Abstract

The compound Ca4Fe1.5Al17.67O32 was synthesized at 2.5 GPa and 1250 °C using a piston-cylinder apparatus. The crystal structure, determined from single-crystal X-ray diffraction data collected at 295 K (tetragonal, space group I4̅2d, a = 20.1847(14) Å, c = 5.6203(6) Å, V = 2289.83(3) Å3, Z = 4) was refined to a final R-index of 0.024 for 1229 independent observed reflections and 130 parameters. The main building units comprising the compound are 11.8 × 7.2 Å wide spinel-type ribbons running parallel to [001], which are connected via corner-sharing (Fe,Al)O6-octahedra. Additional linkage between the spinel units is provided by AlO4-tetrahedra residing on the 4̅-axis as well as by Ca cations, in sevenfold coordination with oxygen attached to the spinel-like building units. Refinement of site occupancies reveals that the incorporation of Fe occurs at two of four octahedral and at two of three tetrahedral sites. Apparently, Ca4Fe1.5Al17.67O32 represents a new structure type exhibiting more pronounced structural modifications relative to the spinel aristotype compared with the so-called spinelloid structures. Furthermore, this synthetic compound contains Fe2+ in tetrahedral coordination, which is relatively uncommon among inorganic materials.

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