Electronic Origin of Non-Zone-Center Phonon Condensations: Octahedral Rotations as A Case Study

TL;DR

This paper explains the electronic origin of non-zone-center phonon condensations, specifically octahedral rotations, through bonding interactions between Bloch states, providing a universal mechanism for structural phase transitions.

Contribution

It introduces a new electronic bonding mechanism based on Jahn--Teller effects that explains zone-boundary phonon instabilities, supported by first-principles calculations.

Findings

Bonding interactions between different wavevector Bloch states cause phonon instabilities.

Octahedral rotations are driven by this electronic mechanism.

The mechanism is universal for non-zone-center phonon condensations.

Abstract

Unstable zone-boundary phonon modes drive atomic displacements linked to a rich array of properties. Yet, the electronic origin of the instability remains to be clearly explained. Here, we propose that bonding interaction between Bloch states belonging to different wavevectors leads to such instability via the pseudo- or second-order Jahn--Teller effect. Our first-principles calculations and representation theory-based analyses show that rotations of anion coordinated octahedra, an archetypal example of zone-boundary phonon condensations, are induced by this bonding mechanism. The proposed mechanism is universal to any non-zone-center phonon condensations and could offer a general approach to understand the origin of structural phase transitions in crystals.