Mott insulating low thermal expansion perovskite TiF3

A class of transition metal trifluorides harbors a structural quantum phase transition, above which extremely large negative thermal expansion (NTE) is shown in ScF${}_{3}$. A related material (TiF${}_{3}$) has reported a much weaker effect and attempts have been made to describe the differing thermal expansion behavior of this system using uncorrelated electronic structure calculations. Here, the authors use a combination of infrared reflectivity, ac magnetometry, and density functional theory calculations to demonstrate that in fact that TiF${}_{3}$ is a strongly correlated electron system, where uncorrelated approaches are ineffective. The work concludes that Mott physics is an important consideration in describing the evolution of the unusual NTE effect in these systems.