On the polymorphism of CaAl 2O 4—structural investigations of two high pressure modifications

Abstract

The crystal structures of two high pressure polymorphs have been studied using X-ray diffraction techniques. The compounds were synthesized from ceramic precursors at 1000 °C / 3.5 GPa (phase A) and 1100 °C / 10 GPa (phase B), respectively. In case of phase A, the structure was determined from a single crystal diffraction data set collected at room conditions and refined to a residual R | F | = 0.027 for 1319 observed reflections ( I > 2 σ ( I ) ). The compound crystallises in the monoclinic space group P 2 1 / m ( a = 5.556 ( 2 ) Å , b = 3.7627 ( 8 ) Å , c = 7.056 ( 3 ) Å , β = 101.36 ( 3 ) ° , V = 144.62 ( 8 ) Å 3 , Z = 2 , D calc = 3.629 g cm −1 ). The crystals were subject to twinning by partial-merohedry which was accounted for during the refinements. Furthermore, a distinct one-dimensional diffuse scattering parallel to c ∗ could be observed. Phase A represents a layer structure containing double sheets of edge sharing AlO 6-octahedra, similar to those observed in boehmite (AlO(OH)) or lepidocrocite (FeO(OH)). Linkage between the layers is provided by the Ca cations occupying the interstitial space between the sheets. They are coordinated by seven oxygen ligands. The structure of phase B has been refined from laboratory powder diffraction data using the Rietveld method (space group Pnam, a = 8.92004 ( 5 ) Å , b = 10.31550 ( 6 ) Å , c = 2.87129 ( 1 ) Å , V = 264.203 ( 1 ) Å 3 , Z = 4 , D calc = 3.973 g cm −1 ). The compound is isostructural with monocalcium ferrite (CaFe 2O 4) and contains two crystallographically independent AlO 6-octahedra forming edge-sharing double chains parallel to [001]. Calcium cations are located in the tunnels formed by the linked double chains, being in eightfold coordination.