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 Cs0.83(Fe0.86Se)2 revealed first order I4/m to I4/mmm structural transformation around 216{\deg}C associated with the disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of intercalated Alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs0.83(Fe1-ySe)2, Rb0.85(Fe1-ySe)2 and K0.8(Fe1-ySe)2 (y ~ 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 non-superconductive states. 
V. Svitlyk, D. Chernyshov, E. Pomjakushina, A. Krzton-Maziopa, K.   Conder, V. Pomjakushin, V. Dmitriev
cond-mat.mtrl-sci