Abstract

The new oxides Ba_2−xSr_xFe₄O₈ have been prepared at 1100 °C in air. XRD characterization gives evidence for a rather wide homogeneity range (0.8 ⩽ x ⩽ 1.3) of hexagonal phases isotypical with the tetrahedral ferrite BaSrFe₄O₈. The stuffed tridymite-like structure of these oxides consists of large tetrahedral double layers connected by octahedra. The formation of the solid solution is based on the existence of a partial disorder (18%) of the distribution of the Ba²⁺ and Sr²⁺ cations between the centers of the crowns—twelvefold coordination—and the octahedra. Surprisingly, the Ba²⁺ cations partly occupy the octahedra whatever the composition x, up to a value close to 34% for the Ba-rich limit. The occurrence of such a partial disorder, which also exists in the solid solution BaSr_1−xCa_xFe₄O₈, seems to be connected to a cooperative rotation of the (FeO₄) tetrahedra resulting in a lengthening of the metal–oxygen distances in the octahedra.