Resonant scattering and multiple Bragg X-ray diffraction in LaMnO₃; a classical view

The mechanism for resonant scattering in LaMnO₃ has been the subject of some controversy between two models. Model (A) is based on orbital ordering, whereas model (B) invokes the Jahn-Teller distortion. From the structural point of view, the two models lead to different d-electron densities. The forbidden reflection 300 is turned on by resonance, leading to a well defined scheme for the lobes of the Mn d electrons [model (A)]. It turns out that the same experimental results could have been interpreted according to model (B). The only difference is that this alternative interpretation would have required a structure factor F₃₀₀ of opposite sign. This is a typical example of a phase problem. An experimental technique has been applied for phase determination based on multiple Bragg scattering. This shows that the multiple-beam results are in agreement with model (B), not with model (A), laying to rest a long overdue controversy.