring the true magnitude of the oxygen isotope shift, and the meaning of the small isotope shift are discussed.