Raman and infrared studies of La_1-ySr_yMn_1-xM_xO_3 (M=Cr, Co, Cu, Zn, Sc or Ga): Oxygen disorder and local vibrational modes

TL;DR

This study investigates how various ion substitutions in La_1-ySr_yMn_1-xM_xO_3 affect oxygen disorder, vibrational modes, and magnetic properties using Raman and infrared spectroscopy across different temperatures.

Contribution

It provides new insights into the influence of different substituents on oxygen lattice distortions and vibrational modes in manganites, highlighting the dominant role of ionic radius over electronic or magnetic effects.

Findings

Substituents cause broad Raman bands indicating oxygen disorder.

The local breathing mode frequency varies with substituent type.

The A_{1g} mode frequency correlates linearly with rhombohedral distortion angle.

Abstract

We present results of our study of polarized Raman scattering and infrared reflectivity of rhombohedral ceramic La_1-ySr_yMn_1-xM_xO_3 manganites in the temperature range between 77 and 320K. 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 the distortions of the oxygen sublattice by the presence of broad bands in the Raman spectra, the increase of d.c. 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 to 77K. The frequency of the bulk A_{1g} mode depends linearly on the angle of the rhombohedral distortion.