The effects of point defects and microstructure on the pseudo-elasticity of ThCr2Si2-type crystals

Abstract

Ternary intermetallic compounds with the ThCr2Si2-type structure, which are known for their high-temperature superconductivity, have recently garnered interest due to the discovery of a pseudo-elastic mechanical response to compression along the c-axis. However, the effects of point defects and doping on this response remain unknown. In this work, these effects are investigated with density functional theory (DFT) in conjunction with continuum-scale models. DFT simulations of hydrostatic and uniaxial compression of pure ThCr2Si2-type crystals were conducted. The magnetic phase transition of CaFe2As2 was reproduced, while LaRu2P2 exhibited a continuous transition into its collapsed tetragonal phase. The two-phase DFT data was used to build a continuum-scale, thermodynamically-driven composite model which predicts the pseudo-elastic response of a large sample under displacement control and load control scenarios. Strain along the c-axis was shown to be the critical parameter in predicting crystal collapse. Then, DFT simulations of defected or doped unit cells were conducted to investigate their energetics and mechanical responses to compression. In some cases, the addition of vacancies effectively suppressed the pseudo-elastic response of the crystals. Simulations of crystals doped with varying concentrations revealed alterations of the mechanical properties as well. Tunable variability of the phase change with respect to dopant concentration was predicted in disordered doped structures, while multiple phase changes were predicted in ordered doped structures. Composite models were then built with the DFT data to predict the response of a sample comprised of multiple microstructures. The models predict a wide range of variability in the mechanical behavior and provide insight into how impurities and defects can be used to tune the response of these materials.