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High-pressure Ca₄Al₆O₁₃: An example of a calcium aluminate with three different types of coordination polyhedra for aluminum

¹ Fachbereich Geowissenschaften (Kristallographie), Universität Bremen, Klagenfurter Strasse, D-28359 Bremen, Germany
² Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794, U.S.A.
³ Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

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

The crystal structure of tetracalcium trialuminate (Ca₄Al₆O₁₃), synthesized at 1250 °C and 2.5 GPa, has been determined from single-crystal X-ray data by direct methods [space group Pcan, Z = 8, a = 5.3002(2) Å, b = 17.7610(5), c = 21.0887(9) Å] and refined to R1 = 6.42%. The unit cell parameters of Ca₄Al₆O₁₃ exhibit a relationship to those of perovskite: , b 5 a _Pv. and . The diffraction data showed the typical features of a pseudotranslational symmetry: all reflections (hkl) with l equal 4n (n is an integer) had significantly higher intensity than the reflections with l 4n. Furthermore, diffuse streaks parallel to b* were observed. The new compound exhibits Al³⁺ in three different kinds of coordination polyhedra: octahedra, tetrahedra, and trigonal bipyramids. One of the two main building units is slightly corrugated sheets of perovskite-type corner sharing AlO₆ octahedra perpendicular to [010]. The octahedral sheets are connected by layers containing tetrahedral zweier single chains. Within these layers the tetrahedral chains are linked by two different kinds of rods containing distorted trigonal bipyramids sharing common corners and edges, respectively. The tetrahedral chains and the bipyramidal rods are parallel to [100]. Charge compensation is achieved by the Ca ions, which are coordinated by 9 or 10 oxygen cations.