Temperature and Pressure Evolution of the Crystal Structure of Ax(Fe1-ySe)2 (A = Cs, Rb, K) Studied by Synchrotron Powder Diffraction

Temperature-dependent synchrotron powder diffraction on Cs-0.83(Fe0.86Se)(2) revealed first-order I4/m to I4/mmm structural transformation around 216 degrees C associated with a disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of the intercalated alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs-0.83(Fe1-ySe)(2), Rb-0.85(Fe1-ySe)(2), and K-0.8(Fe1-ySe)(2) (y similar to 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and nonsuperconductive states.