  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.
