STRUCTURE AND SUPERCONDUCTIVITY IN Y(1-X)LN(X)BA(2-Y)SR(Y)CU(4)O(8) (LN=LU, TB, GD, PR AND CE) - THE INFLUENCE OF IONIC SIZE AND VALENCY

The structural changes occurring upon substitution of several lanthani des (Ln) for Y in the high-T-c superconductor YBa2Cu4O8 have been stud ied by neutron powder diffraction in order to establish the influence of the size and valency of the dopant ions. A series of Lu and Gd dope d Y-124 samples was prepared and the phase stability determined. Simul taneous substitution of Sr for Pa was observed to increase substantial ly the content of Lu that could be introduced into the structure. The effects of varying the size and valency of the ion at the Y site were studied by partial substitution of Tb, Pr and Ce for Y. The bond lengt h changes obtained in this study indicate that while Tb is trivalent, Ce is tetravalent and Pr may have an intermediate valency or take part in hybridization. Measurements of the superconducting transition temp erature showed that T-c decreases with increasing ionic radius and uni t cell volume in the series of Lu and Gd doped Y-124 samples. A drasti c depression of T-c is observed for Ce doped Y-124 due to hole filling , that does not seem to be compensated by charge transfer from the dou ble CuO chains.