Partial substitution of 18O in YBa2Cu3O7: Investigations of inhomogeneities and their effect on Tc

Abstract

The possibility of site-selective substitution of ¹⁸O into YBa₂Cu₃O₇ was explored. Samples containing various quantities of ¹⁸O were prepared by processing in ¹⁸O₂ both at 950 °C and at 400 °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 ¹⁸O substitution and of the ratios of ¹⁸O to ¹⁶O by TPD, TPR, and SIMS show that even for temperatures as low as 400 °C and times as short as 2 h. ¹⁸O is not substituted exclusively into the chain site (O1) in YBa₂Cu₃O₇. In addition, there is no consistent variation in the shifts in T_c with the degree of substitution; therefore, the isotope effect for a sample with 100% ¹⁸O cannot be predicted by a linear extrapolation of data obtained for samples with partial ¹⁸O substitution. The mechanism of oxygen substitution, the difficulties of measuring the true magnitude of the oxygen isotope shift, and the meaning of the small isotope shift are discussed.