Lone Pair Rotational Dynamics in Solids

TL;DR

This paper introduces the concept of electronic plastic crystals, where lone pair electrons exhibit orientational motion within a crystalline structure, revealing a new type of dynamic behavior in solids.

Contribution

It defines electronic plastic crystals by coupling electronic and nuclear structures and predicts their existence in halogen crystals and halide perovskites using ab initio simulations.

Findings

Electronic lone pairs show orientational motion at finite temperatures.

Predicted electronic plastic crystal behavior in halogen crystals and perovskites.

Charge density fluctuations could be observed via synchrotron scattering.

Abstract

Traditional classifications of crystalline phases focus on nuclear degrees of freedom. Through examination of both electronic and nuclear structure, we introduce the concept of an electronic plastic crystal. Such a material is classified by crystalline nuclear structure, while localized electronic degrees of freedom - here lone pairs - exhibit orientational motion at finite temperatures. This orientational motion is an emergent phenomenon arising from the coupling between electronic structure and polarization fluctuations generated by collective motions, such as phonons. Using ab initio molecular dynamics simulations, we predict the existence of electronic plastic crystal motion in halogen crystals and halide perovskites, and suggest that such motion may be found in a broad range of solids with lone pair electrons. Such fluctuations in the charge density should be observable, in principle via synchrotron scattering.