Abstract
Crystal structure of NH 4Fe IIFe IIIF 6 is studied in order to explain further its peculiar antiferromagnetic behavior compared to the spin glass one of the pyrochlore family. NH 4Fe IIFe IIIF 6 is orthorhombic, space group Pnma with a = 7.045 (4) A ̊ , b = 7.454 (4) A ̊ , c = 10.116 (6) A ̊ , Z = 4. Diffraction data on single crystals obtained by hydrothermal synthesis, collected on an automatic four circle diffractometer, have been refined by full matrix least-squares calculations to a weighted value of 0.029 (unweighted R = 0.024) for 798 observed reflections. This structure is derived from the pyrochlore structure, with a cationic order between Fe 2+ and Fe 3+ ions. (Fe IIF 6) 4− octahedra form infinite trans chains along [100] by sharing corners although similar chains of (Fe IIIF 6) 3−) octahedra lie along [010]. This type of Fe IIFe III order is related to a similar one existing in Fe 2F 5 · 2H 2O, the dehydration of which leads to the pyrochlore Fe 2F 5 · H 2O. A mechanism is proposed to explain the formation of this compound.
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