Small Polaron Localization, Jahn-Teller Distortion and Defects in La0.67Ca0.33MnO3: An Infrared Spectroscopy and X-ray Absorption Analysis

TL;DR

This study uses infrared spectroscopy and EXAFS to analyze polaron localization, Jahn-Teller distortions, and defects in La0.67Ca0.33MnO3, revealing structural and electronic changes across the insulator-metal transition.

Contribution

It combines infrared and EXAFS techniques to elucidate the structural and electronic evolution in La0.67Ca0.33MnO3 during the insulator-metal transition, highlighting the role of Jahn-Teller distortions and defects.

Findings

Identification of two non-equivalent sites in the insulating phase.

Observation of dynamic averaging of Mn sites below TIM.

Detection of extra octahedra influencing transition temperature.

Abstract

We combine infrared reflectivity and EXAFS (Extended X-ray Absorption Fine Structure) techniques to study the solid solution La0.67Ca0.33MnO3 prepared by different methods yielding samples with different insulator-metal transition temperatures (TIM). While the small polaron analysis of the optical conductivity provides a natural description for the higher frequency reflectivity tail of conducting samples, our structural results are in accord with two non-equivalent sites in the insulating phase of good-quality samples. Those sites, one for the Mn3+ Jahn-Teller distorted octahedra and another for the Mn4+ ion, gradually turn into one dynamically averaged below the transition TIM. On the other hand, carriers screening antiresonances near infrared longitudinal optical modes, above TIM, mirror thermal activated small polarons weakly smearing EXAFS oscillations. We associate this to the lack of M3+,M4+ explicit structure in the Mn K-edge absorption band. Extra octahedra, detected by EXAFS below TIM in higher resistivity samples, seem to be excluded of participating in the dynamics of the insulator-metal transition shifting TIM toward lower temperatures.