Chemical pressure for optimizing T-c in a given superconducting system

The application of external pressure to cuprate superconductors induces Inc reases in T-c. The same cannot be stated for the chemical pressure, applied through substituting large cations with smaller ones, for example, Sr for Ba. Both types of pressure induce T-c increases in only one system among di e cuprates, namely La2-x(Ba1-ySry)(x)CuO4. For this system, T-c increases e ither for increasing y or for increasing the external pressure. By studying the crystal structure and the superconducting properties of several sample s in the system Y(Ba1-xSrx)(2)Cu3Ow as a function of x and w, we show that in this system the substitution of Sr for Ba reduces the unit cell volume a s does the external pressure. However, it also reduces the strain of the Ba /Sr layers that seem to be needed for optimizing the charge transfer betwee n the chain and planar Cu cations. The strain of the Ba layer seems to scal e well with T-c, at least in the YBCO system. (C) 2000 Elsevier Science B.V . All rights reserved.