New non-superconducting modulation-free BiPbSr2MOy phases (M = Co, Mn, Fe) isotypic with the 10 K Bi2Sr2CuOy superconductor

Abstract Bi can be replaced by Pb in the phases Bi 2 Sr 2 MO y (M = Co, Mn, Fe), which are isostructural to the 10 K superconductor Bi 2 Sr 2 CuO y but have a transition metal M substituted for Cu. New non-superconducting phases with the formula BiPbA 2 MO 6 (A = Sr, Ca; M = Fe, Co and Mn) were prepared in a reducing atmosphere. Single crystals of these phases were grown using excess PbO-Bi 2 O 3 as a flux, and their structural, magnetic and electrical properties were studied. The structure of BiPbSr 2 MnO 6 was solved by single crystal X-ray diffraction studies, and show no modulation, in contrast to Bi 2 Sr 2 MO y , where the structure is strongly modulated by a bending of the perovskite slabs. In the (Pb, Bi)O layers, the (Pb, Bi) and O atoms form ribbons along the a -axis. There are only six oxygens per formula, so the Mn is in the 3+ oxidation state. Mossbauer measurements for M = Fe confirm this oxidation state. The lack of an extra oxygen in the (Pb, Bi)O layers is consistent with the lack of the modulation, since extra oxygen accompanies the structural modulation in Bi 2 Sr 2 MO y . The modulation can be introduced reversibly in BiPbSr 2 MnO 6 by heating it under oxygen near 500°C, directly showing for the first time the correlation between oxygen and the modulation. All the Pb substituted phases are insulating, and some show magnetic phase transitions. Although the magnetic transition temperatures are comparable to those in Bi 2 Sr 2 MO y , the peaks in magnetic susceptibility are weaker, probably because there is no modulation.