Sodium insertion in the tetrahedral layer structure of the ferrites Ba2-xSrxFe4O8 was performed by solid state reaction at 1220K in air. Super-stoichiometric oxides with the actual formula (Ba2-xSrx)1-y/4NayFe4O8, having 0.60 ≤ Ba/Sr ≤ 1.67, were characterized by using X-ray and neutron powder diffraction. The hexagonal unit-cell volume exhibited an increasing dependence on the Na insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter was the likely signature of the location of the inserted Na cations within the interlayer space. One-half of the Na cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. alternating-current conductivity measurements point to a cationic conductivity whose thermally activated regime––Ea # 0.7eV––evidenced from 570K, was insensitive to the Na content. The bottle-neck of the 2D sodium mobility regards the crossing of the O triangular faces shared by the different polyhedra within the interlayer space.

Sodium “Stuffed” Ba2-xSrxFe4O8 Ferrites - New Cationic Conductors. J.Choisnet, V.Caignaert, B.Raveau: Journal of Solid State Chemistry, 2003, 170[2], 435-42