The thermal expansion of Ti-substituted CaAl12O19

CaAl₁₂O₁₉, which can incorporate Ti as both Ti³⁺ and Ti⁴⁺ (charge coupled substitution with Mg²⁺), is one of the first minerals to condense from a gas of solar composition and is used as a ceramic. It is variously known as hibonite, calcium hexaluminate (CaO.6Al₂O₃), and CA₆. The lattice parameters and unit cell volumes of Ti-substituted hibonite (P6₃/mmc) with the formulae CaAl_11.8Ti³⁺_0.2O₁₉ and CaAl_9.8Ti³⁺_0.54Mg_0.83Ti⁴⁺_0.83O₁₉ were determined as a function of temperature from ~ 10 to 275 K by neutron powder diffraction. The thermal expansion is highly anisotropic with the expansion in c a factor of ~ 5 greater than that in a. The change in a is approximately equal for the two compounds whereas the change in c is almost 50% larger for CaAl_11.8Ti³⁺_0.2O₁₉. CaAl_11.8Ti³⁺_0.2O₁₉ also exhibits negative thermal expansion between 10 and 70 K. The change in unit cell volume with temperature of both compositions is well described by a two term Einstein expression. The large change in c is consistent with substitution of Ti onto the M2 and M4 sites of the R-block structural unit.