Hypervalency in amorphous chalcogenides

Mechanism for Local-Atomic Structure Changes in Chalcogenide-based Threshold-Switching Devices

Threshold-switching devices based on amorphous chalcogenides are considered for use as selector devices in 3D crossbar memories. However, the fundamental understanding of amorphous chalcogenide is hindered owing to the complexity of the local structures and difficulties in the trap analysis of multinary compounds. Furthermore, after threshold switching, the local structures gradually evolve to more stable energy states owing to the unstable homopolar bonds. Herein, based on trap analysis, DFT simulations, and operando XPS analysis, it is determined that the threshold switching mechanism is deeply related to the charged state of Se-Se homopolar defects. A threshold switching device is demonstrated with an excellent performance through the modification of the local structure via the addition of alloying elements and investigating the time-dependent trap evolution. The results concerning the trap dynamics of local atomic structures in threshold switching phenomena may be used to improve the design of amorphous chalcogenides.

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.

Chemical bonding in phase-change chalcogenides

Almost all phase-change memory materials (PCM) contain chalcogen atoms, and their chemical bonds have been denoted both as 'electron-deficient' [sometimes referred to as 'metavalent'] and 'electron-rich' ['hypervalent', multicentre]. The latter involve lone-pair electrons. We have performed calculations that can discriminate unambiguously between these two classes of bond and have shown that PCM have electron-rich, 3c-4e ('hypervalent') bonds. Plots of charge transferred between (ET) and shared with (ES) neighbouring atoms cannot on their own distinguish between 'metavalent' and 'hypervalent' bonds, both of which involve single-electron bonds. PCM do not exhibit 'metavalent' bonding and are not electron-deficient; the bonding is electron-rich of the 'hypervalent' or multicentre type.

Structure of Amorphous Selenium: Small Ring, Big Controversy

Selenium (Se) discovered in 1817 belongs to the family of chalcogens. Surprisingly, despite the long history of over two centuries and the chemical simplicity of Se, the structure of amorphous Se (a-Se) remains controversial to date regarding the dominance of chains versus rings. Here, we find that vapor-deposited a-Se is composed of disordered rings rather than chains in melt-quenched a-Se. We further reveal that the main origin of this controversy is the facile transition of rings to chains arising from the inherent instability of rings. This transition can be inadvertently triggered by certain characterization techniques themselves containing above-bandgap illumination (above 2.1 eV) or heating (above 50 °C). We finally build a roadmap for obtaining accurate Raman spectra by using above-bandgap excitation lasers with low photon flux (below 1017 phs m-2 s-1) and below-bandgap excitation lasers measured at low temperatures (below -40 °C) to minimize the photoexcitation- and heat-induced ring-to-chain transitions.

Correlation between the density of defect states (DDS) and cross-linking of corner/edge sharing GeSe4 tetrahedral structural units

We have estimated the DDS in the STSG [Se78-xTe20Sn2Gex (x = 0, 2, 4, 6)] system by using the Correlated Barrier Hopping (CBH) model by performing A.C. conduction measurements in the frequency range (1 kHz-10 kHz) and temperature underneath the glass transition temperature (303-333) K. The detailed analysis reveals that bi-polaron hopping is a leading conduction mechanism over single-polaron hopping. Further, there is a noticeable reduction in DDS with increasing concentration of Ge beyond the composition x = 2. A close inspection indicates that cross-linking of Se with Ge has an important role in controlling the DDS in terms of the corner/edge sharing configurations in the structural unit of GeSe4 tetrahedral.

Metavalent Bonding in Layered Phase-Change Memory Materials

Metavalent bonding (MVB) is characterized by the competition between electron delocalization as in metallic bonding and electron localization as in covalent or ionic bonding, serving as an essential ingredient in phase-change materials for advanced memory applications. The crystalline phase-change materials exhibits MVB, which stems from the highly aligned p orbitals and results in large dielectric constants. Breaking the alignment of these chemical bonds leads to a drastic reduction in dielectric constants. In this work, it is clarified how MVB develops across the so-called van der Waals-like gaps in layered Sb₂ Te₃ and Ge-Sb-Te alloys, where coupling of p orbitals is significantly reduced. A type of extended defect involving such gaps in thin films of trigonal Sb₂ Te₃ is identified by atomic imaging experiments and ab initio simulations. It is shown that this defect has an impact on the structural and optical properties, which is consistent with the presence of non-negligible electron sharing in the gaps. Furthermore, the degree of MVB across the gaps is tailored by applying uniaxial strain, which results in a large variation of dielectric function and reflectivity in the trigonal phase. At last, design strategies are provided for applications utilizing the trigonal phase.

The chemical bond in solids-revisited

This article complements an earlier topical review of the chemical bond (Jones 2018J. Phys.: Condens. Matter30153001), starting in the mid-19th century and seen from the perspective of a condensed matter physicist. The discussion of applications focused on the structure and properties of phase change materials. We review here additional aspects of chemistry, particularly some that have raised interest recently in this context. Concepts such as 'electron-rich', 'electron-deficient (excess orbital)', 'hypervalent', 'three-centre', and 'metavalent' bonds, and 'multicentre hyperbonding' are now found in the condensed matter literature. They are surveyed here, as well as the bond in metals and the 'Peierls' distortion. What are these concepts, are they related, and are they sometimes new labels for established, but unfamiliar ideas? 'Half bonds' and 'fractional valencies' play a central role in this discussion. It is remarkable that they were introduced 100 years ago, but ignored or forgotten, and have needed to be rediscovered more than once.