Vacancy-induced nanoscale phase separation in KxFe2-ySe2 single crystals evidenced by Raman scattering and powder x-ray diffraction

Polarized Raman scattering spectra of KxFe2-ySe2 were analyzed in terms of peculiarities of both I4/m and I4/mmm space group symmetries. The presence of the Raman active modes from both space group symmetries (16 Raman-active modes of the I4/m phase and two Raman-active modes of the I4/mmm phase) confirmed the existence of two crystallographic domains with different space group symmetry in a KxFe2-ySe2 sample. High-resolution synchrotron powder x-ray diffraction structural refinement of the same sample confirmed the two-phase description, and determined the atomic positions and occupancies for both domains.