W\"ustite: Electric, thermodynamic and optical properties of FeO
  We report on a systematic optical investigation of w\"ustite. In addition, the sample under consideration, Fe0.93O, has been characterized in detail by electrical transport, dielectric, magnetic and thermodynamic measurements. From infrared reflectivity experiments, phonon properties, Drude-like conductivity contributions and electronic transitions have been systematically investigated. The phonon modes reveal a clear splitting below the antiferromagnetic ordering temperature, similar to observations in other transition-metal monoxides and in spinel compounds which have been explained in terms of a spin-driven Jahn-Teller effect. The electronic transitions can best be described assuming a crystal-field parameter Dq = 750 cm-1 and a spin-orbit coupling constant \lambda = 95 cm-1. A well defined crystal field excitation at low temperatures reveals significant broadening on increasing temperature with an overall transfer of optical weight into dc conductivity contributions. This fact seems to indicate a melting of the on-site excitation into a Drude behavior of delocalized charge carriers. The optical band gap in w\"ustite is close to 1.0 eV at room temperature. With decreasing temperatures and passing the magnetic phase transition we have detected a strong blue shift of the correlation-induced band edge, which amounts more than 15% and has been rarely observed in antiferromagnets. 
F. Schrettle, Ch. Kant, P. Lunkenheimer, F. Mayr, J. Deisenhofer, A.   Loidl
cond-mat.str-el cond-mat.mtrl-sci