Thermal Disorder and Bond Anharmonicity in Cesium Lead Iodide Studied by Neutron Total Scattering and the Reverse Monte Carlo Method

Cesium lead iodide, CsPbI3, combines promising optoelectronic applications with long-term chemical stability. It has a cubic perovskite crystal structure at temperatures above 600 K. Previous experimental work showed that the I atoms move substantially in directions perpendicular to the Pb-I-Pb linkages, while computational modeling shows that there are soft, anharmonic distortion modes for wave vectors in both the Brillouin zone center and zone boundary. However, there has been no direct experimental probe of the energy profile associated with these distortions. We report here total neutron scattering data from a powder sample of this material as a function of temperature. We use reverse Monte Carlo modeling to construct configurations of atoms consistent with both the local and long-range structure. Our key finding is that the Pb-I bonds are extremely anharmonic, and thus it follows that all near-neighbor bond distributions are significantly asymmetric. The distribution of nearest-neighbor Pb-I distances, with a long tail to distances up to around 120% of the distance corresponding to the peak in the distribution function, can be described as reflecting an underlying anharmonic bond potential energy function which is well represented by the standard Morse potential with reasonable parameters.