Synthesis, magnetic properties and Mössbauer spectroscopy for the pyrochlore family Bi2BB′O7 with B=Cr and Fe and B′=Nb, Ta and Sb

The samples Bi2BB′O7, with B=Cr and Fe and B′=Nb, Ta and Sb were prepared by solid state method. The crystallographic structure was investigated on the basis of X-ray powder diffraction data. Rietveld refinements show that the crystal structure is cubic, space group Fd-3m. The Bi3+ cation on the eight-coordinate pyrochlore A-site shows displacive disorder, as a consequence of its lone pair electron configuration. There is also a considerable A-site disorder shown by Rietveld Analysis and confirmed in the case of the iron containing samples with Mössbauer spectroscopy. The magnetic measurements show paramagnetic behavior at all temperatures for the Cr oxides. The Fe pyrochlores show antiferromagnetic order around 10 K.