Evolution of the Optical Properties and the Electrical Resistivity of CaMnO3 during the Substitution of Mo for Mn Ions

The reflectance, the optical density, and the optical conductivity spectra of CaMn1-xMoxO3 (x <= 0.07) polycrystals are studied over a wide spectral region. The substitution of Mo6+ ions for Mn4+ ions leads to a strong decrease in the first high-energy phonon band, which is related to vibrations in the MnO6 octahedron. The appearance of band-type charge carriers is detected in CaMn1-xMoxO3 with x <= 0.04, and carrier localization is observed in CaMn0.93Mo0.07O3. This localization increases at T < 160 K, i.e., during the transition from the paramagnetic to the antiferromagnetic state with orbital ordering and a monoclinic structure. The optical conductivity spectrum of CaMn0.93Mo0.07O3 exhibits a strong shift of fundamental absorption bands, which is associated with a change in the band structure because of a decrease in the splitting of the 3d levels of Mn in the crystal field.