Intrinsic crystal phase separation in the antiferromagnetic superconductor RbyFe2-xSe2: a diffraction study

The crystal and magnetic structures of the superconducting iron-based chalcogenides RbyFe2-xSe2 have been studied by means of single-crystal synchrotron x-ray and high-resolution neutron powder diffraction in the temperature range 2-570 K. The ground state of the crystal is an intrinsically phase-separated state with two distinct-by-symmetry phases. The main phase has the iron vacancy ordered root 5 x root 5 superstructure (I4/m space group) with AFM ordered Fe spins. The minority phase does not have root 5 x root 5-type of ordering and has a smaller in-plane lattice constant a and larger tetragonal c-axis and can be well described by assuming the parent average vacancy disordered structure (I4/mmm space group) with the refined stoichiometry Rb-0.60(5)(Fe1.10(5)Se)(2). The minority phase amounts to 8-10% mass fraction. The unit cell volume of the minority phase is 3.2% smaller than the one of the main phase at T = 2 K and has quite different temperature dependence. The minority phase merges with the main vacancy ordered phase on heating above the phase separation temperature T-P = 475 K. The spatial dimensions of the phase domains strongly increase above T-P from 1000 to > 2500 angstrom due to the integration of the regions of the main phase that were separated by the second phase at low temperatures. Additional annealing of the crystals at a temperature T = 488 K, close to T-P, for a long time drastically reduces the amount of the minority phase.