OPTICAL STUDY OF THE EVOLUTION OF CHARGE AND SPIN ORDERING IN THE MANGANESE PEROVSKITE BI1-XCAXMNO3 (X-GREATER-THAN-0.5)

We report an optical reflectivity study of charge and spin ordering in Bi1-xCaxMnO3 (x = 0.74 and 0.82) single crystals. In die high tempera ture phase, T > T-co, which is characterized by ferromagnetic correlat ions, the low-energy contribution to the optical conductivity sigma(1) (omega) is dominated by small polaron dynamics. For T-co > T > TN, the presence of both small polaron and charge-gap-like features signifies ''phase separation'' behavior in which domains of ferromagnetic and a ntiferromagnetic order coexist. Below T-N, the polaron response is com pletely suppressed, and a complete charge gap develops, coincident wit h the onset of long-range antiferromagnetic order. [S003I-9007(98)0769 8-4].