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

TitleTransition to collapsed tetragonal phase in CaFe2As2 single crystals as seen by Fe-57 Mossbauer spectroscopy
Publication TypeJournal Article
Year of Publication2016
AuthorsBud'ko, SL, Ma, XM, Tomic, M, Ran, S, Valenti, R, Canfield, PC
JournalPhysical Review B
Date Published01
Type of ArticleArticle
ISBN Number1098-0121
Accession NumberWOS:000368484300003
Keywordscomplexes, density-functional theory, dft, iron compounds, isomer-shifts, methane monooxygenase, model, prediction, quadrupole splittings, spectra

Temperature dependent measurements of Fe-57 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 similar to 25% on cooling from room temperature to similar 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.

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Complex States