  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.
