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See detailOpto-electronic properties and solar cell efficiency modelling of Cu2ZnXS4 (X=Sn,Ge,Si) kesterites
Ratz, Thomas ULiege; Raty, Jean-Yves ULiege; Brammertz, Guy et al

in Journal of Physics : Energy (in press)

In this work, first-principles calculations of Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 are per- formed to highlight the impact of the cationic substitution on the structural, electronic and optical properties ... [more ▼]

In this work, first-principles calculations of Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 are per- formed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 and absorption coefficients of the order of 10^4 cm−1 are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cells. In that perspective, we used an improved version of the Shockley-Queisser model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.71, 19.85 and 3.10 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using an internal quantum efficiency value providing experimentally comparable VOC values, cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4. We confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of nearly 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu2ZnGeS4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu2ZnSiS4 might be interesting for transparent photovoltaic windows. [less ▲]

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See detailTheoretical study of the opto-electronic properties of Cu2ZnXS4 (X=Sn,Ge,Si) kesterites for solar cell efficiency modelling.
Ratz, Thomas ULiege; Raty, Jean-Yves ULiege; Brammertz, Guy et al

Poster (2021, June 02)

In this work, first-principles calculations of Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 are per- formed to highlight the impact of the cationic substitution on the structural, electronic and optical properties ... [more ▼]

In this work, first-principles calculations of Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 are per- formed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 and absorption coefficients of the order of 10^4 cm−1 are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cells. In that perspective, we used an improved version of the Shockley-Queisser model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.71, 19.85 and 3.10 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using an internal quantum efficiency value providing experimentally comparable VOC values, cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4. We confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of nearly 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu2ZnGeS4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu2ZnSiS4 might be interesting for transparent photovoltaic windows. [less ▲]

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See detailStudy of the opto-electronic properties of Cu2ZnXS4 (X=Sn,Ge,Si) kesterites as input data for solar cell efficiency modelling
Ratz, Thomas ULiege; Raty, Jean-Yves ULiege; Brammertz, Guy et al

Poster (2020, November 26)

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic ... [more ▼]

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of 10^4 cm^{−1} are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results (absorption coefficient, refractive index and reflectivity) are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical model including non-radiative recombination via an external parameter defined as the in- ternal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.88 %, 19.94 % and 3.11 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using a realistic internal quantum efficiency which provides 𝑉OC values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 (for an optimal thickness of 1.15 𝜇m). With this methodology we confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu2ZnGeS4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu2ZnSiS4 might be interesting for transparent photovoltaic windows. [less ▲]

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See detailOpto-electronic properties and solar cell efficiency modelling of Cu2ZnXS4 (X=Sn,Ge,Si) kesterites
Ratz, Thomas ULiege; Raty, Jean-Yves ULiege; Brammertz, Guy et al

E-print/Working paper (2020)

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic ... [more ▼]

In this work, first principle calculations of Cu2 ZnSnS4 (CZTS), Cu2 ZnGeS4 (CZGS) and Cu2 ZnSiS4 (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of 10^4 cm^{−1} are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results (absorption coefficient, refractive index and reflectivity) are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.88 %, 19.94 % and 3.11 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 for vanishing non-radiative recombination rate. Using a realistic internal quantum efficiency which provides 𝑉OC values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4 (for an optimal thickness of 1.15 𝜇m). With this methodology we confirm the suitability of Cu2ZnSnS4 in single junction solar cells, with a possible efficiency improvement of 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu2ZnGeS4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu2ZnSiS4 might be interesting for transparent photovoltaic windows. [less ▲]

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See detailLaser Generation of Sub-Micrometer Wrinkles in a Chalcogenide Glass Film as Physical Unclonable Functions
Martinez, Paloma; Papagiannouli, Irene; Descamps, Dominique et al

in Advanced Materials (2020), 32(n/a), 2003032

Abstract Laser interaction with solids is routinely used for functionalizing materials' surfaces. In most cases, the generation of patterns/structures is the key feature to endow materials with specific ... [more ▼]

