Thermal disorder and anharmonicity of cesium iodide in EXAFS theory

In extended X-ray absorption fine structure (EXAFS) theory, the thermal disorder can be described by means of the moments of atomic displacement so-called EXAFS cumulants. In this work, thermal disorder and anharmonicity of cesium iodide (CsI) have been studied based on the anharmonic correlated Debye model in EXAFS theory. Analytical expressions of the first three EXAFS cumulants and the anharmonic factor were derived. Numerical calculations have been performed to determine these thermodynamic quantities of CsI up to temperature 700 K using the Lennard-Jones potential combined with Coulomb interaction contribution. It showed that the anharmonicity of thermal lattice vibrations gave significant contributions to the EXAFS cumulants. By investigation of the anharmonic factor in EXAFS theory we highlight the importance of anharmonicity not only at high temperature but also at low temperature in CsI crystal. This behavior of cesium iodide departs from copper metal, where the anharmonic contribution was unveiled almost zero at temperature below 100 K. •Anharmonicity and thermal disorder of CsI in EXAFS theory have been studied.•Anharmonicity gives significant contributions to EXAFS cumulants.•Importance of anharmonicity has been highlighted at both high and low temperature.