Phase coexistence in Cs$_0.8$Fe$_1.6$Se$_2$ as seen by x-ray mapping of reciprocal space

AxFe2−ySe2 (A = K, Rb, Cs) superconductors are frequently complex mixtures of phases even in the form usually described as a single crystal. Phases, resulting from phase separation, metrically are very similar, so their detailed characterization is a delicate task, and the existing data are typically scarce. We characterize the room-temperature phase-separated state of Cs0.8Fe1.6Se2 in reciprocal space by means of single-crystal diffraction of synchrotron radiation. Using the arguments of commensurability and detailed analysis of twinning patterns, we augment the previous findings by quantifying the intergrowth state, consisting of the tetragonal phase with ordered Fe vacancies and the minor disordered phase. Compared to the main phase, the minor one is compressed in the tetragonal a-b plane and expanded along the c direction; a set of modulated Bragg rods evidences a planar disorder. Fourfold splitting of the rods and main Bragg peaks implies a rotational twinning; close inspection of the lattice metric indicates that the symmetry of the minor phase is not higher than monoclinic, with a deviation from the orthogonal basis of ∼0.25°.