Atomistic Description of Spin Crossover Under Pressure and its Giant Barocaloric Effect

TL;DR

This study uses computational methods to analyze how pressure affects spin crossover transitions in a specific complex, revealing mechanisms behind the giant barocaloric effect and proposing a protocol for discovering new materials with enhanced BCE.

Contribution

It introduces a computational protocol to accurately model the barocaloric effect in spin crossover materials, aiding rapid discovery of high-performance SCO compounds.

Findings

Pressure influences the SCO transition and phase behavior.

Low-frequency phonons play a key role in the barocaloric effect.

The study provides a computational approach for material discovery.

Abstract

The pressure-dependent evolution of the spin crossover (SCO) transition has garnered significant interest due to its connection to the giant barocaloric effect (BCE) near room temperature. Pressure alters both the molecular and solid-state structures of SCO materials, affecting the relative stability of low- and high-spin states and, consequently, the transition temperature ($T_{1/2}$). Crucially, the shape of the $T_{1/2}$ vs. pressure curve dictates the magnitude of the BCE, making its accurate characterization essential for identifying high-performance materials. In this work, we investigate the nonlinear $T_{1/2}$ vs. pressure behavior of the prototypical SCO complex [FeL$_2$][BF$_4$]$_2$ [L = 2,6-di(pyrazol-1-yl)pyridine] using solid-state PBE+U computations. Our results unveil the mechanisms by which pressure influences its SCO transition, including the onset of a phase transition, as well as the key role of low-frequency phonons in the BCE. Furthermore, we establish a computational protocol for accurately modeling the BCE in SCO crystals, providing a powerful tool for the rapid and efficient discovery of new materials with enhanced barocaloric performance.