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

TL;DR

This study reveals that RbyFe2-xSe2 superconductors intrinsically separate into two distinct crystal phases with different structures and magnetic orderings, and these phases can merge or change with temperature and annealing.

Contribution

It provides detailed diffraction evidence of intrinsic phase separation and temperature-dependent phase evolution in RbyFe2-xSe2 superconductors, highlighting the complex structural behavior.

Findings

Main phase has x superstructure with AFM order

Minority phase is vacancy disordered with smaller in-plane lattice

Phase domains grow and merge above T_P = 475 K

Abstract

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 \surd5x\surd5 superstructure (I4/m space group) with AFM ordered Fe spins. The minority phase does not have \surd5x\surd5-type of ordering and has smaller in plane lattice constant a and larger tetragonal c-axis and can be well described assuming the parent average vacancy disordered structure (I4/mmm space group) with the refined stoichiometry Rb0.60(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 transforms to 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 TP from 1000 \AA\ to > 2500 \AA\ due to the merging of the regions of the main phase that were separated by the second phase at low temperatures. Additional annealing of the crystals at the temperature T=488 K close to T_P for the long time drastically reduces the amount of the minority phase.