Uniaxial compression of [001]-oriented CaFe2As2 single crystals: the effect of microstructure and temperature on superelasticity Part II: Modeling

Density functional theory (DFT) simulations are combined with analytical models to describe the impact that defects and temperature have on the mechanical response of [001]-oriented compression of CaFe₂As₂. Our experiments, described in a companion paper, demonstrate that the solution in which CaFe₂As₂ is grown (either in a Sn or FeAs solution), as well as post-growth heat treatment, can affect the mechanical response of these materials. To address these questions, we use DFT to understand the phase equilibria in the Ca-Fe-As systems and to determine which defect structures and precipitates should form in the FeAs-grown CaFe₂As₂. Our results demonstrate that FeAs and Fe should precipitate out of Fe-rich CaFe₂As₂ and that there should be a low-energy coherent interface between the precipitate and the CaFe₂As₂ matrix that influences what actually precipitates. Additionally, the simulations show that off-stoichiometric CaFe₂As₂ should occur through the formation of vacancies in the structure. The simulations of the mechanical response of CaFe₂As₂ demonstrate that the mechanical stiffening observed in experiments can be a result of point defects, the most likely source being As vacancies. Finally, by using free energy calculations within DFT, we show that the temperature-dependent stress-strain curves can be partially explained by the inclusion of vibrational entropy differences between the orthorhombic and collapsed tetragonal phases in CaFe₂As₂.