Structural studies on a stuffed framework high pressure polymorph of CaAl2O4

Abstract

The crystal structure of a high pressure polymorph of CaAl2O4 has been investigated using laboratory X-ray powder diffraction data. The compound was prepared from a ceramic precursor in a piston cylinder apparatus at 800 °C and 20 kbar. The quenchable so-called phase-II of monocalcium aluminate adopts the monoclinic space group P21/c (a = 7.97187(7) Å, b = 8.62844(7) Å, c = 10.26276(10) Å, β = 94.801(4)°, V = 703.44(1) Å3, Z = 4, D calc = 2.99 g/cm3). The main structural features of the high-P form are layers of AlO4-tetrahedra perpendicular to the a-axis. Stacking of the layers parallel to [100] results in a three-dimensional framework containing channels, where the calcium cations are accommodated for charge compensation. Individual sheets can be described as being built by condensation of ditrigonally-shaped six-membered rings (S6R). The sequence of up (U) and down (D) pointing apices within a single ring is UUDUDD. The calcium cations in the tunnels are coordinated by six to seven oxygen ligands. Topologically the framework belongs to the group of three-dimensional 4-connected nets. The coordination sequences (4-11-24-41-63-91-123-160-202-249) and the vertex symbols (61.61.62.41.61.61) are identical for the four crystallographically independent Al-atoms. The framework density has a value of 17.06 T-atoms/1000 Å3.