Fluorination of Bi1.8Fe1.2SbO7 pyrochlore solid solutions

Abstract

A technique has been developed for fluorinating the pyrochlore oxide Bi_1.8Fe_0.2FeSbO₇, and a compound with the composition Bi_1.8Fe_1.2SbO_7–x/2F_x has been obtained. The synthesized oxyfluoride also has the pyrochlore structure (sp. gr. Fd3m), with a lattice parameter a = 10.4443(1) Å (R_wp = 5.2). It has been shown that the charge balance upon fluorine substitution for oxygen is maintained not through partial reduction of Fe³⁺ to Fe²⁺ but through the incorporation of fluorine into oxygen vacancies. The magnetic behavior of the fluorinated pyrochlore phase is determined by the persisting frustration of the octahedral sublattice, which is responsible for the development of a spin glass state below T_f = 12 K. The fluorination-induced changes in the anion sublattice led to an increase in the antiferromagnetic exchange interaction between neighboring Fe³⁺ ions and changes in the dynamic properties of the spin glass phase.