Schofield, Paul F.Berry, Andrew J.Doyle, Patricia M.Knight, Kevin S.2025-05-232025-05-230342-1791http://www.scopus.com/inward/record.url?scp=85200479369&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733752704CaAl12O19, 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 CaAl11.8Ti3+0.2O19 and CaAl9.8Ti3+0.54Mg0.83Ti4+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 CaAl11.8Ti3+0.2O19. CaAl11.8Ti3+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.A.J.B. thanks the Engineering and Physical Sciences Research Council (EPSRC) for the award of a CNE Studentship, which was used to support P.M.D. We thank ISIS Neutron Spallation Source\u00A0for the award of beamtime for proposal RB920090.enPublisher Copyright: © The Author(s) 2024.CACalcium hexaluminateHiboniteNegative thermal expansionNeutron powder diffractionTi202410.1007/s00269-024-01286-585200479369