Vacancy-induced nanoscale phase separation in KxFe2−ySe2

Polarized Raman scattering spectra of K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ 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 K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ 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.