Synthesis, crystal structure, and superconductivity of Ba (Bi0.25Pb0.75)(1-x)MgxO3-delta

Magnesium has been doped into the B-site of BaBi0.25Pb0.75O3-delta to form a solid solution Ba (Bi0.25Pb0.75)(1-x)MgxO3-delta (0 <= x <= 0.10) by solid-state reaction. The X-ray, neutron, and selected area electron diffraction are used to confirm that this solid solution crystallizes in triclinic space group P1. The unit cell volume of Ba(Bi0.25Pb0.75)(1-x)MgxO3-delta increases with Mg, which is surprising because the radius of Mg2+ is smaller than any radius of Bi3+, Bi5+, Pb2+, and Pb4+. XPS and iodometric titration data of samples show that the oxygen vacancies increase with Mg in Ba(Bi0.25Pb0.75)(1-x)MgxO3-delta. The increased oxygen vacancies result in increase of Bi3+:Bi5+ and Pb2+:Pb4+ ratios, which is the utmost possible reason for unit cell volume increase. They are all superconductors with T-C(zero) (the highest temperature at which the electrical resistivity becomes "zero") between 11.2 and 9.2 K.