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Recently, nanostructuration has been proposed to improve the performance of phase change memories. This is the case of superlattices composed of amorphous carbon and crystalline germanium telluride, which we have investigated by molecular dynamics. For this, a modified Stillinger–Weber potential is adapted to reproduce their stiffness contrast/impedance ratio. In order to study the effect of the interface interaction, two sets of parameters are used to model the interfaces with different interactions between the two materials using the properties of the softer material or the average properties between the two creating an adaptation of impedance across the layers. The effects of interface roughness and carbon diffusion at grain boundaries are studied. Using equilibrium molecular dynamics as well as the propagation of wave-packets, we show first that without impedance adaptation, the anisotropy is high, and the roughness has a marked impact on the properties. However, the introduction of impedance adaptation destroys those effects on the thermal conductivity. Finally, we show that the periodic texturing of the interface increases the transmission of in-plane transverse phonons. [ABSTRACT FROM AUTHOR]

Scanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out‐of‐plane oriented Sb2Te3 thin films deposited by sputtering on SiO2 and Si substrates. Nanobeam precession electron diffraction, energy‐dispersive X‐ray spectroscopy, and high‐angle‐annular dark‐field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb2Te3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb2Te3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo‐2D Sb2Te3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low‐frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo‐2D materials, such as Sb2Te3/GeTe superlattices. [ABSTRACT FROM AUTHOR]

A family of solids including crystalline phase change materials such as GeTe and Sb2Te3, topological insulators like Bi2Se3, and halide perovskites such as CsPbI3 possesses an unconventional property portfolio that seems incompatible with ionic, metallic, or covalent bonding. Instead, evidence is found for a bonding mechanism characterized by half‐filled p‐bands and a competition between electron localization and delocalization. Different bonding concepts have recently been suggested based on quantum chemical bonding descriptors which either define the bonds in these solids as electron‐deficient (metavalent) or electron‐rich (hypervalent). This disagreement raises concerns about the accuracy of quantum–chemical bonding descriptors is showed. Here independent of the approach chosen, electron‐deficient bonds govern the materials mentioned above is showed. A detailed analysis of bonding in electron‐rich XeF2 and electron‐deficient GeTe shows that in both cases p‐electrons govern bonding, while s‐electrons only play a minor role. Yet, the properties of the electron‐deficient crystals are very different from molecular crystals of electron‐rich XeF2 or electron‐deficient B2H6. The unique properties of phase change materials and related solids can be attributed to an extended system of half‐filled bonds, providing further arguments as to why a distinct nomenclature such as metavalent bonding is adequate and appropriate for these solids. [ABSTRACT FROM AUTHOR]

There is much to learn from simulation studies of polyamorphism achieved for systems with different bonding environments. Chalcogenide glasses such as Ge–Se glasses undergo an elastic phase transition involving important changes in network connectivity. Stimulated by recent developments of topological constraint theory, we show that the concept of rigidity can be extended to a broader range of thermodynamic conditions including densified glasses. After having validated our structural first principles molecular dynamics models with experimental data over a broad pressure range for GeSe4, we show that the onset of polyamorphism is strongly related to the constraint density measuring the degree of rigidity of the network backbone, while voids and cavities in the structure collapse at very small pressures. This leads to the identification that the progressive onset of higher coordinated species typical of high pressure phases is responsible for the onset of stressed rigidity, although the constraint analysis also indicates progressive stiffening of bonding angles. Results are compared to stoichiometric and stressed rigid GeSe2 and to isostatic As2Se3 and then generalized to other compositions in the Ge–Se binary under pressure. [ABSTRACT FROM AUTHOR]

At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure (1-6). It was predicted [...]

We investigate the dynamical properties of liquid GexSe100-x as a function of Ge content by first-principles molecular dynamic simulations for a certain number of temperatures in the liquid state. The focus is set on ten compositions (where x ≤ 33%) encompassing the reported flexible to rigid and rigid to stressed-rigid transitions. We examine diffusion coefficients, diffusion activation energies, glassy relaxation behavior, and viscosity of these liquids from Van Hove correlation and intermediate scattering functions. At fixed temperature, all properties/functions exhibit an anomalous behavior with Ge content in the region 18%-22%, and provide a direct and quantitative link to the network rigidity. [ABSTRACT FROM AUTHOR]

This document is a reply to a letter questioning the existence of a distinct class of glass-forming materials that do not conform to Zachariasen's conjecture. The authors confirm that there are glass-forming materials, such as GeTe and related compounds, that do not follow Zachariasen's conjecture and exhibit significant changes in bonding upon crystallization. The authors provide evidence supporting their claim, including data on bond rupture obtained by atom probe tomography. They argue that these findings demonstrate that most phase change materials alter their atomic arrangement, bonding, and properties upon crystallization, contradicting the claims made in the letter they are responding to. [Extracted from the article]

Outstanding photovoltaic (PV) materials combine a set of advantageous properties including large optical absorption and high charge carrier mobility, facilitated by small effective masses. Halide perovskites (ABX3, where X = I, Br, or Cl) are among the most promising PV materials. Their optoelectronic properties are governed by the BX bond, which is responsible for the pronounced optical absorption and the small effective masses of the charge carriers. These properties are frequently attributed to the ns2 configuration of the B atom, i.e., Pb 6s2 or Sn 5s2 ("lone‐pair") states. The analysis of the PV properties in conjunction with a quantum‐chemical bond analysis reveals a different scenario. The BX bond differs significantly from ionic, metallic, or conventional 2c2e covalent bonds. Instead it is better regarded as metavalent, since it shares about one p‐electron between adjacent atoms. The resulting σ‐bond, formally a 2c1e bond, is half‐filled, causing pronounced optical absorption. Electron transfer between B and X atoms and lattice distortions open a moderate bandgap resulting in charge carriers with small effective masses. Hence, metavalent bonding explains favorable PV properties of halide perovskites, as summarized in a map for different bond types, which provides a blueprint to design PV materials. [ABSTRACT FROM AUTHOR]

The ovonic threshold switching (OTS) phenomenon, a unique discontinuity of conductivity upon electric‐field application, has been observed in many chalcogenide glasses, some of which are presently used as selector elements in latest ultimate phase‐change memory devices. Herein, ab initio molecular dynamics is used to simulate the structure of two prototypical glasses that are shown to exhibit significantly different OTS properties and switching performance in OTS devices. The first glass, Ge30Se70 (GS), has a typical structure of connected Ge tetrahedra, whereas in the second GS‐based glass that contains antimony and nitrogen, the structure around Ge atoms is quite more complex. By the simulation of the excitation of electrons in the conduction band, slight modifications of the local order are shown to be sufficient to delocalize electronic states. The electron delocalization involving both Ge and Se (as well as Sb atoms in the case of Sb‐containing glass) ensures the percolation of conductive paths for electrons, giving, therefore, to the excited material a metallic behavior. These conductive channels result from the local formation of "metavalent" bonds in the amorphous structure as characterized geometrically and with associated Born effective charges. [ABSTRACT FROM AUTHOR]