Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral ${R}$-phases of LaMnO$_3$

TL;DR

This study investigates the local and intermediate structures of LaMnO$_3$ in different phases at high temperatures, revealing persistent local orbital order and Jahn-Teller distortions above the transition temperature.

Contribution

It provides detailed neutron diffraction analysis showing local orbital order persists in high-temperature phases of LaMnO$_3$, highlighting strong JT anti-ferrodistortive coupling.

Findings

Local orbitally ordered clusters of about 16 Å diameter exist at high temperatures.

Jahn-Teller distortions remain locally present above the orbital order-disorder transition.

Orbital correlations are maintained in both pseudocubic and rhombohedral phases.

Abstract

The local and intermediate structure of stoichiometric LaMnO$_3$ has been studied in the pseudocubic and rhombohedral phases at high temperatures (300 to 1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high temperature orthorhombic ($O$) and rhombohedral ($R$) phases the MnO$_6$ octahedra are still fully distorted locally. The data suggest the presence of local orbitally ordered clusters of diameter $\sim 16$ \AA ($\sim$four MnO$_6$ octahedra) implying strong nearest neighbor JT anti-ferrodistortive coupling.