Comment on "X-ray resonant scattering studies of orbital and charge ordering in Pr1-xCaxMnO3"

TL;DR

This paper critiques a recent x-ray scattering study on Pr1-xCaxMnO3, arguing that the observed reflections are better explained by structural distortions rather than charge and orbital ordering models.

Contribution

It provides a rigorous analysis showing the inadequacy of the charge-orbital model and supports a structural distortion explanation for the observed reflections.

Findings

Charge-orbital model is inconsistent with electronic balance.

Reflections are better explained by local structural distortions.

The analysis challenges previous interpretations of orbital and charge ordering.

Abstract

In a recent published paper [Phys. Rev. B 64, 195133 (2001)], Zimmermann et al. present a systematic x-ray scattering study of charge and orbital ordering phenomena in the Pr1-xCaxMnO3 series with x= 0.25, 0.4 and 0.5. They propose that for Ca concentrations x=0.4 and 0.5, the appearance of (0, k+1/2, 0) reflections are originated by the orbital ordering of the eg electrons in the a-b plane while the (0, 2k+1, 0) reflections are due to the charge ordering among the Mn3+ and Mn4+ ions. Moreover, for small Ca concentrations (x<0.3), the orbital ordering is only considered and it occurs at (0, k, 0) reflections. A rigorous analysis of all these resonance reflections will show the inadequacy of the charge-orbital model proposed to explain the experimental results. In addition, this charge-orbital model is highly inconsistent with the electronic balance. On the contrary, these reflections can be easily understood as arising from the anisotropy of charge distribution induced by the presence of local distortions, i.e. due to a structural phase transition.