A Raman Spectroscopy Study of Disorder and Local Vibrational Modes in la_0.7Sr_0.3Mn_1-xM_xO₃ (M=Fe, Co)

We present results of Raman scattering and X-ray diffraction studies of La_0.7Sr_0.3Mn_1-xM_xO₃ manganites. Some Mn atoms were replaced by Fe and Co with x in the range of 0.0 - 0.75. XRD analysis of these samples indicates a change in the cell parameters with doping content. Our main focus is the Raman peak at high frequency (>630 cm^-1), which is somewhat controversial. The Raman spectra exhibit an additional high-frequency mode attributed to a local breathing vibration of the oxygen surrounding the substituent ion. The frequency and the intensity of the Raman spectra in these manganites are very sensitive to the type and the amount of the substituent. From the above results, a critical substituent content at which the structural phase changed is shown.