Substitution of Co3+ in YBa2Fe3O8

Abstract The accommodation of Co in the oxygen-saturated solid-solution phase YBa 2 (Fe 1− z Co z ) 3 O 8+ w has been investigated by powder X-ray and neutron diffraction techniques, as well as by 57 Fe Mossbauer spectroscopy. Of the nominal composition range 0.00⩽ z ⩽1.00 tested, the solid-solution limit under syntheses at 950°C in 1 bar O 2 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 Co 3+ replaces Fe 3+ . Despite the nearly constant value of w , Mossbauer 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 Fe 3+ and Co 3+ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe 3+ by the smaller Co 3+ ion. A contrast is pointed out with the substitution of cobalt in YBa 2 Cu 3 O 7 where it is a larger Co 2+ ion that replaces smaller Cu 2+ .