Extended Antibonding States and Phonon Localization Induce Ultralow Thermal Conductivity in Low Dimensional Metal Halide

Thermal conductivity, which measures the ease at which heat passes through a crystalline solid, is controlled by the nature of the chemical bonding and periodicity in the solid. This necessitates an in-depth understanding of the crystal structure and chemical bonding to tailor materials with notable lattice thermal conductivity (κ_L). Herein, the nature of chemical bonding and its influence on the thermal transport properties (2–523 K) of all-inorganic halide perovskite Cs₃Bi₂I₉ are studied. The κ_L exhibits an ultralow value of ≈0.20  W m⁻¹K⁻¹ in 30–523 K temperature range. The antibonding states just below the Fermi level in the electronic structure arising from the interaction between bismuth 6s and iodine 5p orbitals, weakens the bond and causes soft elasticity in Cs₃Bi₂I₉. First-principles density functional theory (DFT) calculations reveal highly localized soft optical phonon modes originating from Cs-rattling and dynamic double octahedral distortion of 0D [Bi₂I₉]³⁻ in Cs₃Bi₂I₉. These low energy nearly flat optical phonons strongly interact with transverse acoustic modes creating an ultrashort phonon lifetime of ≈1 ps. While the presence of extended antibonding states gives rise to soft anharmonic lattice; Cs rattling provides sharp localized optical phonon modes, which altogether result in strong lattice anharmonicity and ultralow κ_L.     

Electronic Band-Structure Calculations of Ba8Me x Si46-x Clathrates with Me = Mg,Pd, Ni,Au, Ag,Cu, Zn,Al, Sn

Clathrate materials of AlSi, CuSi or NiSi type consisting of abundant elements have a realistic chance of becoming useful thermoelectrics in the near future, because the rattling effect due to their crystal cage structure provides a large figure of merit ZT even in experiments measured under large temperature gradients. In the search for better thermoelectrics, new element combinations in the clathrate type I structure with cubic space group Pm3n were calculated using VASP ab initio software. Predictions of the Seebeck coefficient were made by checking the electronic band structure and density of states for a large variety of input data. For x values around 4 to 6 in the structural formula Ba₈Me _x Si_46?x the substituents Cu, Au, and Ag are best for good thermoelectric behavior, which is discussed in this paper as a result of the low electron–phonon interaction parameter.     

Nitrogen‐Dopant‐Induced Organic–Inorganic Hybrid Perovskite Crystal Growth on Carbon Nanotubes

Interfacial engineering of organic–inorganic halide perovskites in conjunction with different functional materials is anticipated to offer novel heterojunction structures with unique functionalities. Unfortunately, complex material compositions and structures of the organic–inorganic hybrid materials make it difficult to tailor a desirable intermolecular interaction at the interface. Spontaneous and highly specific nucleation of perovskite crystals, that is, methylammonium lead iodide perovskite (CH₃NH₃PbI₃, MAPbI₃) at nitrogen‐doped carbon nanotube (NCNT) surfaces for the self‐assembly of MAPbI₃/NCNT hybrids is reported. It is demonstrated that the lone‐pair electrons of pyridinic nitrogen‐dopant sites at NCNTs mediate specific interactions with the cationic component in the perovskite structure and serve as the nucleation sites via coordinate bonding formation, as supported by X‐ray photoelectron spectroscopy and density functional theory calculation. The potential suitability of MAPbI₃/NCNT hybrids is presented for highly sensitive and selective NO₂ sensing layer. This work suggests a reliable self‐assembly route to the molecular level hybridization of organic–inorganic halide perovskites by employing the substitutional dopant sites at graphene‐based nanomaterials.     

