STRUCTURE OF YSR(2)CU(3-X)M(X)O(7-CRYSTAL X-RAY AND POWDER-NEUTRON DIFFRACTION - SUBSTITUENT SITE PREFERENCE AND OXYGEN DEFECT STRUCTURE(DELTA) (M=MO,W AND RE) FROM SINGLE)

Unlike YBa2Cu3O7, the Sr analog YSr2Cu3O7 cannot be made at ambient pr essures without partial substitution of some small, higher-oxidation s tate ions for the Cu. We have prepared doped YSr(2)Cu(3-x)M(x)O(7+delt a) (M = Mo, W and Re) samples with x values in the range 0.1 to 0.3 an d with superconducting transition temperatures up to 50 K. We have det ermined 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 th at the M(+6) ions substitute preferentially for the chain Cu+2 ions an d that they substantially disrupt the Cu-O chain structure. The substi tution of M(+6) also increases the oxygen stoichiometry and leads to a n increase in the effective oxidation state of the Cu ions. This incre ase in oxidation state plays the very important dual role of stabilizi ng the crystal structure with the smaller Cu+3 ions and of turning YSr (2)Cu(3-x)M(x)O(7+delta) into superconductor.