Substitution of Co³⁺ in YBa₂Fe₃O₈

The accommodation of Co in the oxygen-saturated solid-solution phase YBa₂(Fe_1-zCo_z)₃O_8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by ⁵⁷Fe Mössbauer spectroscopy. Of the nominal composition range 0.00≤z≤1.00 tested, the solid-solution limit under syntheses at 950°C in 1 barO₂ is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co³⁺ replaces Fe³⁺. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe³⁺ and Co³⁺ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe³⁺ by the smaller Co³⁺ ion. A contrast is pointed out with the substitution of cobalt in YBa₂Cu₃O₇ where it is a larger Co²⁺ ion that replaces smaller Cu²⁺.