Stabilizing low symmetry-based functions of materials at room temperature through isosymmetric electronic bistability

TL;DR

This paper demonstrates how electronic bistability can stabilize low-symmetry ferroic phases at room temperature through high-temperature ferroelastic symmetry-breaking driven by spin-state switching and Jahn-Teller distortions.

Contribution

It reveals a new mechanism for stabilizing low-symmetry ferroic functions at room temperature via isosymmetric electronic bistability in spin-crossover materials.

Findings

Ferroelastic symmetry-breaking occurs at high temperature in a spin-crossover material.

Electronic bistability provides entropy gain to stabilize low-symmetry phases.

DFT and Landau theory explain the cooperative spin-state switching mechanism.

Abstract

Symmetry-breaking is pivotal for controlling ferroelectric, ferroelastic and/or ferromagnetic functions of materials, which enables applications in sensors, memories, transducers or actuators. Commonly, ferroic phases emerge from descending symmetry-breaking, as the laws of thermodynamics dictate that the ordered low entropy phases form at low temperature, which limits practical applications of many materials at room temperature. Rare examples of ascending symmetry-breakings have been observed, but the driving force remains often unclear. Here, we report on a ferroelastic symmetry-breaking occurring at high temperature in a spin-crossover material, studied by magnetic, DSC and X-ray diffraction measurements. Our DFT calculations and our model, based on the Landau theory of phase transitions, explain how the cooperative thermal switching of molecular spin state drives a ferroelastic symmetry breaking at high temperature, through a coupled Jahn-Teller distortion. Ferroelastic materials have rich properties, with important applications in memory, multifunctional and novel controllable devices. The electronic bistability in soft functional materials represents an important source of entropy gain, capable of overcoming the cost of symmetry-breaking entropy, which opens up new perspectives for stabilizing high-temperature and low-symmetry ferroic functions of advanced materials