Orbital Correlations in the PseudocubicOand RhombohedralRPhases ofLaMnO3

The local and intermediate structure of stoichiometric ${\mathrm{LaMnO}}_{3}$ has been studied in the pseudocubic and rhombohedral phases at high temperatures (300--1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis was 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 ${\mathrm{MnO}}_{6}$ octahedra are still fully distorted locally. More importantly, the intermediate structure suggests the presence of local ordered clusters of diameter $\ensuremath{\sim}16\text{ }\AA{}$ ($\ensuremath{\sim}4$ ${\mathrm{MnO}}_{6}$ octahedra) implying strong nearest-neighbor JT antiferrodistortive coupling. These clusters persist well above the JT transition temperature even into the high-temperature $R$ phase.