Mössbauer study of cubic pyrochlores,Bi2?xYxFeSbO7

The solid solutions Bi_2?xY_xFeSbO₇ (x = 0,1,2) which are cubic pyrochlores of the type A₂B₂O₇ have been prepared for the first time and the lattice parameters determined. The ⁵⁷Fe Mössbauer spectra have been recorded both in the paramagnetic and magnetically ordered states. From the values of the Mössbauer parameters it is found that Fe ions are in the high spin trivalent state. The large value for the quadrupole splitting of these compounds is attributed to the highly distorted BO₆ octahedron in these compounds. The quadrupole splitting decreases with increase of Bi concentration and therefore, the BO₆ octahedron is found to have the lowest distortion for the composition Bi₂FeSbO₇. The magnetic transition temperature and the value of the magnetic hyperfine fields have been determined. The distribution of Fe³⁺ and Sb⁵⁺ ions in B sites gives rise to the distribution in magnetic hyperfine fields.