Uniaxial Compression of [001]-Oriented CaFe 2As 2 Single Crystal the Effect of Microstructure and Temperature on Superelasticity Part II: Modeling

Abstract

Density functional theory simulations are combined with analytical models to describe the impact that defects and temperature have on the mechanical response of [001]-orientated compression of CaFe 2 As 2 . Our experiments, described in a companion paper, demonstrate that the solution in which CaFe 2 As 2 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 determine which defect structures and precipitates should form in the FeAs-grown CaFe 2 As 2 . Our results demonstrate that FeAs and iron should precipitate out of iron-rich CaFe 2 As 2 and that there should be a low-energy coherent interface between the precipitate and the CaFe 2 As 2 matrix that influences what actually precipitates. Additionally, the simulations show that off-stoichiometric CaFe 2 As 2 should occur through the formation of vacancies in the structure. The simulations of the mechanical response of CaFe 2 As 2 demonstrate that 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 CaFe 2 As 2 .