Strain and Mosaic Distribution Measurements in Superelastic CaFe₂As₂

Strain engineering is an important process for tuning material properties for both fundamental understanding and applications. Much higher strains can be achieved in nanomaterials than in bulk samples, which comes with the promise of much stronger property tuning. Even beyond the typical strain limits of nanomaterials, nanopillars of CaFe₂As₂ and related materials have shown elastic strain up to 17% under uniaxial stress; this is known as superelasticity and is associated with a known tetragonal to collapsed tetragonal phase transition [1]. However, strain distribution in nanomaterials can be complex, difficult to measure, and can have a profound effect on properties. In this study, we mapped the lattice parameter and mosaic of single-crystalline CaFe₂As₂ micropillars under progressive loads using X-ray nanodiffraction at the NanoMAX beamline at the MAX-IV synchrotron. Our results show a continuous shift of (002) diffraction peak and corresponding c-axis lattice contraction, consistent with a second order phase transition rather than the first order transition that had been expected. We also measure a continuous strain variation from the highly strained pillar top to a far lesser strained base, with the strain evolving over several microns. The scanning geometry at NanoMAX also allowed us to measure a strain induced mosaic spread to a resolution better than 1/10^th of a degree. Our results are a direct measure of strain distribution with high spatial and angular resolution critical for interpreting strain tuned properties in nanomaterials.

1. 1. John T. Sypek et al., Nature Communications 8. 1083 (2017).