Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Novel structural phases and the properties of LaX (X = P, As) under high pressure: first-principles study

The particle swarm optimization algorithm and density functional theory (DFT) are extensively performed to determine the structures, phase transition, mechanical stability, electronic structures, and thermodynamic properties of lanthanide phosphates (LaP and LaAs) in the pressure range of 0 to 100 GPa. Two novel high-pressure structures of LaP and LaAs are first reported here. It is found that LaX (X = P, As) undergo a phase transition from NaCl-type structure (Fm3m) to CsCl-type structure (P4/mmm) at 19.04 GPa and 17.22 GPa, respectively. With the elevation of the pressure, C2/m-LaP and Imma-LaAs are the most stable structures up to 70.08 GPa and 85.53 GPa, respectively. Finally, the analysis of the elastic constants and hardness confirms that the C2/m-LaP possesses hardness values up to 23.24 GPa due to the strong covalent P-P bonding and ionic La-P bonding, indicating that it is a potential hard material.

Two-Dimensional Ferroics and Multiferroics: Platforms for New Physics and Applications

Two-dimensional (2D) ferroics, including ferromagnets, ferroelectrics, ferroelastics, and multiferroics, recently have been theoretically proposed or experimentally revealed. The research has attracted tremendous attention because of the novel physics and promising applications for nanoelectronics, revealing ferroics in the 2D limit. In the present Perspective, we comprehensively review the recent research progress and also the proposed applications of 2D ferromagnetic, ferroelectric, and ferroelastic materials from theoretical and experimental viewpoints. We then introduce the coupling between ferroic orders and highlight the latest research on 2D multiferroic materials. The promising research directions and outlooks are discussed at the end of the Perspective. It is expected that the comprehensive overview of 2D ferroic materials can provide guidelines for researchers in the area and inspire further explorations of new physics and ferroic devices.