The oxygen isotope effect in Pr, Ca, and Zn substituted YBa 2Cu 3O 7-δ and EuBa 2Cu 3O 7-δ

The oxygen isotope effect α was studied for Pr and Ca substituted YBCO, and for Zn substituted EuBCO and YBCO, α increases to values near 0.5 for the highest Pr concentrations, and lowest transition temperatures. In the Zn substituted system α remains small to the lowest transition temperatures. It is shown that the isotope exponent is a function of mobile hole concentration, it decreases with increasing hole concentration and remains low for over-doping.