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

Bi can be replaced by Pb in the phases Bi2Sr2MOy (M = Co, Mn, Fe), which are isostructural to the 10 K superconductor Bi2Sr2CuOy but have a transition metal M substituted for Cu. New non-superconducting phases with the formula BiPbA2MO6 (A = Sr, Ca; M = Fe, Co and Mn) were prepared in a reducing atmosphere. Single crystals of these phases were grown using excess PbO-Bi2O3 as a flux, and their structural, magnetic and electrical properties were studied. The structure of BiPbSr2MnO6 was solved by single crystal X-ray diffraction studies, and show no modulation, in contrast to Bi2Sr2MOy, 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. Mössbauer 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 Bi2Sr2MOy. The modulation can be introduced reversibly in BiPbSr2MnO6 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 Bi2Sr2MOy, the peaks in magnetic susceptibility are weaker, probably because there is no modulation. © 1990.