Mott insulating negative thermal expansion perovskite TiF₃

We characterize perovskite TiF₃, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S=1/2 local moment per Ti⁺³ ion and an antiferromagnetic exchange coupling. Together, these results show that TiF₃ is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc_1-xTi_xF₃ system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition.