Structural Phase Transitions and Metallized Phenomena in Arsenic Telluride under High Pressure

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Phase Transition and Metallization of Orpiment by Raman Spectroscopy, Electrical Conductivity and Theoretical Calculation under High Pressure

The structural, vibrational, and electronic characteristics in orpiment were performed in the diamond anvil cell (DAC), combined with a series of experimental and theoretical research, including Raman spectroscopy, impedance spectroscopy, atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and first-principles theoretical calculations. The isostructural phase transition at ~25.0 GPa was manifested as noticeable changes in the compressibility, bond lengths, and slope of the conductivity, as well as in a continuous change in the pressure dependence of the unit cell volume. Furthermore, a pressure-induced metallization occurred at ~42.0 GPa, accompanied by reversible electrical conductivity. We also determined the metallicity of orpiment at 45.0 GPa by first-principles theoretical calculations, and the results were in good agreement with the results of the temperature-dependent conductivity measurements. The HRTEM and AFM images of the recovered sample confirmed that orpiment remains in the crystalline phase with an intact layered structure and available crystal-shaped clusters. These high-pressure behaviors of orpiment present some crucial information on the structural phase transition, metallization, amorphization and superconductivity for the A₂B₃-type of engineering materials at high pressure.

Monolayer AsTe2 : Stable Robust Metal in 2D, 1D and 0D

The structural, phononic, and electronic properties of the monolayer structures of AsTe₂ are characterized by performing density functional theory (DFT) calculations. Total energy optimization and phonon calculations reveal that single layers of the 2H-AsTe₂ and 1T-AsTe₂ phases form dynamically stable crystal structures. Electronic structure analysis also shows that both 2H and 1T phases have nonmagnetic metallic character. It is also predicted that the metallic nature of the ultra-thin both 2H-AsTe₂ and 1T-AsTe₂ structures remain unchanged even under high biaxial strain values. For further examination of the dimensionality effect in the robust metallicity in 2D AsTe₂ phases, electronic characteristics of 1D nanoribbons and 0D quantum dots are also investigated. It is found that independent from the dimension and crystallographic orientations 0D and 1D structures of 2H- and 1T-AsTe₂ structures have metallic behavior. It is found that single layers of AsTe₂ are quite promising materials for nanodevice applications owing to the robust metallic character.

Structural phase transitions of (Bi1-xSbx)2(Te1-ySey)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

Recently, A₂B₃-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi₂Te₂Se, BiSbTeSe₂, and Sb₂Te₂Se tetradymites under high pressure. Bi₂Te₂Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi₂Te₃. Thus, the compression behavior of Bi₂Te₂Se is the same as that of Bi₂Se₃, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe₂ and Sb₂Te₂Se undergo similar structural phase transitions to Bi₂Te₂Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A₂B₃-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. The influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.