Abstract Laser interaction with solids is routinely used for functionalizing materials' surfaces. In most cases, the generation of patterns/structures is the key feature to endow materials with specific properties like hardening, superhydrophobicity, plasmonic color-enhancement, or dedicated functions like anti-counterfeiting tags. A way to generate random patterns, by means of generation of wrinkles on surfaces resulting from laser melting of amorphous Ge-based chalcogenide thin films, is presented. These patterns, similar to fingerprints, are modulations of the surface height by a few tens of nanometers with a sub-micrometer periodicity. It is shown that the patterns' spatial frequency depends on the melted layer thickness, which can be tuned by varying the impinging laser fluence. The randomness of these patterns makes them an excellent candidate for the generation of physical unclonable function tags (PUF-tags) for anti-counterfeiting applications. Two specific ways are tested to identify the obtained PUF-tag: cross-correlation procedure or using a neural network. In both cases, it is demonstrated that the PUF-tag can be compared to a reference image (PUF-key) and identified with a high recognition ratio on most real application conditions. This paves the way to straightforward non-deterministic PUF-tag generation dedicated to small sensitive parts such as, for example, electronic devices/components, jewelry, or watchmak. [less ▲]

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See detailOvonic Threshold Switching in Se-Rich GexSe1−x Glasses from an Atomistic Point of View: The Crucial Role of the Metavalent Bonding Mechanism
Raty, Jean-Yves ULiege; Noé, Pierre

in Physica Status Solidi. Rapid Research Letters (2020), 14(5), 1900581

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 ... [more ▼]

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. [less ▲]

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See detailToward ultimate nonvolatile resistive memories: The mechanism behind ovonic threshold switching revealed
Noé, Pierre; Verdy, Anthonin; d'Acapito, Francesco et al

in Science Advances (2020), 6(9 eaay2830),

Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real ... [more ▼]

Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real technological breakthrough in the field of data storage memories. This breakthrough was achieved because of the successful 3D integration of so-called OTS selector devices with innovative phase-change memories, both based on chalcogenide materials. This paves the way for storage class memories as well as neuromorphic circuits. We elucidate the mechanism behind OTS switching by new state-of-the-art materials using electrical, optical, and x-ray absorption experiments, as well as ab initio molecular dynamics simulations. The model explaining the switching mechanism occurring in amorphous OTS materials under electric field involves the metastable formation of newly introduced metavalent bonds. This model opens the way for design of improved OTS materials and for future types of applications such as brain-inspired computing. [less ▲]

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See detailDiscovering Electron-Transfer-Driven Changes in Chemical Bonding in Lead Chalcogenides (PbX, where X = Te, Se, S, O)
Maier, Stefan; Steinberg, Simon; Cheng, Yudong et al

in Advanced Materials (2020), 32(49), 2005533

Abstract Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead ... [more ▼]

Abstract Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O), a combination of property-, bond-breaking-, and quantum-mechanical bonding descriptors are applied. The outcome of the explorations reveals an electron-transfer-driven transition from metavalent bonding in PbX (X = Te, Se, S) to iono-covalent bonding in β-PbO. Metavalent bonding is characterized by adjacent atoms being held together by sharing about a single electron (ES ≈ 1) and small electron transfer (ET). The transition from metavalent to iono-covalent bonding manifests itself in clear changes in these quantum-mechanical descriptors (ES and ET), as well as in property-based descriptors (i.e., Born effective charge (Z*), dielectric function ε(ω), effective coordination number (ECoN), and mode-specific Grüneisen parameter (γTO)), and in bond-breaking descriptors. Metavalent bonding collapses if significant charge localization occurs at the ion cores (ET) and/or in the interatomic region (ES). Predominantly changing the degree of electron transfer opens possibilities to tailor material properties such as the chemical bond (Z*) and electronic (ε∞) polarizability, optical bandgap, and optical interband transitions characterized by ε2(ω). Hence, the insights gained from this study highlight the technological relevance of the concept of metavalent bonding and its potential for materials design. [less ▲]

