  The temperature dependence of the reflectance spectra of magnesium oxide
(MgO) and ferropericlase (Mg1-xFexO, for x=0.06 and x=0.27) have been measured
over a wide frequency range (~50 to 32000 cm-1) at 295 and 6 K. The complex
dielectric function has been determined from a Kramers-Kronig analysis of the
reflectance. The spectra of the doped materials resembles pure MgO in the
infrared region, but with much broader resonances. We use a shell model to
calculate the dielectric function of ferropericlase, including both anharmonic
phonon-phonon interactions and disorder scattering. These data are relevant to
understanding the heat conductivity of ferropericlase in the earth's lower
mantle.
