Phase separation in Ca1-xLaxFe2As2 superconductors: a Fe-57 Mossbauer study

We report a detailed Fe-57 Mossbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mossbauer spectra of Ca1-xLaxFe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s(-1) and -0.32 mm s(-1) at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1-yLayFe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1-xLaxFe2As2 superconductors.