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See detailLocal structure of [(GeTe)<sub>2</sub>/(Sb<sub>2</sub>Te<sub>3</sub>)<sub>m</sub>]<sub>n</sub> super-lattices by X-ray Absorption Spectroscopy
dacapito, Francesco; KOWALCZYK, Philippe; Raty, Jean-Yves ULiege et al

in Journal of Physics D: Applied Physics (2020)

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by X-ray Absorption ... [more ▼]

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by X-ray Absorption Spectroscopy (XAS) at the Ge-K edge. The quantitative analysis of the first coordination shells reveal that the SLs appear to possess a structure very similar to that of thin film of the canonical Ge2Sb2Te5 (GST225) phase-change alloy. By comparing experimental data with ab initio Molecular Dynamics simulations of the EXAFS spectra, we show that chemical disorder is mandatory in order to reproduce the experimental data in the full spectral range. As a result, we can unambiguously conclude that Ge/Sb intermixing resulting from inter-diffusion of the GeTe and Sb2Te3 layers within SLs is inherent to SLs and is not induced by sample preparation method nor by interaction with the electron beam of electron microscopes used in all the previous studies that were suggesting such a phenomenon. We further evidence that the short Ge-Te distance is the same in GeTe and GST225 films, as well as in SLs. The main difference is the impact of disorder in GST225 and SLs. Intermixing being definitively present in [(GeTe)2/(Sb2Te3)m]n SLs, this parameter must be considered in future models aiming at going further in the understanding and the development of iPCM technology. This seems mandatory in order to allow such technology to emerge in the near future on the non-volatile memory market. [less ▲]

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See detailThe interplay between Peierls distortions and metavalent bonding in IV-VI compounds: Comparing GeTe with related monochalcogenides
Raty, Jean-Yves ULiege; Wuttig, M.

in Journal of Physics D: Applied Physics (2020), 53(23), 234002

In this article, we revisit bonding in crystalline GeTe, a simple binary alloy that is also a popular phase change material, and use an ab initio approach that goes beyond the usual one electron ... [more ▼]

In this article, we revisit bonding in crystalline GeTe, a simple binary alloy that is also a popular phase change material, and use an ab initio approach that goes beyond the usual one electron description obtained with density functional theory. By considering the electron pair density, we obtain a measure of the number of pairs of electrons that are shared between neighbors. Employing the charge transfer between adjacent atoms as the second quantifier of chemical bonding, we obtain a map which separates ionic, covalent and metallic bonding. Interestingly, GeTe is not located in any of these regions, but instead is located in a region where materials with a peculiar set of properties prevails. The corresponding materials have been coined incipient metals and their bonding 'metavalent bonding' (MVB). They often possess a Peierls distortion, which stabilizes the rhombohedral crystal structure by breaking the cubic symmetry. For these materials, the electron population of longer and shorter bonds is close to one-half, and charge transfer between adjacent atoms is quasi-independent of the degree of distortion. The energy gained by the Peierls distortion is much smaller than the energy gained by creating the cubic structure, delocalizing one electron over two bonds. Such Peierls distortions are not observed for aromatic compounds which utilize resonant bonding and have properties which differ significantly from the property portfolio of metavalently bonded materials. This stresses the difference between metavalent bonding and the resonant valence bond view of aromatic compounds and molecules. MVB is also responsible for the anomalies in dielectric properties and the anharmonicity of the solids. The comparison between PbTe, GeTe and GeS is particularly instructive, showing that bonding in these materials shows interesting differences, where metavalent bonds govern the behavior of PbTe and GeTe, while GeS is dominated by the Peierls distortion. © IOP Publishing Ltd. [less ▲]

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See detailPhase Change Materials : Characterisation and implications of ‘Metavalent’ Bonding.
Raty, Jean-Yves ULiege

