Characterization and Decomposition of the Natural van der Waals SnSb2Te4 under Compression

Synergistic Improvement of BiI3 and In on Thermoelectric Properties of Zone-Melted n-Type Bi2Te2.7Se0.3

Bismuth telluride (Bi2Te3) is the only commercial thermoelectric material so far, and it is also the best thermoelectric material with the best performance at room temperature. However, up to now, the zT value of n-type materials used on a large scale is only about 1.0; this makes the thermoelectric conversion efficiency of thermoelectric devices and thermoelectric applications stagnant. Therefore, under the synergistic action of BiI3 and In, the properties of n-type Bi2Te2.7Se0.3 material are improved. The experiments show that BiI3, which is nontoxic and non-absorbent, can effectively improve the power factor of the material and inhibit the bipolar effect and is an effective dopant. After the inclusion of In, due to the low bond energy of the In-Te bond, it is easy to form the InTe phase in the matrix material and then introduce the second phase, and the presence of the second phase in the material will scatter phonons and reduce the lattice thermal conductivity so that it can reach 0.31 W m-1 K-1 at 350 K. Ultimately, a high maximum zT of 1.20 at 325 K and a remarkable average zT of 1.04 (300-500 K) are attained in the In0.005Bi1.995Te2.7Se0.3 + 0.13 wt % BiI3 sample.

Electron-Deficient Multicenter Bonding in Phase Change Materials: A Chance for Reconciliation

In the last few years, a controversy has been raised regarding the nature of the chemical bonding present in phase change materials (PCMs), many of which are minerals such as galena (PbS), clausthalite (PbSe), and altaite (PbTe). Two opposite bonding models have claimed to be able to explain the extraordinary properties of PCMs in the last decade: the hypervalent (electron-rich multicenter) bonding model and the metavalent (electron-deficient) bonding model. In this context, a third bonding model, the electron-deficient multicenter bonding model, has been recently added. In this work, we comment on the pros and cons of the hypervalent and metavalent bonding models and briefly review the three approaches. We suggest that both hypervalent and metavalent bonding models can be reconciled with the third way, which considers that PCMs are governed by electron-deficient multicenter bonds. To help supporters of the metavalent and hypervalent bonding model to change their minds, we have commented on the chemical bonding in GeSe and SnSe under pressure and in several polyiodides with different sizes and geometries.

Pressure-driven dome-shaped superconductivity in topological insulator GeBi2Te4

The discovery of new superconductors based on topological insulators always captures special attention due to their unique structural and electronic properties. High pressure is an effective way to regulate the lattice as well as electronic states in the topological insulators, thus altering their electronic properties. Herein, we report the structural and electrical transport properties of the topological insulator GeBi2Te4by using high-pressure techniques. The synchrotron x-ray diffraction revealed that GeBi2Te4underwent two structural phase transitions fromR-3m(phase I) toC2/m(phase II) and then intoIm-3m(phase III). Superconductivity was observed at 6.6 GPa to be associated with the first structural phase transition. The superconducting transition temperatureTcreached a maximum value of 8.4 K, accompanied by theRHsign changing from negative to positive at 14.6 GPa, then gradually decreased with increasing pressure in phase III, showing a dome-shaped phase diagram. The present results provide a platform for understanding the interplay between the crystal structure and superconductivity by the regulation of pressure in the topological insulator materials.

Phase Separation Prevents the Synthesis of VBi2Te4 by Molecular Beam Epitaxy

Intrinsic magnetic topological insulators (IMTIs) have a non-trivial band topology in combination with magnetic order. This potentially leads to fascinating states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. One of the theoretically predicted IMTIs is VBi2Te4, but experimental evidence of this material is lacking so far. Here, we report on our attempts to synthesise VBi2Te4 by molecular beam epitaxy (MBE). X-ray diffraction reveals that in the thermodynamic phase space reachable by MBE, there is no region where VBi2Te4 is stably synthesised. Moreover, scanning transmission electron microscopy shows a clear phase separation to Bi2Te3 and VTe2 instead of the formation of VBi2Te4. We suggest the phase instability to be due to either the large lattice mismatch between VTe2 and Bi2Te3 or the unfavourable valence state of vanadium.

Revisiting the Nature of Chemical Bonding in Chalcogenides to Explain and Design their Properties

Quantum chemical bonding descriptors have recently been utilized to design materials with tailored properties. Their usage to facilitate a quantitative description of bonding in chalcogenides as well as the transition between different bonding mechanisms is reviewed. More importantly, these descriptors can also be employed as property predictors for several important material characteristics, including optical and transport properties. Hence, these quantum chemical bonding descriptors can be utilized to tailor material properties of chalcogenides relevant for thermoelectrics, photovoltaics, and phase-change memories. Relating material properties to bonding mechanisms also shows that there is a class of materials, which are characterized by unconventional properties such as a pronounced anharmonicity, a large chemical bond polarizability, and strong optical absorption. This unusual property portfolio is attributed to a novel bonding mechanism, fundamentally different from ionic, metallic, and covalent bonding, which is called "metavalent." In the concluding section, a number of promising research directions are sketched, which explore the nature of the property changes upon changing bonding mechanism and extend the concept of quantum chemical property predictors to more complex compounds.

