Sodium 'stuffed' Ba2-xSrxFe4O8 ferrites: new cationic conductors

摘要

Sodium insertion in the tetrahedral layer structure of the ferrites Ba2-xSrxFe4O8 was performed by solid state reaction at 1220 K in air. Superstoichiometric oxides with the actual formula (Ba2-xSrx)(1-y/4)NayFe4O8-y less than or equal to 0.56; 0.60 less than or equal to Ba/Sr less than or equal to 1.67-were characterized by X-ray and neutron powder diffraction. The hexagonal unit-cell volume shows an increasing dependence on the sodium insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter is the likely signature of the location of the inserted sodium cations within the interlayer space. One-half of the sodium cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. ac conductivity measurements point to a cationic conductivity whose thermally activated regime-E-a 0.7 eV-evidenced from 570 K, is unsensitive to the sodium content. The bottleneck of the 2D sodium mobility regards the crossing of the oxygen triangular faces shared by the different polyhedra within the interlayer space. (C) 2002 Elsevier Science (USA). All rights reserved. [References: 9]