Structure of YSr(2)Cu(3-x)M(x)O(7+delta) (M=Mo,W and Re) from single-crystal X-ray and powder-neutron diffraction. Substituent site preference and oxygen defect structure

Unlike YBa2Cu3O7, the Sr analog YSr2Cu3O7 cannot be made at ambient pressures without partial substitution of some small, higher-oxidation state ions for the Cu. We have prepared doped YSr(2)Cu(3-x)M(x)O(7+delta) (M = Mo, W and Re) samples with x values in the range 0.1 to 0.3 and with superconducting transition temperatures up to 50 K. We have determined the structures of YSr2Cu2.82Mo0.18O7.25 and YSr2Cu2.77Re0.23O7.26 using X-ray single-crystal diffraction and of YSr(2)Cu(3-x)M(x)O(7+delta)(M = Mo, W and Re) using neutron powder diffraction. We find that the M(+6) ions substitute preferentially for the chain Cu+2 ions and that they substantially disrupt the Cu-O chain structure. The substitution of M(+6) also increases the oxygen stoichiometry and leads to an increase in the effective oxidation state of the Cu ions. This increase in oxidation state plays the very important dual role of stabilizing the crystal structure with the smaller Cu+3 ions and of turning YSr(2)Cu(3-x)M(x)O(7+delta) into superconductor.