Pressure induced topological and topological crystalline insulators

Research on topological and topological crystalline insulators (TCIs) is one of the most intense and exciting topics due to its fascinating fundamental science and potential technological applications. Pressure (strain) is one potential pathway to induce the non-trivial topological phases in some topologically trivial (normal) insulating or semiconducting materials. In the last ten years, there have been substantial theoretical and experimental efforts from condensed-matter scientists to characterize and understand pressure-induced topological quantum phase transitions (TQPTs). In particular, a promising enhancement of the thermoelectric performance through pressure-induced TQPT has been recently realized; thus evidencing the importance of this subject in society. Since the pressure effect can be mimicked by chemical doping or substitution in many cases, these results have opened a new route to develop more efficient materials for harvesting green energy at ambient conditions. Therefore, a detailed understanding of the mechanism of pressure-induced TQPTs in various classes of materials with spin-orbit interaction is crucial to improve their properties for technological implementations. Hence, this review focuses on the emerging area of pressure-induced TQPTs to provide a comprehensive understanding of this subject from both theoretical and experimental points of view. In particular, it covers the Raman signatures of detecting the topological transitions (under pressure), some of the important pressure-induced topological and TCIs of the various classes of spin-orbit coupling materials, and provide future research directions in this interesting field.

Metavalent bonding in chalcogenides: DFT-chemical pressure approach

Understanding the chemical bond nature has attracted considerable attention as it is crucial to analyze and comprehend the different physical and chemical properties of materials. This work is considered a complementary part of our previous work in studying the nature of different types of bonding interactions in a wide variety of molecules and materials using the DFT Chemical Pressure (CP) approach. Recently, a new type of chemical bond, the metavalent bond (MVB), has been defined. We show how the CP formalism can be used to analyze and study the establishment of MVB in two chalcogenides, GeSe and PbSe, in a similar fashion as the electron localization function (ELF) profiles. This is accomplished by analyzing the CP maps of these two chalcogenides at different pressures (up to 40 GPa for GeSe and 10 GPa for PbSe). The CP maps show distinctive features related to the MVB, providing insights into the existence of such chemical interaction in the crystal structure of the two compounds. Similar to ELF profiles, CP maps can visualize and track the strength of the MVB in GeSe and PbSe under pressure.

Combined Experimental and Theoretical Studies: Lattice-Dynamical Studies at High Pressures with the Help of Ab Initio Calculations

Lattice dynamics studies are important for the proper characterization of materials, since these studies provide information on the structure and chemistry of materials via their vibrational properties. These studies are complementary to structural characterization, usually by means of electron, neutron, or X-ray diffraction measurements. In particular, Raman scattering and infrared absorption measurements are very powerful, and are the most common and easy techniques to obtain information on the vibrational modes at the Brillouin zone center. Unfortunately, many materials, like most minerals, cannot be obtained in a single crystal form, and one cannot play with the different scattering geometries in order to make a complete characterization of the Raman scattering tensor of the material. For this reason, the vibrational properties of many materials, some of them known for millennia, are poorly known even under room conditions. In this paper, we show that, although it seems contradictory, the combination of experimental and theoretical studies, like Raman scattering experiments conducted at high pressure and ab initio calculations, is of great help to obtain information on the vibrational properties of materials at different pressures, including at room pressure. The present paper does not include new experimental or computational results. Its focus is on stressing the importance of combined experimental and computational approaches to understand materials properties. For this purpose, we show examples of materials already studied in different fields, including some hot topic areas such as phase change materials, thermoelectric materials, topological insulators, and new subjects as metavalent bonding.

Structural, vibrational and electronic properties of α'-Ga2S3 under compression

We report a joint experimental and theoretical study of the low-pressure phase of α'-Ga₂S₃ under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α'-Ga₂S₃ at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga-S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α'-Ga₂S₃. We found that the structural channels are responsible for the anisotropic properties of α'-Ga₂S₃ and the A'(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α'-Ga₂S₃ at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga₂Se₃. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α'-Ga₂S₃ is stressed.

High-Pressure Spectroscopy Study of Zn(IO3)2 Using Far-Infrared Synchrotron Radiation

We report the first high-pressure spectroscopy study on Zn(IO3)2 using synchrotron far-infrared radiation. Spectroscopy was conducted up to pressures of 17 GPa at room temperature. Twenty-five phonons were identified below 600 cm−1 for the initial monoclinic low-pressure polymorph of Zn(IO3)2. The pressure response of the modes with wavenumbers above 150 cm−1 has been characterized, with modes exhibiting non-linear responses and frequency discontinuities that have been proposed to be related to the existence of phase transitions. Analysis of the high-pressure spectra acquired on compression indicates that Zn(IO3)2 undergoes subtle phase transitions around 3 and 8 GPa, followed by a more drastic transition around 13 GPa.