Silica-free mullite structures in the Al2O3-B2O3-P2O5 ternary system

The stability of silica-free mullite structures in the Al2O3-B2O3-P2O5 ternary system was investigated at T = 900−1000 °C. A monophasic mullite was obtained for the composition Al8BPO16, showing that substitution of P and B for Si is possible for 2:1 mullite, Al4SiO8, while the same substitution for the 3:2 mullite yielded a biphasic mixture. Another monophasic sample was obtained in the same temperature range, with composition Al8.6P0.4B1.6O16.3. Further preparations close to the above compositions enabled us to assess the homogeneity range for these silica-free mullites. At 1000 °C it can be expressed by the following notation:  Al8+xP1-xB1+xO16+x/2 with 0 ≤ x ≤ 0.6 ± 0.1. The stability range expressed by the above relation results lie on the join 2:1 silica-free mullite−Al9B2O16.5. Neutron and X-ray powder diffraction were performed on the two limit compositions (with x = 0 and x = 0.6), showing that phosporus is likely to enter the tetrahedral T site. Boron occupies a site with triangular coordination, between T and T* sites, at a distance of 0.6 Å from the center of the T polyhedron.