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Temperature dependent measurements of 57Fe Mossbauer spectra on CaFe2As2single crystals in the tetragonal and collapsed tetragonal phases are reported.Clear features in the temperature dependencies of the isomer shift, relativespectra area and quadrupole splitting are observed at the transition from thetetragonal to the collapsed tetragonal phase. From the temperature dependentisomer shift and spectral area data, an average stiffening of the phonon modesin the collapsed tetragonal phase is inferred. The quadrupole splittingincreases by ~25% on cooling from room temperature to ~100 K in the tetragonalphase and is only weakly temperature dependent at low temperatures in thecollapsed tetragonal phase, in agreement with the anisotropic thermal expansionin this material. In order to gain microscopic insight about these measurementswe perform ab initio density functional theory calculations of the electricfield gradient and the electron density of CaFe2As2 in both phases. Bycomparing the experimental data with the calculations we are able to fullycharacterize the crystal structure of the samples in the collapsed-tetragonalphase through determination of the As z-coordinate. Based on the obtainedtemperature dependent structural data we are able to propose charge saturationof the Fe - As bond region as the mechanism behind the stabilization of thecollapsed-tetragonal phase at ambient pressure.

Transition to collapsed tetragonal phase in CaFe2As2 single crystals as seen by 57Fe Mossbauer spectroscopy