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Crystal structure analysis of synthetic Ca₄Fe_1.5Al_17.67O₃₂: A high-pressure, spinel-related phase

¹ Fachbereich Geowissenschaften (Kristallographie), Universität Bremen, Klagenfurter Str., D-28359 Bremen, Germany
² Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
³ Center for High Pressure Research, Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, U.S.A.

Correspondence: ^* E-mail: vkahlen{at}zfn.uni-bremen.de

The compound Ca₄Fe_1.5Al_17.67O₃₂ 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 I2d, a = 20.1847(14) Å, c = 5.6203(6) Å, V = 2289.83(3) ų, 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 x 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 -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, Ca₄Fe_1.5Al_17.67O₃₂ 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.