Minimum Thermal Conductivity in Weak Topological Insulators with Bismuth‐Based Stack Structure

Contrary to the conventional belief that the consideration for topological insulators (TIs) as potential thermoelectrics is due to their excellent electrical properties benefiting from the topological surface states, this work shows that the 3D weak TIs, formed by alternating stacks of quantum spin Hall layers and normal insulator (NI) layers, can also be decent thermoelectrics because of their focus on minimum thermal conductivity. The minimum lattice thermal conductivity is experimentally confirmed in Bi₁₄Rh₃I₉ and theoretically predicted for Bi₂TeI at room temperature. It is revealed that the topologically “trivial” NI layers play a surprisingly critical role in hindering phonon propagation. The weak bonding in the NI layers gives rise to significantly low sound velocity, and the localized low‐frequency vibrations of the NI layers cause strong acoustic–optical interactions and lattice anharmonicity. All these features are favorable for the realization of exceptionally low lattice thermal conductivity, and therefore present remarkable opportunities for developing high‐performance thermoelectrics in weak TIs.     

High Performance Mg2(Si,Sn) Solid Solutions: a Point Defect Chemistry Approach to Enhancing Thermoelectric Properties

A point defect chemistry approach to improving thermoelectric (TE) properties is introduced, and its effectiveness in the emerging mid‐temperature TE material Mg₂(Si,Sn) is demonstrated. The TE properties of Mg₂(Si,Sn) are enhanced via the synergistical implementation of three types of point defects, that is, Sb dopants, Mg vacancies, and Mg interstitials in Mg₂Si_0.4Sn_0.6‐xSb_x with high Sb content (x > 0.1), and it is found that i) Sb doping at low ratios tunes the carrier concentration while it facilitates the formation of Mg vacancies at high doping ratios (x > 0.1). Mg vacancies act as acceptors and phonon scatters; ii) the concentration of Mg vacancies is effectively controlled by the Sb doping ratio; iii) excess Mg facilitates the formation of Mg interstitials that also tunes the carrier concentration; vi) at the optimal Sb‐doping ratio near x ≈ 0.10 the lattice thermal conductivity is significantly reduced, and a state‐of‐the‐art figure of merit ZT > 1.1 is attained at 750 K in 2 at% Zn doped Mg₂Si_0.4Sn_0.5Sb_0.1 specimen. These results demonstrate the significance of point defects in thermoelectrics, and the promise of point defect chemistry as a new approach in optimizing TE properties.     

The Criteria for Beneficial Disorder in Thermoelectric Solid Solutions

Forming solid solutions has long been considered an effective approach for good thermoelectrics because the lattice thermal conductivities are lower than those of the constituent compounds due to phonon scattering from disordered atoms. However, this effect could also be compensated by a reduction in carrier mobility due to electron scattering from the same disorder. Using a detailed study of n‐type (PbTe)_1–x (PbSe)_x solid solution (0 ≤ x ≤ 1) as a function of composition, temperature, and doping level, quantitative modeling of transport properties reveals the important parameters characterizing these effects. Based on this analysis, a general criterion for the improvement of zT due to atomic disorder in solid solutions is derived and can be applied to several thermoelectric solid solutions, allowing a convenient prediction of whether better thermoelectric performance could be achieved in a given solid solution. Alloying is shown to be most effective at low temperatures and in materials that are unfavorable for thermoelectrics in their unalloyed forms: high lattice thermal conductivity (stiff materials with low Grüneisen parameters) and high deformation potential.     

Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms

In contrast to polar cation displacements driving oxides into noncentrosymmetric and ferroelectric states, inversion‐preserving anion displacements, such as rotations or tilts of oxygen octahedra about cation coordination centers, are exceedingly common. More than one nonpolar rotational mode in layered perovskites can lift inversion symmetry and combine to induce an electric polarization through a hybrid improper ferroelectric (HIF) mechanism. This form of ferroelectricity expands the compositional palette to new ferroelectric oxides because its activity derives from geometric rather than electronic origins. Here, the new Ruddlesden–Popper HIF Sr₃Zr₂O₇, which is the first ternary lead‐free zirconate ferroelectric, is reported and room‐temperature polarization switching is demonstrated. This compound undergoes a first‐order ferroelectric‐to‐paraelectric transition, involving an unusual change in the “sense” of octahedral rotation while the octahedral tilt remains unchanged. Our experimental and first‐principles study shows that the paraelectric polymorph competes with the polar phase and emerges from a trilinear coupling of rotation and tilt modes interacting with an antipolar mode. This form of hybrid improper “antiferroelectricity” is recently predicted theoretically but has remained undetected. This work establishes the importance of understanding anharmonic interactions among lattice degrees of freedom, which is important for the discovery of new ferroelectrics and likely to influence the design of next‐generation thermoelectrics.     

Bifurcated Polarization Rotation in Bismuth‐Based Piezoelectrics

ABO₃ perovskite‐type solid solutions display a large variety of structural and physical properties, which can be tuned by chemical composition or external parameters such as temperature, pressure, strain, electric, or magnetic fields. Some solid solutions show remarkably enhanced physical properties including colossal magnetoresistance or giant piezoelectricity. It has been recognized that structural distortions, competing on the local level, are key to understanding and tuning these remarkable properties, yet, it remains a challenge to experimentally observe such local structural details. Here, from neutron pair‐distribution analysis, a temperature‐dependent 3D atomic‐level model of the lead‐free piezoelectric perovskite Na_0.5Bi_0.5TiO₃ (NBT) is reported. The statistical analysis of this model shows how local distortions compete, how this competition develops with temperature, and, in particular, how different polar displacements of Bi³⁺ cations coexist as a bifurcated polarization, highlighting the interest of Bi‐based materials in the search for new lead‐free piezoelectrics.     

Investigation on the atomic layer deposition of the Se doped Sb–Te phase change films using an alkyl-silyl precursor

We used TDMASb, Te(t-Bu)₂, and Se(SiMe₃)₂ as precursors to investigate the behavior of atomic vapor deposition (AVD) during formation of Se doped Sb–Te phase change films. The deposition was prepared by a sequential recipe for the atomic layer deposition (ALD) reaction with consideration of the electrostatic interactions between the Se alkyl-silyl precursor and the Sb and Te precursors. The Sb–Te–Se ternary films could be deposited by AVD at temperatures of 80–300 °C with fine controllability and high surface quality. The elemental composition of the films was controlled by varying the deposition temperature. The Se doping effect on the crystallization temperature and set resistance in the crystalline state was evaluated via electrical measurements and theoretical analyses by applying the glass transition temperature model and resonance bonding model. The results indicated that the incorporation of Se into Sb–Te films may provide for improved thermal stability and power consumption efficiency in phase-change random access memory (PCRAM) devices.     

Structural characterization of AlxGa1−xSb grown by LPE

Al_xGa_1−xSb ternary solid solutions lattice-matched to the GaSb (001) substrate with composition in the range 0.05≤x≤0.2 were grown by liquid phase epitaxy. High resolution X-ray diffraction and Raman scattering techniques were applied to characterize Al_xGa_1−xSb alloys. The out of plane lattice parameter was estimated directly from the asymmetrical diffractions of planes (115) and (−1−15). The out of plane lattice parameter as a function of Aluminium content is higher than the corresponding bulk lattice parameter of Al_xGa_1−xSb layers obtained with Vegard's law. These results show that some of the layers are more strained than others. Two peaks are observed in their Raman spectra over this composition range. The assignment of the observed modes to GaSb-like modes is discussed. The studies of the chemical composition of the layers by SIMS exhibit the presence of tellurium, carbon and oxygen like the main residual impurities.     

