STRUCTURAL-CHANGES AND REDISTRIBUTION OF HOLES IN CA-DOPED YBA2CU4O8

The crystal structure of (Y1-xCax)Ba2Cu4O8 at room temperature has bee n refined by Rietveld analysis of time-of-flight (TOF) neutron powder diffraction data. The largest changes upon Ca doping involve the O(2) and O(3) ions in the Cu(2)O-2 plane which move towards the Pa ion, whi le the Cu(2)-Cu(2) plane distance decreases. The complex magnetic susc eptibility measurements have shown that the superconducting transition temperature, T-c, increases from 81.3 K for x=0.0 to 89.9 K for x=0.1 . Calculation of Madelung energies based upon the structural data has shown that the number of holes in the conducting Cu(2)O-2 plane increa ses with increasing Ca content, which is qualitatively consistent with the observed increase in T-c.