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We characterize perovskite TiF_3, a material which displays significantnegative thermal expansion at elevated temperatures above itscubic-to-rhombohedral structural phase transition at 330 K. We find the opticalresponse favors an insulating state in both structural phases, which we showcan be produced in density functional theory calculations only through theintroduction of an on-site Coulomb repulsion. Analysis of the magneticsusceptibility data gives a S=1/2 local moment per Ti+3 ion and anantiferromagnetic exchange coupling. Together, these results show that TiF_3 isa strongly correlated electron system, a fact which constrains possiblemechanisms of strong negative thermal expansion in the Sc_1-xTi_xF3 system. Weconsider the relative strength of the Jahn-Teller and electric dipoleinteractions in driving the structural transition.

Mott insulating negative thermal expansion perovskite TiF3