Raman spectra of YBa 2(Cu 1−xM x) 3O 7−δ with transition metal dopants of Fe, Co, and Zn

Abstract

Raman spectra of YBa 2(Cu 1− x M x ) 3O 7δ with 3d-metals substituting for Cu in the high- T c superconductor were studied systematically as a function of the doping concentration x. Four substituents Fe, Co, Ni, and Zn were investigated with concentrations ranging up to 12% for Fe and up to 9% for the others. The 500 cm -1 axial symmetric vibration of O(4) along the c-axis shows softening for increasing Co and Fe doping concentration, while it is unchanged for Ni and Zn substitutions. The sensitivity of this mode to the local changes in the chains is discussed. The 435 cm -1 in-phase and bond bending mode associated with out-of-plane vibration of O(2) and O(3) displays a general upward shift in frequency for Co, Fe and Zn. This mode is affected by the relative displacement of the Ba and Y atoms with respect to the CuO 2 plane brought about by the dopants, and therefore changes in the corresponding frequency do not necessarily indicate a perturbation in the Cu(2) site. Our Raman results are discussed on the basis of recent studies elucidating the local structures of similarly doped samples and the bond length changes brought about by doping, and in the context of the results of recent normal-coordinate calculations. We emphasize the interrelationship of charge distribution, bond length changes, Raman mode force constants and transition temperature, pointing out the similarity in the results between trivalent doping on the Cu(1)-O(1) chains and the creation of oxygen deficiency in the chains for the case of the pure material.