Scientific conference (2019, December 03)

Phase Change Materials are emerging as active components of non-volatile memories thanks to their ability to switch extremely rapidly from a conducting crystal to a more semiconducting glassy. Using Ab ... [more ▼]

Phase Change Materials are emerging as active components of non-volatile memories thanks to their ability to switch extremely rapidly from a conducting crystal to a more semiconducting glassy. Using Ab Initio simulations, we address the structure and some properties of the glassy phase, like aging, but also go back to the description of bonding in the crystalline phase. We develop a two-dimensional map based on a quantum-topological description of electron sharing and electron transfer in binary solids. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary materials. Furthermore, it highlights a distinct region where phase change materials are found and for which bonding has been qualified as ‘metavalent’. Extending this map into the third dimension by including physical properties of application interest, we show that bonding in metavalent compounds differs from the our usual views of bonding. This map could be used to help designing new materials: by searching for desired properties in a 3D space and then mapping this back onto the 2D plane of bonding. [1] J.Y. Raty, M. Schumacher, P. Golub, V. Deringer, C. Gatti and M. Wuttig Advanced Materials, (2018) 1806280 [2] M. Wuttig, V. Deringer, X. Gonze, C. Bichara and J.Y. Raty, Advanced Mater. (2018) 1803777 Work funded by F.R.S.-FNRS & the Region Wallonne (CECI Tier-1) the Federation Wallonie-Bruxelles (ARC AIMED) [less ▲]

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See detailExperimental and ab-intio investigations of sub-ps optical excitation effects in amorphous GeTe films
Martinez, Paloma; Gaudin, Jerome; Papagiannouli, A. et al

Poster (2019, September 10)

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See detailUnderstanding the structure and properties of Pn2Ch3 (V2VI3) compounds from a bonding perspective
Chen, Yudong; Cojocaru-Miredin, Oana; Keutgen, Jens et al

Poster (2019, September 09)

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See detailX-ray absorption spectroscopy study of the complex structure of phase change materials
d'Acapito, Francesco; Raty, Jean-Yves ULiege; Hippert, Françoise et al

Conference (2019, September 09)

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See detailDesign rules for chalcogenide thin films towards on-chip highly non-linear optical components in the mid-infrared
Dory, Jean-Baptiste; Raty, Jean-Yves ULiege; Jager, Jean-Baptiste et al

Conference (2019, September 09)

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See detailUltrafast dynamics of phase transition in Chalcogenide Phase-Change Materials
Martinez, Paloma; Gaudin, Jerome; Papagiannouli, E. et al

Conference (2019, May 20)

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See detailBonding and properties in phase change materials through ab initio simulations
Raty, Jean-Yves ULiege

Scientific conference (2019, May 08)

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See detailBonding and Properties of Phase Change Materials through Ab Initio Simulations
Raty, Jean-Yves ULiege

Scientific conference (2019, March 20)

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See detailA quantum-mechanical map for bonding and properties in materials
Raty, Jean-Yves ULiege; Schumacher, Mathias; Golub, Pavlo et al

Conference (2019, March 04)

Materials with rationally controlled properties play important parts in the development of new and advanced technologies. For instance, the properties of thermoelectric, phase-change, or topologically ... [more ▼]

Materials with rationally controlled properties play important parts in the development of new and advanced technologies. For instance, the properties of thermoelectric, phase-change, or topologically insulating materials can be traced back, to a significant extent, to the nature of bonding in materials. Here, we develop a two-dimensional map based on a quantum-topological description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary materials [1]. Furthermore, it highlights a distinct region for a mechanism recently termed “metavalent” bonding [2]. Extending this map into the third dimension by including physical properties of application interest, we show that bonding in metavalent compounds differs from the classical textbooks views of bonding. This map could be used to help designing new materials: by searching for desired properties in a 3D space and then mapping this back onto the 2D plane of bonding. [1]Advanced Materials, accepted. [2]Advanced Materials, accepted [less ▲]

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