Abstract

An optical study has been made of La_0·7Ca_0·3MnO₃ and La_0·8Li_0·2MnO₃ in the infrared spectral region (100 cm^-1 to 4000 cm^-1). The transmission of all samples is negligible. The photoconductive response is also unmeasurable. The data reported are therefore restricted to reflectivity measurements. At room temperature, both materials exhibit similar ‘bending’ and ‘stretching’ phonon modes, associated with the MnO₆ octahedra, at ~330 and ~580 cm^-1, respectively. The ‘external’ phonon mode observed at ~170 cm^-1, corresponding to the (La,Ca/Li) cations beating against the MnO₆ octahedra, is slightly shifted between the two materials, as would be expected on the basis of the different compositions. At elevated temperature (~600°C) the intensity of the light reflected decreases across the infrared spectrum, and the ‘stretching’ mode moves to lower energy.