Crystal-chemistry of mullite-type aluminoborates Al18B4O33 and Al5BO9: A stoichiometry puzzle

Orthorhombic Al2O3-rich aluminoborate is an important ceramic material for which two slightly different compositions have been assumed: Al5BO9 (5Al2O3:B2O3) and Al18B4O33 (9Al2O3:2B2O3). The formula Al18B4O33 (=Al4.91B1.09O9) was derived from results of chemical analyses when crystal structure data were not yet available. Subsequent structural investigations indicated Al5BO9 composition. Nevertheless, Al18B4O33 was still accepted as the correct stoichiometry assuming that additional B replaces 9% Al. Powder samples of both compositions and ones with excess boron were prepared by solid state reactions between α-Al2O3 and B2O3/H3BO3 at temperatures above 1100 °C and single-crystals were grown from flux at 1100 and 1550 °C. Products were investigated by single-crystal and powder XRD, 11B and 27Al solid-state MAS-NMR, Raman and FTIR spectroscopy as well as Laser-ablation ICP-MS. No indication of the predicted 9% B→Al substitution was found. LA ICP-MS indicated 12.36(27) wt% B2O3 corresponding to Al4.97B1.03O9. Hence, the suggested Al18B4O33 stoichiometry can be excluded for all synthesized samples. A very low amount of Al vacancies at a five-fold coordinated site are likely, charge balanced by an additional nearby three-fold coordinated B site. All evidences indicate that the title compound should be reported as Al5−xB1+xO9 with x<0.038(6), which is close to Al5BO9.