Prediction of anomalous LA-TA splitting in electrides

TL;DR

This paper predicts a novel LA-TA phonon splitting in electrides caused by long-range Coulomb interactions, revealing new physical phenomena and potential applications in these unique materials.

Contribution

It introduces a new nonanalytic force correction in electrides leading to LA-TA splitting, expanding understanding of phonon behavior in these materials.

Findings

LA-TA splitting occurs near the zone center in electrides

Not all electrides exhibit detectable splitting

Criteria for the occurrence of splitting are established

Abstract

Electrides are an emerging class of materials with excess electrons localized in interstices and acting as anionic interstitial quasi-atoms (ISQs). The spatial ion-electron separation means that electrides can be treated physically as ionic crystals, and this unusual behavior leads to extraordinary physical and chemical phenomena. Here, a completely different effect in electrides is predicted. By recognizing the long-range Coulomb interactions between matrix atoms and ISQs that are unique in electrides, a nonanalytic correction to the forces exerted on matrix atoms is proposed. This correction gives rise to an LA-TA splitting in the acoustic branch of lattice phonons near the zone center, similar to the well-known LO-TO splitting in the phonon spectra of ionic compounds. The factors that govern this splitting are investigated, with isotropic fcc-Li and anisotropic hP4-Na as the typical examples. It is found that not all electrides can induce a detectable splitting, and criteria are given for this type of splitting. The present prediction unveils the rich phenomena in electrides and could lead to unprecedented applications.