Temperature and pressure evolution of the crystal structure of Ax(Fe1-ySe)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction

TL;DR

This study investigates how temperature and pressure affect the crystal structure of Ax(Fe1-ySe)2 compounds with A = Cs, Rb, K, revealing a temperature-induced structural transition and persistent Fe vacancy ordering under high pressure.

Contribution

It provides new insights into the structural transformations and vacancy ordering in Ax(Fe1-ySe)2 compounds under varying temperature and pressure conditions.

Findings

Structural transition from I4/m to I4/mmm at 216°C

Fe vacancy ordering persists up to 120 kbar

Irreversible transition likely due to alkali atom mobility

Abstract

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.