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 mullit e show a smooth reversible behaviour which can be described by a rotation o f more or less rigid TO4 (T = Al(2),Si)-tetrahedra and accompanying distort ions of the Al(l)O-6-octahedra. Large atomic displacement parameters along the c-direction, including static parts, match to respective structural mod ulations in the mullite structure. A comparison of a pristine and a preheat ed sample reveals an irreversible depopulation of the Al(2) atomic site in favour of the Al(*) site.