Structural Aspects of the Lattice Thermal Expansion of Hexagonal Cordierite

The mechanism of lattice thermal expansion in pure hexagonal cordierite prepared by glass devitrification was investigated by time‐of‐flight neutron powder diffraction at temperatures from 22° to 750°C under vacuum. Full matrix least‐squares refinement of the diffraction spectra was carried out in space group P6/mcc using the Rietveld method. The structural data from the refinements were resolved into components parallel and normal to the c axis. The negative c axis expansion results from two effects: (1) increasing distortion of the T 2 tetrahedral (ring tetrahedra) with temperature, primarily due to displacement of the T 2 cations toward the c‐axis channels in the structure; (2) decreasing distortion of the T 1 /M tetrahedra/ octahedra with increasing temperature, due to octahedral I (M‐O 1 ) bond expansion and an associated decrease in torsional distortion of the T 1 tetrahedra.