  Temperature dependent measurements of 57Fe Mossbauer spectra on CaFe2As2
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 ~25% on cooling from room temperature to ~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 CaFe2As2 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.
