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

The stability of silica-free mullite structures in the Al2O3-B2O3-P2O5 tern ary system was investigated at T = 900-1000 degreesC. A monophasic mullite was obtained for the composition Al8BPO16, showing that substitution of P a nd B for Si is possible for 2:1 mullite, Al4SiO8, while the same substituti on-for the 3:2 mullite yielded a biphasic mixture. Another monophasic sampl e was obtained in the same temperature range, with composition Al8.6P0.4B1. 6O16.3 Further preparations close to the above compositions enabled us to a ssess the homogeneity range for these silica-free mullites. At 1000 degrees C it can be expressed by the following notation: Al8+xP1-xB1+xO16+x/2 with 0 less than or equal to x less than or equal to 0.6 +/- 0.1. The stability range expressed by the above relation results lie on the join 2:1 silica-fr ee 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 phospo rus is likely to enter the tetrahedral T site. Boron occupies a site with t riangular coordination, between T and T* sites, at a distance of 0.6 Angstr om from the center of the T polyhedron.