Phonon density of states of magnetite (\ce{Fe3O4}) nanoparticles via molecular dynamics simulations
Pablo Galaviz, Kyle A. Portwin, Dehong Yu, Kirrily C. Rule, David L. Cortie, Zhenxiang Cheng
TL;DR
This work probes the phonon density of states (PDOS) in Fe3O4 magnetite nanoparticles using classical molecular dynamics across multiple force fields, benchmarked against experimental X-ray and neutron scattering and density functional theory references. It systematically analyzes how nanoparticle size, temperature, and surface-adsorbed water alter the PDOS, revealing pronounced broadening and softening with decreasing size and substantial water-induced broadening and high-energy extension; temperature yields only minor changes, mainly in the oxygen-dominated region. DFT calculations for bulk magnetite provide a reference to evaluate force-field performance, showing that simple Lennard-Jones potentials capture overall PDOS reasonably well and align with DFT, while ReaxFF variants better reproduce Fe features but shift O contributions. The study also highlights that single-particle models suffice to represent clusters, while surface water can induce significant chemical interaction and diffusion depending on the force field. Overall, these findings advance understanding of magnetite nanoparticle vibrational behavior under realistic surface conditions and offer practical guidance for MD modeling of PDOS in similar systems.
Abstract
This study presents a comprehensive computational investigation of magnetite nanoparticles, systematically evaluating a range of force fields against experimental results. We analyze the influence of particle size, temperature, and surface-adsorbed water molecules on the structural and dynamic properties of the nanoparticles. We performed classical molecular dynamics of nanoparticles and bulk magnetite and utilized density functional theory calculations for bulk magnetite for comparison. Our results reveal that nanoparticle size and the presence of adsorbed water molecules have a pronounced impact on the density of states. Specifically, as the nanoparticle size is decreased, phonon modes exhibit significant broadening and softening, which is attributable to reduced phonon lifetimes resulting from enhanced boundary scattering. The incorporation of water further broadens the density of states and extends the spectra to higher energy regions. Temperature variations result in a slight broadening and softening of the phonon density of states, particularly in the oxygen-dominated region, which is attributed to phonon anharmonicity.