Role of Surface Effects in the Vibrational Density of States and the Vibrational Entropy in Spin Crossover Nanomaterials: A Molecular Dynamics Investigation

Alaa Fahs,Gábor Molnár,Azzedine Bousseksou,William Nicolazzi

doi:10.3390/magnetochemistry7020027

Abstract

Size reduction effects on the lattice dynamics of spin crossover (SCO) thin films have been investigated through molecular dynamics (MD) simulations of the density of vibrational states. The proposed simple model structure and reduced force field allows us to obtain good orders of magnitude of the sound velocity in both spin states and takes into account the contribution of free surfaces in the vibrational properties of very thin films (below a thickness of 12 nm). The slab method issue from the field of surface physico-chemistry has been employed to extract surface thermodynamic quantities. In combination with the related slab-adapted method, the slab approach provides a powerful numerical tool to separate surface contributions from finite-size effects. Due to the relatively low stiffness of SCO materials, the lattice dynamics seems to be governed by surface instead of confinement effects. The size evolution of thermodynamic quantities is successfully reproduced, especially the increase of the vibrational entropy with the size reduction, in good agreement with experimental observations.

Highlights

Size reduction effects on the lattice dynamics of spin crossover (SCO) thin films have been investigated through molecular dynamics (MD) simulations of the density of vibrational states
Even if the slab adapted method assures the absence of surface effects and simulates a virtually infinite medium, the system needs to contain a sufficient number of lattice degrees of freedom to obtain a large number of vibrational states
The lattice dynamics of SCO nanomaterials have been studied through molecular dynamics simulations by adapting the so-called slab method largely employed in surface physics

Summary

Introduction

Switchable molecular phenomena are destined to play a key role in various societal applications [1,2,3]. Collective elastic behaviors, usually called cooperativity, originated from the molecular structural difference between the two spin states are able to induce first order phase transitions with hysteretic phenomena [7,8]. Owing to these properties, SCO materials are expected to be candidates for future applications in data processing, photonic or spintronic devices as well as actuators [9,10,11].

Molecular Dynamics Simulations and Slab Method

Results

Conclusions