Quantum‐Chemical Studies on the Geometric and Electronic Structures of Bertholloide Cobalt Oxynitrides

We report electronic structure calculations using density‐functional theory (local density approximation (LDA) and generalized gradient approximation (GGA); plane waves and muffin‐tin orbitals; pseudopotentials and all‐electron approaches) on non‐stoichiometric CoN_xO_1–x oxynitride phases. The preference of the experimentally suggested zinc‐blende structure type over the rock‐salt type is confirmed and explained, on the basis of COHP (crystal orbital Hamilton population) chemical bonding analyses, by reduced Co–Co antibonding interactions in the ZnS structural alternative. A pressure‐induced phase transition into the NaCl type, however, is predicted at approximately 30 GPa. Supercell calculations touching upon the exact composition and local structure of CoN_xO_1–x provide evidence for a broad range of energetically metastable compositions with respect to the zinc‐blende‐type boundary phases CoN and CoO, especially for the more oxygen‐rich phases. All non‐stoichiometric compounds are predicted to be metallic materials which do not exhibit significant magnetic moments. Likewise, there is no indication for anionic ordering such that random anion arrangements are preferred.     

Trend for Thermoelectric Materials and Their Earth Abundance

The low crustal abundance of materials such as tellurium (Te) (0.001?ppm by weight), antimony (Sb) (0.2?ppm), and germanium (Ge) (1.4?ppm) contributes to their price volatility as applications (competing with thermoelectrics) continue to grow, for example, cadmium telluride photovoltaics, antimony-lead alloy for batteries, and Ge for fiber optics and infrared optical technologies. Previous consideration of material scarcity has focused on Te-based thermoelectrics. Here, we broaden the analysis to include recent high-figure-of-merit (ZT) materials such as skutterudites, Zintl phase compounds, and clathrates that employ Sb, ytterbium (2.8?ppm), and Ge. The maximum demonstrated ZT for each particular alloy exhibits an empirical dependence on the crustal abundance, A, such that ZT?=?A ^?b , where b is in the range from 0.05 to 0.10. This analysis shows that no material with crustal abundance of 30?ppm (~4?× 10¹⁸ metric tons) has ZT greater than 0.8.     

Disordered Atomic Packing Structure of Metallic Glass: Toward Ultrafast Hydroxyl Radicals Production Rate and Strong Electron Transfer Ability in Catalytic Performance

Developing new functional applications of metallic glasses in catalysis is an active and pivotal topic for materials science as well as novel environmental catalysis processes. Compared to the crystalline materials with highly ordered atomic packing, metallic glass has a simply disordered atomic structure. Recent reports have demonstrated that the metallic glasses are indeed having many superiorly catalytic properties, yet the understanding of the mechanism is insufficient. In this work, the structural relaxation (α‐relaxation) by annealing in an amorphous Fe₇₈Si₉B₁₃ alloy is studied for unraveling the catalytic mechanism at the atomic scale. The volume fractions of the crystalline structures, such as α‐Fe, Fe₂Si, and Fe₂B, in the as‐received and annealed metallic glasses are fully characterized. It is found that the randomly atomic packing structure with weak atomic bonding in the as‐received metallic glass has an efficient electron transfer capability, presenting advanced superiorities in the aspects of production rate of hydroxyl radicals (?OH), dye degradation rate (k ), and essential degradation ability (K _SA) for water treatment. The discovery of this critically important work unveils why using metallic glasses as catalysts has higher reactivity than the crystalline materials, and more importantly, it provides new research opportunities into the study of synthetic catalysts.     

In Situ Electrochemical Activation of Atomic Layer Deposition Coated MoS2 Basal Planes for Efficient Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂), which is composed of active edge sites and a catalytically inert basal plane, is a promising catalyst to replace the state‐of‐the‐art Pt for electrochemically catalyzing hydrogen evolution reaction (HER). Because the basal plane consists of the majority of the MoS₂ bulk materials, activation of basal plane sites is an important challenge to further enhance HER performance. Here, an in situ electrochemical activation process of the MoS₂ basal planes by using the atomic layer deposition (ALD) technique to improve the HER performance of commercial bulk MoS₂ is first demonstrated. The ALD technique is used to form islands of titanium dioxide (TiO₂) on the surface of the MoS₂ basal plane. The coated TiO₂ on the MoS₂ surface (ALD(TiO₂)‐MoS₂) is then leached out using an in situ electrochemical activation method, producing highly localized surface distortions on the MoS₂ basal plane. The MoS₂ catalysts with activated basal plane surfaces (ALD(Act.)‐MoS₂) have dramatically enhanced HER kinetics, resulting from more favorable hydrogen‐binding.     

