Transition to collapsed tetragonal phase inCaFe2As2single crystals as seen byFe57Mössbauer spectroscopy

Temperature dependent measurements of $⁵⁷\mathrm{Fe}$ M\"ossbauer spectra on ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ single crystals in the tetragonal and collapsed tetragonal phases are reported. Clear features in the temperature dependencies of the isomer shift, relative spectra area, and quadrupole splitting are observed at the transition from the tetragonal to the collapsed tetragonal phase. From the temperature dependent isomer shift and spectral area data, an average stiffening of the phonon modes in the collapsed tetragonal phase is inferred. The quadrupole splitting increases by $\ensuremath{\sim}25%$ on cooling from room temperature to $\ensuremath{\sim}100$ K in the tetragonal phase and is only weakly temperature dependent at low temperatures in the collapsed tetragonal phase, in agreement with the anisotropic thermal expansion in this material. In order to gain microscopic insight about these measurements, we perform ab initio density functional theory calculations of the electric field gradient and the electron density of ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ in both phases. By comparing the experimental data with the calculations we are able to fully characterize the crystal structure of the samples in the collapsed-tetragonal phase through determination of the As $z$ coordinate. Based on the obtained temperature dependent structural data we are able to propose charge saturation of the Fe-As bond region as the mechanism behind the stabilization of the collapsed-tetragonal phase at ambient pressure.