Abstract
CaAl 12 O 19 , which can incorporate Ti as both Ti 3+ and Ti 4+ (charge coupled substitution with Mg 2+ ), 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.6Al 2 O 3 ), and CA 6 . The lattice parameters and unit cell volumes of Ti-substituted hibonite ( P 6 3 / mmc ) with the formulae CaAl 11.8 Ti 3+ 0.2 O 19 and CaAl 9.8 Ti 3+ 0.54 Mg 0.83 Ti 4+ 0.83 O 19 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 11.8 Ti 3+ 0.2 O 19 . CaAl 11.8 Ti 3+ 0.2 O 19 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.