A germanoferrite with the real stuffed tridymite type structure: K0.75Ba0.25Fe1.25Ge0.75O4

Abstract

The new oxide K_0.75Ba_0.25Fe_1.25Ge_0.75O₄ has been prepared at 1000 °C in air. This germanoferrite exhibits the real tridymite type structure—hexagonal cell, a = 5.38 Å, c = 8.92 Å—which is fully stuffed by K⁺ and Ba²⁺ cations. A combined X-ray and neutron powder diffraction Rietveld analysis gives evidence for a large off-centering of the bridging oxygen atoms along the c axis. This is likely to be connected with the absence of long range ordering of the tetrahedral and stuffed cations, as well. An analysis of the directional sequences of the hexagonal rings in the parent oxides KFeGeO₄ and BaFe₂O₄ allows an understanding of why the real tridymite type is retained for a composition much closer to KFeGeO₄ than BaFe₂O₄. More generally, it is proved that the stuffed tridymite-like oxides show a dependence of the expansion of their tetrahedral framework on their directedness: the larger the directedness, the lower the expansion of the structure.