Ambient and high‐pressure synthesis, composition, and crystal structure of B‐mullites

The chemical compositions of several B‐doped mullite samples were analyzed using prompt gamma activation analyses (PGAA) indicating that 15% of the Si in the crystal structure of mullite can be replaced by B during sol‐gel synthesis at ambient pressure and 1200°C without the formation of impurities. Furthermore the PGAA results agree very well with the chemical compositions derived from Rietveld refinements based on neutron diffraction data. High‐pressure and high‐temperature synthesis yielded a B‐mullite with significantly higher B‐content than observed before (close to composition Al 8 Si 2 B 2 O 19 ). The results of PGAA, XRD, and neutron diffraction experiments show linear behavior of lattice parameters b and c as well as of the inclination angle ω of the AlO 6 octahedra in the ab ‐plane with increasing B‐content. The Rietveld refinements support the substitution mechanism known for B‐mullites, involving the formation of oxygen vacancies and the replacement of 40% of the tetrahedral Si by BO 3 units during the synthesis at 875°C and 10 kbar. However, the refined chemical composition as well as a very low lattice parameter a suggest a second mechanism for the incorporation of B into mullite. This is clearly supported by the 11 B MAS NMR experiment indicating the presence of B in BO 4 configuration but BO 3 being dominant. Here for the first time a B‐mullite crystal structure is presented yielding BO 3 and BO 4 units in space group Pbam .