NEUTRON-DIFFRACTION ANALYSIS OF CA-44 AND CA SUBSTITUTED SUPERCONDUCTORS YBA2CU4O8 WITH TC-APPROXIMATE-TO-90-K

Low-temperature neutron powder diffraction investigations performed on YBa2Cu4O8 ceramic samples, with T(c) increased to almost-equal-to 90 K by Ca-44 and Ca substitution, show that the Ca is distributed on the Y sites rather than the Ba sites, according to the chemical formula Y1-xCaxBa2Cu4O8. Use of the isotope Ca-44 (scattering length b=1.42 F) increased the contrast with Y (b=7.75 F) and Ba (b=5.07 F). Associated changes of the structure are discussed. In particular interatomic distances Cu(2)-O(1) and Cu(2)-O(2,3) appear to decrease with increasing Ca doping.