Study of oxygen vacancy ordering in mullite at high temperatures

High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al2O3:SiO2 ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature T-c > 3000 degrees C.