Metavalent or Hypervalent Bonding: Is There a Chance for Reconciliation?

A family of solids including crystalline phase change materials such as GeTe and Sb2 Te3 , topological insulators like Bi2 Se3, 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 B2 H6 . 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.

Tailoring chemical bonds to design unconventional glasses

Glasses are commonly described as disordered counterparts of the corresponding crystals; both usually share the same short-range order, but glasses lack long-range order. Here, a quantification of chemical bonding in a series of glasses and their corresponding crystals is performed, employing two quantum-chemical bonding descriptors, the number of electrons transferred and shared between adjacent atoms. For popular glasses like SiO2, GeSe2, and GeSe, the quantum-chemical bonding descriptors of the glass and the corresponding crystal hardly differ. This explains why these glasses possess a similar short-range order as their crystals. Unconventional glasses, which differ significantly in their short-range order and optical properties from the corresponding crystals are only found in a distinct region of the map spanned by the two bonding descriptors. This region contains crystals of GeTe, Sb2Te3, and GeSb2Te4, which employ metavalent bonding. Hence, unconventional glasses are only obtained for solids, whose crystals employ theses peculiar bonds.

Gibbs Adsorption and Zener Pinning Enable Mechanically Robust High-Performance Bi2 Te3 -Based Thermoelectric Devices

Bi2 Te3 -based alloys have great market demand in miniaturized thermoelectric (TE) devices for solid-state refrigeration and power generation. However, their poor mechanical properties increase the fabrication cost and decrease the service durability. Here, this work reports on strengthened mechanical robustness in Bi2 Te3 -based alloys due to thermodynamic Gibbs adsorption and kinetic Zener pinning at grain boundaries enabled by MgB2 decomposition. These effects result in much-refined grain size and twofold enhancement of the compressive strength and Vickers hardness in (Bi0.5 Sb1.5 Te3 )0.97 (MgB2 )0.03 compared with that of traditional powder-metallurgy-derived Bi0.5 Sb1.5 Te3 . High mechanical properties enable excellent cutting machinability in the MgB2 -added samples, showing no missing corners or cracks. Moreover, adding MgB2 facilitates the simultaneous optimization of electron and phonon transport for enhancing the TE figure of merit (ZT). By further optimizing the Bi/Sb ratio, the sample (Bi0.4 Sb1.6 Te3 )0.97 (MgB2 )0.03 shows a maximum ZT of ≈1.3 at 350 K and an average ZT of 1.1 within 300-473 K. As a consequence, robust TE devices with an energy conversion efficiency of 4.2% at a temperature difference of 215 K are fabricated. This work paves a new way for enhancing the machinability and durability of TE materials, which is especially promising for miniature devices.

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.

Doping by Design: Enhanced Thermoelectric Performance of GeSe Alloys Through Metavalent Bonding

Doping is usually the first step to tailor thermoelectrics. It enables precise control of the charge-carrier concentration and concomitant transport properties. Doping should also turn GeSe, which features an intrinsically a low carrier concentration, into a competitive thermoelectric. Yet, elemental doping fails to improve the carrier concentration. In contrast, alloying with Ag-V-VI₂ compounds causes a remarkable enhancement of thermoelectric performance. This advance is closely related to a transition in the bonding mechanism, as evidenced by sudden changes in the optical dielectric constant ε_∞ , the Born effective charge, the maximum of the optical absorption ε₂ (ω), and the bond-breaking behavior. These property changes are indicative of the formation of metavalent bonding (MVB), leading to an octahedral-like atomic arrangement. MVB is accompanied by a thermoelectric-favorable band structure featuring anisotropic bands with small effective masses and a large degeneracy. A quantum-mechanical map, which distinguishes different types of chemical bonding, reveals that orthorhombic GeSe employs covalent bonding, while rhombohedral and cubic GeSe utilize MVB. The transition from covalent to MVB goes along with a pronounced improvement in thermoelectric performance. The failure or success of different dopants can be explained by this concept, which redefines doping rules and provides a "treasure map" to tailor p-bonded chalcogenides.

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.

Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Conversion between Metavalent and Covalent Bond in Metastable Superlattices Composed of 2D and 3D Sublayers

Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb₂Te₃ gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb₂Te₃ (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb₂Te₃ (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.

Scaling and Confinement in Ultrathin Chalcogenide Films as Exemplified by GeTe

Chalcogenides such as GeTe, PbTe, Sb₂ Te₃ , and Bi₂ Se₃ are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo-electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15 nm. X-ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected.

