High temperature crystal structure of a 3:2 mullite from neutron diffraction data

High temperature neutron investigations up to 1950 K of 3:2-sintered mullite show a smooth reversible behaviour which can be described by a rotation of more or less rigid TO₄ (T = Al(2),Si)-tetrahedra and accompanying distortions of the Al(1)O₆-octahedra. Large atomic displacement parameters along the c-direction, including static parts, match to respective structural modulations in the mullite structure. A comparison of a pristine and a preheated sample reveals an irreversible depopulation of the Al(2) atomic site in favour of the Al(*) site.