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

Abstract

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 Al8Si2B2O19). 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 AlO6 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 BO3 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 11B MAS NMR experiment indicating the presence of B in BO4 configuration but BO3 being dominant. Here for the first time a B‐mullite crystal structure is presented yielding BO3 and BO4 units in space group Pbam.