Structural Evolution of Corundum at High Temperatures

The evolution of the corundum (α-Al 2 O 3 ) structure was investigated at 297, 573, 873, and 1173 K using the single-crystal X-ray diffraction method and molecular dynamics simulation. The results were smoothly extrapolated to the previously reported high-temperature data of the compound at 2170 K [Ishizawa et al. : Acta Crystallogr., Sect. B 36 (1980) 228], revealing a significant shift of the fractional coordinate z of the Al atom as a function of temperature. The Al atom shift indicates that the Al...Al intermetallic repulsion force across a pair of face-sharing octahedra becomes conspicuous over other interatomic forces at high temperatures. The increasing Al...Al repulsion is relieved by both the preferential expansion of a unit cell along the c -axis and the change in the z coordinate of the Al atom. The results of molecular dynamics simulation experiments qualitatively agreed with the X-ray diffraction data. The thermal expansion of the crystal was well described by the quadratic functions, a = 4.75385 (1.0 + 5.05841 × 10 -6 T + 1.69084 × 10 -9 T 2 ) Å, and c = 12.9782 (1.0 + 4.61344 × 10 -6 T + 2.63802 × 10 -9 T 2 ) Å.