Incorporation of chromium into aluminium borate 9Al2O3⋅ 2B2O3 (A9B2)

Abstract

Aluminium borate 9Al 2 O 3 .2B 2 O 3 (A 9 B 2 ) and Cr 3+ -rich and A 9 B 2 (≃10 wt% Cr 2 O 3 ) were synthesized from oxide powders by solid state reactions. The fractional atomic coordinates determined from single crystal diffraction data agree comparatively well with those obtained by the Rietveld method using powder X-ray diffraction data. The crystal structure of A 9 B 2 is similar to the structures of sillimanite, mullite and andalusite. The main structural units common to all four phases are the infinite chains of edge-sharing AlO 6 -octahedra running parallel to c. Chromium incorporation causes an expansion of the structure. Refinement of occupancy factors has shown the preference of Cr 3+ for the octahedrally coordinated Al (1)-position