Pressure-tuning the spatial extension of polarons and charge mobilities in polar insulators

Originating from the coupling of excess charge with the ionic vibrations, polaron formation is a pervasive phenomenon that has been observed in different types of materials. Depending on their spatial extensions, these quasiparticles have conventionally been categorized into small or large polarons typically for polarizable materials with major implications on their physical properties such as charge mobilities. However, a lack of comprehensive and mechanistic understanding of polarons has limited the ability to systematically control the sizes of these quasiparticles. Herein, we employ density functional theory-based calculations to show that hydrostatic pressure can be used as a tool to control the size of polaron wave functions in some lithium halides. More specifically, we show that a transition from a small to a large hole polaron is achievable in lithium bromide through the application of pressure, which can drastically improve their charge mobilities. We attribute the increase in the size of hole polarons to the systematically increasing contributions from states at the top of the valence band (and their increased localization near the Brillouin zone center) with rising pressures. Moreover, our spectral analysis shows that contrary to the traditional theories that predict the coupling of the excess charge primarily with optical phonons, contributions from low-wave vector acoustic modes dominate the large hole polarons at higher pressures in lithium bromide. Our study provides critical insights into the intrinsic mechanisms controlling the size of polarons in pressurized lithium halides, thus opening up an enticing platform for manipulating the material functionalities dictated by polarons in polar semiconductors and insulators.