Phase transitions and iron-ordered moment form factor in LaFeAsO

Elastic neutron-scattering studies of single-crystal LaFeAsO reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O) structural transition occurs at TS≈156 K, and it exhibits a sharp transition at TP≈148 K. We argue that in the temperature range TS to TP, T and O structures may dynamically coexist possibly due to nematic spin correlations recently proposed for the iron pnictides, and we attribute TP to the formation of long-range O domains from the finite local precursors. The antiferromagnetic structure emerges at TN≈140 K, with the iron moment direction along the O a axis. We extract the iron magnetic form factor and use the tabulated ⟨j0⟩ of Fe, Fe2+, and Fe3+ to obtain a magnetic moment size of ∼0.8 μB at 9.5 K.