Anisotropic Thermal Expansion in CaAl₄O₇

Abstract

Crystals of CaAl₄O₇ (space group C2/c) have been examined by high-temperature powder x-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 1473 K. The lattice deformation induced by thermal expansion has been investigated by matrix algebra analysis to determine the directions and magnitudes of the principal distortions. During heating from 298 K, two of the three principal distortions, λ1 and λ2, invariably showed a positive thermal expansion, the magnitudes of which at 1473 K are respectively approximately 1.0% and approximately 0.8%. The direction of λ1 was nearly parallel to [$30\overline 4$], and the λ2 axis invariably coincides with the crystallographic b axis. On the other hand, the distortion λ3 showed a negative thermal expansion below approximately 1173 K; the maximum contraction of approximately 0.2% occurred at approximately 673 K nearly along [207]. During further heating to 1473 K, λ3 showed, to the contrary, a positive expansion of approximately 0.2% nearly along [50, 16]. The negative thermal expansion along the λ3 axis substantially accounts for the mean linear thermal expansion being very low at temperatures below approximately 473 K.