Raman and infrared studies ofLa1−ySryMn1−xMxO3(M=Cr, Co, Cu, Zn, Sc or Ga): Oxygen disorder and local vibrational modes

Abstract

We present results of our study of polarized Raman scattering and infrared reflectivity of rhombohedral ceramic ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{\mathrm{Mn}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{3}$ manganites in the temperature range between 77 and $320\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In our samples, a part of the Mn atoms is substituted by $M=\mathrm{Cr}$, 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 ${A}_{g}$-like high frequency mode attributed to the local breathing vibration of oxygens surrounding the substituent ion. Its frequency and intensity strongly depend on the type of the substituent. In the Co-substituted sample, the mode anomalously softens when going from $300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The frequency of the bulk ${A}_{1g}$ mode depends linearly on the angle of the rhombohedral distortion.