Neutron diffraction study of average and local structure in La0.5Ca0.5MnO3

TL;DR

This neutron diffraction study investigates the local and average structures of La0.5Ca0.5MnO3 at different temperatures, revealing local distortions and partial charge ordering that are not distinguishable by average structure analysis alone.

Contribution

The paper combines Rietveld and pair distribution function analyses to differentiate local structural features and charge states in La0.5Ca0.5MnO3, providing insights into its partial charge ordering.

Findings

Local Mn octahedral distortions are observed.

The system exhibits partial charge ordering at 20K.

Structural models fit neutron data equally well at low temperature.

Abstract

We used neutron powder diffraction to obtain the local and long-range structure of La0.5Ca0.5MnO3 at room temperature and 20K. By combining Rietveld and pair distribution function analysis of the total neutron scattering data, we have analyzed the structure of the compound using two competing models describing the low temperature phase: first the charge ordered/orbital ordered model and second the Mn-Mn dimer model. These structural models fit the 20K neutron powder diffraction pattern equally well using Rietveld analysis. Therefore, pair distribution function analysis is used to probe the local and medium-range structure revealing a system with two distinctly distorted Mn octahedra and Mn ions with non-integral valence states. The distorted octahedra differ with the structural model for the Zener polaron type Mn-Mn dimer picture proposed for Pr0.6Ca0.4MnO3 and order in a similar checkerboard configuration associated with the CE-type anti-ferromagnetic structure. Therefore, locally the charge difference and structural ordering between the two Mn is appreciable enough to describe the system at 20K as "partially charge ordered".