Local structure study of the orbital order/disorder transition in LaMnO3

We use a combination of neutron and x-ray total scattering measurements together with pair distribution function (PDF) analysis to characterize the variation in local structure across the orbital order/disorder transition in LaMnO3. Our experimental data are inconsistent with a conventional order/disorder description of the transition, and reflect instead the existence of a discontinuous change in local structure between ordered and disordered states. Within the orbital ordered regime, the neutron and x-ray PDFs are best described by a local structure model with the same local orbital arrangements as those observed in the average (long-range) crystal structure. We show that a variety of meaningfully different local orbital arrangement models can give fits of comparable quality to the experimental PDFs collected within the disordered regime; nevertheless, our data show a subtle but consistent preference for the anisotropic Potts model proposed previously [M. R. Ahmed and G. A. Gehring, Phys. Rev. B 79, 174106 (2009)]. The key implications of this model are electronic and magnetic isotropy together with the loss of local inversion symmetry at the Mn site. We conclude with a critical assessment of the interpretation of PDF measurements when characterizing local symmetry breaking in functional materials.