Low-temperature neutron powder diffraction study of chromium dioxide and the validity of the Jahn-Teller viewpoint

Abstract

The structure of CrO{sub 2} has been determined at 10, 173, and 295 K by powder neutron diffraction. The structure of this material is interesting because the mode of distortion around the metal center is the reverse of that expected on the basis of local Jahn-Teller considerations. Band-structure calculations however, show that although the structure with two long and four short metal-oxygen distances is that expected for the d{sup 0} configuration, where O-O repulsions control the picture, the opposite distortion is predicted for d counts leading to t{sub 2g} block degeneracies. We explain this result by constructing the d band dispersion curve using the angular overlap model. It is shown how through-bond coupling in the lower than octahedral local environment allows mixing between a component of each of the e{sub g} and t{sub 2g} orbital sets and frustrates the energetic preferences predicted for the t{sub 2g} block alone. The factors that control the details of the geometries of the transition-metal oxides and fluorides with the rutile structure are explored. 35 refs., 7 figs., 5 tabs.