Elastic neutron scattering studies of an optimized LaFeAsO single crystal reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O) structural transition occurs at $T_\texttt{ S} \approx 156$ K, and it exhibits a sharp transition at $T_\texttt{ P} \approx 148$ K. We argue that in the temperature range $T_\texttt{ S}$ to $T_\texttt{ P}$, T and O structures may dynamically coexist possibly due to nematic spin correlations recently proposed for the iron pnictides, and we attribute $T_\texttt{ P}$ to the formation of long-range O domains from the finite local precursors. The antiferromagnetic structure emerges at $T_\texttt{ N} \approx 140$ K, with the iron moment direction along the O \emph{ a} axis. We extract the iron magnetic form factor and use the tabulated $\langle j_0\rangle$ of Fe, Fe$^{ 2+}$ and Fe$^{ 3+}$ to obtain a magnetic moment size of $\sim$0.8 $\mu_\texttt{ B}$ at 9.5 K.
Categories: cond-mat.supr-con cond-mat.str-el