Anomalous transport properties in a YBa2Cu4O8 crystal

Resistivity rho(T) and thermoelectric power alpha(T) have been measured under hydrostatic pressure on a 1% Ni-doped YBa2Cu4O8 crystal gown under 600 bars O-2 pressure. The Ni are shown to substitute predominantly for the chain Cu and to Suppress the chain superconductivity. A giant dT(c)/dP=1.2K/kbar and d alpha(300 K)/dP=0.2 mu V/K kbar are interpreted to signal a reduction of Ni(III) to N(II) in the chains by the application of pressure. The temperature of the transition in rho(a)(T) and rho(c)(T) remains near 170 K independent of significant changes in the charge distribution between chains and bilayers, consistent with a structure-induced transition rather than the opening of a spin gap.