Crystal structure analysis of synthetic Ca4Fe1.5Al17.67O32: A high-pressure, spinel-related phase

The compound Ca4Fe1.5Al17.67O32 was synthesized at 2.5 GPa and 1250 degrees C using a piston-cylinder apparatus. The crystal structure, determined from single-crystal X-ray diffraction data collected at 295 K (tetragonal, spac e group I (4) over bar 2d, a = 20.1947(14) Angstrom, c = 5.6203(6) Angstrom , V = 2289.83(3) Angstrom (3), Z = 4) was refined to a final R-index of 0.0 24 for 1229 independent observed reflections and 130 parameters. The main b uilding units comprising the compound are 11.8 x 7.2 Angstrom wide spinel-t ype ribbons running parallel to [001], which are connected via corner-shari ng (Fe,Al)O-6-octahedra. Additional linkage between the spinel units is pro vided by AlO4-tetrahedra residing on the (4) over bar -axis as well as by C a cations, in sevenfold coordination with oxygen attached to the spinel-lik e building units. Refinement of site occupancies reveals that the incorpora tion of Fe occurs at two of four octahedral and at two of three tetrahedral sites. Apparently, Ca4Fe1.5Al17.67O32 represents a new structure type exhi biting more pronounced structural modifications relative to the spinel aris totype compared with the so-called spinelloid structures. Furthermore, this synthetic compound contains Fell in tetrahedral coordination, which is rel atively uncommon among inorganic materials.