Pressure-induced two-step spin crossover in double-layered elastic model

TL;DR

This study models pressure-induced two-step spin crossover in a double-layered elastic system, revealing an intermediate phase with striped clusters, relevant to transition metal materials and multistep transitions.

Contribution

It introduces a simplified elastic model showing how pressure induces a two-step spin crossover with an intermediate striped phase, connecting to real materials.

Findings

Pressure transforms the transition into a two-step process.

An intermediate striped spin phase emerges under pressure.

The model explains multistep spin transitions in materials.

Abstract

We study the two-step spin crossover in a double-layered elastic model based on transition metal complexes each taking high spin (HS) and low spin (LS) states. Here, only the simplest elastic interactions between adjacent molecules are considered and the system is exposed to the external pressure within the framework of $NPT$-Monte Carlo method. As a certain amount of pressure is applied, the first order thermal transition between uniform HS and LS phases transforms to a two-step transition with an emergent intermediate spin (IS) phase, where the HS and LS molecules are paired face to face between layers and form diagonally striped clusters within the layer. The difference in the size of HS and LS molecules is reflected both in the elastic interactions and in the enthalpy, and the IS phase could gain the latter over the loss of the former by significantly reducing its volume. The present pressure effect is interpreted to the chemical one in double-layered transition metal materials, which actually reveals a variety of multistep spin crossover transitions relevant to our numerical result.