Pressure effects on the electronic, optical, and thermodynamic properties in van der waals multiferroic NiI2

Abstract

Recently, NiI₂ was demonstrated as the first purely 2D multiferroic material, offering new insights into the interplay between magnetic and ferroelectric order in two-dimensional systems, and showcasing its potential as a versatile material for next-generation spintronic and multifunctional devices. In this work, we investigate the electronic, optical, and thermodynamic properties of the newly identified van der Waals multiferroic material NiI₂ using ab-initio calculations, focusing on the impact of applied pressure on its properties. From an electronic perspective, NiI₂ undergoes a transition from a semiconductor (1.1 eV) to a metallic state at 25 GPa due to pressure-induced modifications of the electronic structure, particularly the hybridization of Ni 3d and I 5p orbitals. The study also investigates the anisotropic behavior of NiI₂ under pressure, the transition occurring at 15 GPa along the ab-plane and at 13 GPa along the c-axis. This demonstrates the material's distinct response to pressure in different crystallographic directions. The effect of hydrostatic pressure on the magnetic properties of NiI₂ was also examined, revealing a reinforcement of antiferromagnetic interactions with increasing pressure. Moreover, significant enhancements are observed in the material's optical characteristics, particularly in its optical conductivity, the material exhibits an increase from 1221 (Ω cm)⁻¹ at 0 GPa to 2169 (Ω cm)⁻¹ at 9 GPa. Similarly, the absorption coefficient spectrum improves from 1.2 × 10⁵ cm⁻¹ at 0 GPa to 1.8 × 10⁵ cm⁻¹ at 9 GPa, around 2 eV. Interestingly, the static refractive index also shows a significant enhancement, increasing from 3.6 under ambient pressure to 4.2 at 9 GPa. These changes are attributed to structural modifications and increased electron delocalization. Additionally, thermodynamic analysis, including heat capacity and entropy, provides key insights into the material's behavior under pressure. The heat capacity increases linearly with temperature, following the Dulong-Petit law, while the normalized heat capacity exhibits a peak that is in line with a magnetic phase transition at around 59 K, a peak that shifts with pressure. These findings highlight the potential of NiI₂ for multifunctional devices, where pressure plays a crucial role in tuning its properties.