Switching to Hidden Metallic Crystal Phase in Phase-Change Materials by Photoenhanced Metavalent Bonding

Metavalent bonding is crucial for the determination of phase transition and improvement of device performance in phase-change materials, which are attracting interest for use in memory devices. Although monitoring dielectric and phononic parameters provides a direct measure of the metavalent bonding, the control of phase-change phenomena and metavalent bonding in the dynamical regime has yet to be demonstrated. This study reports the photoenhanced metavalent bonding and resulting hidden metallic crystalline state of Ti-doped Sb₂Te₃, a representative phase-change material with ultralong sustainability. Using ultrafast terahertz spectroscopy, Ti_0.4Sb₂Te₃ was discovered to possess ultralong pump-probe dynamics, which is retained over hundreds of picoseconds, unlike the short-lived state of undoped Sb₂Te₃. Moreover, for Ti_0.4Sb₂Te₃ during the long-lived transmission change, the infrared-active phonon is highly softened, even more than the amount of a thermal phonon shift, indicating the photoenhancement of lattice anharmonicity. Such a long-lived relaxation implies photoinduced transition into a crystalline state of ultrastrong metavalent bonding in Ti_0.4Sb₂Te₃, on the basis of comparisons of the dynamical dielectric constant and temporal phonon shift. Our results show the realization of photoengineering of phase-change materials by tuning electron sharing or transferring.

Thermally Controlled Charge-Carrier Transitions in Disordered PbSbTe Chalcogenides

Binary and ternary chalcogenides have recently attracted much attention due to their wide range of applications including phase-change memory materials, topological insulators, photonic switches, and thermoelectrics. These applications require a precise control of the number and mobility of charge carriers. Here, an unexpected charge-carrier transition in ternary compounds from the PbTe-Sb₂ Te₃ pseudo-binary line is reported. Upon thermal annealing, sputtered thin films of PbSb₂ Te₄ and Pb₂ Sb₂ Te₅ undergo a transition in the temperature coefficient of resistance and in the type of the majority charge carriers from n-type to p-type. These transitions are observed upon increasing structural order within one crystallographic phase. To account for this striking observation, it is proposed that the Fermi energy shifts from the tail of the conduction band to the valence band because different levels of overall structural disorder lead to different predominant types of native point defects. This view is confirmed by an extensive computational study, revealing a transition from excess cations and Sb_Pb for high levels of disorder to Pb_Sb prevailing for low disorder. The findings will help fine-tune transport properties in certain chalcogenides via proper thermal treatment, with potential benefits for memories, thermoelectric material optimization, and neuromorphic devices.

Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge₃Sb₂Te₆ (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

Metavalent Bonding in Crystalline Solids: How Does It Collapse?

The chemical bond is one of the most powerful, yet much debated concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding was identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking, and sharing of about one electron between adjacent atoms. This metavalent bond is a fundamental type of bonding in solids, besides covalent, ionic, and metallic bonding, raising the pertinent question as to whether there is a well-defined transition between metavalent and covalent bonds. Here, three different pseudo-binary lines, namely, GeTe_{1- x} Se_x , Sb₂ Te_{3(1- x )} Se_{3 x} , and Bi_{2-2 x} Sb_{2 x} Se₃ , are studied, and a sudden change in several properties, including optical absorption ε₂ (ω), optical dielectric constant ε_∞ , Born effective charge Z*, electrical conductivity, as well as bond breaking behavior for a critical Se or Sb concentration, is evidenced. These findings provide a blueprint to experimentally explore the influence of metavalent bonding on attractive properties of phase-change materials and thermoelectrics. Particularly important is its impact on optical properties, which can be tailored by the amount of electrons shared between adjacent atoms. This correlation can be used to design optoelectronic materials and to explore systematic changes in chemical bonding with stoichiometry and atomic arrangement.

Long-Range Forces in Rock-Salt-Type Tellurides and How they Mirror the Underlying Chemical Bonding

Chemical bonding in main-group IV chalcogenides is an intensely discussed topic, easily understandable because of their remarkable physical properties that predestine these solid-state materials for their widespread use in, for instance, thermoelectrics and phase-change memory applications. The atomistic origin of their unusual property portfolio remains somewhat unclear, however, even though different and sometimes conflicting chemical-bonding concepts have been introduced in the recent years. Here, it is proposed that projecting phononic force-constant tensors for pairs of atoms along differing directions and ranges provide a suitable and quantitative descriptor of the bonding nature for these materials. In combination with orbital-based quantitative measures of covalency such as crystal orbital Hamilton populations (COHP), it is concluded that the well-established many-center and even n-center bonding is an appropriate picture of the underlying quantum-chemical bonding mechanism, supporting the recent proposal of hyperbonded phase-change materials.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe)_0.9 (AgBiTe₂ )_0.1  , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe)_1-x (AgBiTe₂ )_x  . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)_1-x (AgBiTe₂ )_x  . (GeSe)_0.9 (AgBiTe₂ )_0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 μW cm^-1  K^-2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm^-1  K^-1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)_0.9 (AgBiTe₂ )_0.1 . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.

