micropillars.*

Applied strain has become a critical tool in the study of complex materials. High levels of elastic strain can be achieved by applying uniaxial stress to micropillars. Due to the mechanical size effect, typical materials can be elastically strained thirty to forty times more as micropillars than in the bulk. In a few cases the effect can be an order of magnitude greater than that, dubbed superelasticity.¹ One material of this kind is CaFe₂As₂, a parent material for the high T_C superconductivity. Superelasticity in CaFe₂As₂ is associated with a well-known tetragonal to collapsed tetragonal phase transition that occurs under compressive stress. The transition under strain control occurs over a range of strain with DFT calculations indicating that the transition is locally abrupt but spatially dispersed forming (001) oriented phase boundaries.¹ Measuring this transition is the subject of our study. In addition to some microstructural changes, we found that above a critical level of strain a second (002) peak appears corresponding regions of the crystals with a second, shorter c-axis length. This sudden change is consistent with the proposed spatially dispersed transition and allows us to map the distribution of the two phases.

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