# Strain–Phonon Cooperation as a Necessary Ingredient to Understand the Jahn–Teller Effect in Solids

## TL;DR

This paper explains how strain and vibrations in KCuF3 crystals are key to understanding their structure and properties, challenging previous theories.

## Contribution

The paper introduces a new understanding of lattice properties in KCuF3 based on electron-vibration interactions and strain-phonon coupling.

## Key findings

- Structural and magnetic properties of KCuF3 arise from electron-vibration interactions.
- Coupling between lattice strain and phonon distortions is essential for stable configurations.
- A new low-energy phase in KCuF3 is predicted, alterable via strain engineering.

## Abstract

Spatial degeneracy
is the cause of the complex electronic, geometrical,
and magnetic structures found in a number of materials whose more
representative example is KCuF3. In the literature the
properties of this lattice are usually explained through the Kugel–-Khomskii
model, based on superexchange interactions. Here we provide rigorous
theoretical and computational arguments against this view proving
that structural and magnetic properties essentially arise from electron–vibration
(vibronic) interactions. Moreover, based on the work of Öpik
and Pryce, we show that the coupling between lattice (homogeneous
strain) and motif (phonons) distortions is essential to understand
the main stable configurations of the lattice. Using this information,
we predict a new low-energy phase in KCuF3 that could strongly
alter its properties and provide guidance on how to stabilize it through
strain engineering.