  We characterize perovskite TiF_3, 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 TiF_3 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.
