Departmental Bulletin Paper Structural Basis for the Anisotropic Thermal Expansion of Aluminum Titanate,Al2TiO5,at Elevated Temperatures

Alexandra, Doncieux  ,  Keisuke, Ninomiya  ,  Nobuo, Ishizawa  ,  Toshitaka, Ota  ,  Marc, huger

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Supplementary File, CIF http://www.crl.nitech.ac.jp/ar/2016/Al2TiO5_ACRC_Report_2017.cif
Evolution of the undoped and Mg-doped Al2TiO5 crystal structures at elevated temperatures up to ~1300 K was investigated by the single-crystal X-ray diffraction technique. The compounds crystallize in the pseudobrookite-type [4c]M1[8f]M22O5 structure with the space group Cmcm, containing crystallographically independent M1 site at the Wyckoff position of 4c and M2 site at 8f for the metal atoms. The structural formula was found to be [4c][Al0.615(3)Ti0.385(3)][8f][Al0.693(2)Ti0.307(2)]2O5 for the undoped Al2TiO5 and [4c][Al0.616(9)Ti0.384(9)][8f][Al0.562(4)Ti0.373(4)Mg0.065(10)]2O5 for the Mg-doped one from the population analysis of the metal atoms. The mean thermal expansion coefficients were approximately —3.8 × 10—6 along the a-axis, 11.0 × 10—6 along the b-axis and 20.0 × 10—6 along the c-axis. During heating, the tunnels running along the c-axis in the structure change shape gradually with a characteristic buckling deformation of the chevron-shaped pillars composed of metal-oxygen bonds, resulting in an expansion of the tunnel along all directions perpendicular to the a-axis and the associated Poisson contraction along the a-axis. The large anisotropy in thermal expansion of the compounds, especially the negative thermal expansion along the a-axis, was thus explained from atomistic point of view.
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