The crystal structure of (Y1-xCax)Ba2Cu4O8 at room temperature has been 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)O2 plane which move towards the Ba ion, while the Cu(2)-Cu(2) plane distance decreases. The complex magnetic susceptibility measurements have shown that the superconducting transition temperature, Tc 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)O2 plane increases with increasing Ca content, which is qualitatively consistent with the observed increase in Tc.
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