Discovering Electron-Transfer-Driven Changes in Chemical Bonding in Lead Chalcogenides (PbX, where X = Te, Se, S, O)

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 (γ_TO )), 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.

Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films

Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few-layer crystalline films of monochalcogenides display robust in-plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first-principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te-based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te-based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few-layers structures offers a powerful tool to tune materials properties for applications in information technology.

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

A unified picture of different application areas for incipient metals is presented. This unconventional material class includes several main-group chalcogenides, such as GeTe, PbTe, Sb₂ Te₃ , Bi₂ Se₃ , AgSbTe₂ and Ge₂ Sb₂ Te₅ . These compounds and related materials show a unique portfolio of physical properties. A novel map is discussed, which helps to explain these properties and separates the different fundamental bonding mechanisms (e.g., ionic, metallic, and covalent). The map also provides evidence for an unconventional, new bonding mechanism, coined metavalent bonding (MVB). Incipient metals, employing this bonding mechanism, also show a special bond breaking mechanism. MVB differs considerably from resonant bonding encountered in benzene or graphite. The concept of MVB is employed to explain the unique properties of materials utilizing it. Then, the link is made from fundamental insights to application-relevant properties, crucial for the use of these materials as thermoelectrics, phase change materials, topological insulators or as active photonic components. The close relationship of the materials' properties and their application potential provides optimization schemes for different applications. Finally, evidence will be presented that for metavalently bonded materials interesting effects arise in reduced dimensions. In particular, the consequences for the crystallization kinetics of thin films and nanoparticles will be discussed in detail.

Investigating Bond Rupture in Resonantly Bonded Solids by Field Evaporation of Carbon Nanotubes

Carbon nanotubes, which possess an atomic arrangement that closely resembles graphene, form a class of nanomaterials with an exceptional application portfolio including electronics, batteries, sensors, etc. Both carbon nanotubes and graphene have exceptional mechanical and electronic properties. These exceptional properties of graphene are attributed to the combined effect of σ and π bonds which form upon sp² hybridization, resulting in what is known as resonant bonding. Here, we use atom probe tomography (APT, a technique based on controlled desorption of atoms under high electric field) to observe its bond-rupture characteristics. Results show that the bond rupture of carbon nanotubes, which are resonantly bonded, is similar to that observed for covalently bonded systems. However, a significant difference is observed when compared with those solids which are metavalently bonded. This clearly justifies that resonant bonding, a sub-branch of covalent bonding, is very different from "metavalent" bonding.

A Quantum-Mechanical Map for Bonding and Properties in Solids

A 2D map is created for solid-state materials based on a quantum-mechanical 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 compounds; furthermore, it highlights a distinct region for a mechanism recently termed "metavalent" bonding. Then, it is shown how this materials map can be extended in the third dimension by including physical properties of application interest. Finally, it is shown how the map coordinates yield new insight into the nature of the Peierls distortion in phase-change materials and thermoelectrics. These findings and conceptual approaches provide a novel avenue to tailor material properties.

Incipient Metals: Functional Materials with a Unique Bonding Mechanism

While solid-state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. Phase-change materials (PCMs) for data storage are a prominent example: they have been claimed to show "resonant bonding," but a clear definition of this mechanism has been lacking. Here, it is shown that these solids are fundamentally different from resonant bonding in the π-orbital systems of benzene and graphene, based on first-principles data for vibrational, optical, and polarizability properties. It is shown that PCMs and related materials exhibit a unique mechanism between covalent and metallic bonding. It is suggested that these materials be called "incipient metals," and their bonding nature "metavalent". Data for a diverse set of 58 materials show that metavalent bonding is not just a superposition of covalent and metallic cases, but instead gives rise to a unique and anomalous set of physical properties. This allows the derivation of a characteristic fingerprint of metavalent bonding, composed of five individual components and firmly rooted in physical properties. These findings are expected to accelerate the discovery and design of functional materials with attractive properties and applications, including nonvolatile memories, thermoelectrics, photonics, and quantum materials.