We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr1-xNaxFe2As2. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels (x=0.48, near optimal superconductivity) and high temperatures (above room temperature for x=0, decreasing to 150 K for x=0.48), with a typical length scale of 1-3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

We report on temperature-dependent pair distribution function measurements of Sr_{1-x}Na_{x}Fe_{2}As_{2}, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C_{4} phase. Quantitative refinements indicate that the instantaneous local structure in the C_{4} phase comprises fluctuating orthorhombic regions with a length scale of ∼2  nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C_{4} phase and the neighboring C_{2} and superconducting phases.