Raman and infrared studies of La1- ySryMn1- xMxO3 (M=Cr, Co, Cu, Zn, Sc or Ga) : Oxygen disorder and local vibrational modes

We present results of our study of polarized Raman scattering and infrared reflectivity of rhombohedral ceramic La1-ySryMn1-xMxO3 manganites in the temperature range between 77 and 320 K. In our samples, a part of the Mn atoms is substituted by M = Cr, Co, Cu, Zn, Sc, or Ga with x in the range 0-0.1. The hole concentration was kept at the optimal value of about 32% by tuning the Sr content y. We have monitored distortions of the oxygen sublattice by the presence of broad bands in the Raman spectra, the increase of dc resistivity extracted from the infrared reflectivity, and the change of the critical temperature of the ferromagnetic transition. Our results support the idea that these properties are mainly determined by the radius of the substituent ion, its electronic and magnetic structure playing only a minor role. Furthermore, the Raman spectra exhibit an additional Ag-like high frequency mode attributed to the local breathing vibration of oxygens surrounding the substituent ion.