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The crystal and magnetic structures of the superconducting iron basedchalcogenides RbyFe2-xSe2 have been studied by means of single crystalsynchrotron x-ray and high resolution neutron powder diffraction in thetemperature range 2-570 K. The ground state of the crystal is an intrinsicallyphase separated state with two distinct by symmetry phases. The main phase hasthe iron vacancy ordered \surd5x\surd5 superstructure (I4/m space group) withAFM ordered Fe spins. The minority phase does not have \surd5x\surd5-type ofordering and has smaller in plane lattice constant a and larger tetragonalc-axis and can be well described assuming the parent average vacancy disorderedstructure (I4/mmm space group) with the refined stoichiometryRb0.60(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 themain phase at T = 2 K and has quite different temperature dependence. Theminority phase transforms to the main vacancy ordered phase on heating abovethe phase separation temperature T_P = 475 K. The spatial dimensions of thephase domains strongly increase above TP from 1000 \AA\ to > 2500 \AA\ due tothe merging of the regions of the main phase that were separated by the secondphase at low temperatures. Additional annealing of the crystals at thetemperature T=488 K close to T_P for the long time drastically reduces theamount of the minority phase.

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