Jahn–Teller Driven Electronic Instability in Thermoelectric Tetrahedrite

Tetrahedrite, Cu₁₂Sb₄S₁₃, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal‐to‐semiconductor transition (MST), remains largely unknown. This work presents the first determination of the low‐temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, the results show that the trigonal–planar copper cations remain in planar coordination below the MST. The atomic displacement parameters of the trigonal–planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the MST. The phase transition is a consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found in the cubic phase. This study reveals that a Jahn–Teller electronic instability leads to the formation of “molecular‐like” Cu₅⁷⁺ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper–copper interactions. First principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. These results provide insights on the interplay between phonon transport, electronic properties, and crystal structure in mixed‐valence compounds.     

Employing Interfaces with Metavalently Bonded Materials for Phonon Scattering and Control of the Thermal Conductivity in TAGS‐x Thermoelectric Materials

The thermoelectric compound (GeTe)_x(AgSbTe₂)_1?x, in short (TAGS‐x), is investigated with a focus on two stoichiometries, i.e., TAGS‐50 and TAGS‐85. TAGS‐85 is currently one of the most studied thermoelectric materials with great potential for thermoelectric applications. Yet, surprisingly, the lowest thermal conductivity is measured for TAGS‐50, instead of TAGS‐85. To explain this unexpected observation, atom probe tomography (APT) measurements are conducted on both samples, revealing clusters of various compositions and sizes. The most important role is attributed to Ag₂Te nanoprecipitates (NPs) found in TAGS‐50. In contrast to the Ag₂Te NPs, the matrix reveals an unconventional bond breaking mechanism. More specifically, a high probability of multiple events (PME) of ≈60% is observed for the matrix by APT. Surprisingly, the PME value decreases abruptly to ≈20–30% for the Ag₂Te NPs. These differences can be attributed to differences in chemical bonding. The precipitates' PME value is indicative of normal bonding, i.e., covalent bonding with normal optical modes, while materials with this unconventional bond breaking found in the matrix are characterized by metavalent bonding. This implies that the interface between the metavalently bonded matrix and covalently bonded Ag₂Te NP is partly responsible for the reduced thermal conductivity in TAGS‐50.     

Effect of High-Pressure Torsion on Texture,Microstructure, and Raman Spectroscopy: Case Study of Fe- and Te-Substituted CoSb3

Fe_0.05Co_0.95Sb_2.875Te_0.125, a double-element-substituted skutterudite, was prepared by induction melting, annealing, and hot pressing (HP). The hot-pressed sample was subjected to high-pressure torsion (HPT) with 4 GPa pressure at 673 K. X-ray diffraction was performed before and after HPT processing of the sample; the skutterudite phase was observed as a main phase, but an additional impurity phase (CoSb₂) was observed in the HPT-processed sample. Surface morphology was determined by high-resolution scanning electron microscopy. In the HP sample, coarse grains with sizes in the range of approximately 100 nm to 300 nm were obtained. They changed to fine grains with a reduction in grain size to 75 nm to 125 nm after HPT due to severe plastic deformation. Crystallographic texture, as measured by x-ray diffraction, indicated strengthening of (112), (102) poles and weakening of the (123) pole of the HPT-processed sample. Raman-active vibrational modes showed a peak position shift towards the lower energy side, indicating softening of the modes after HPT. The distortion of the rectangular Sb–Sb rings leads to broadening of Sb–Sb vibrational modes due to local strain fluctuation. In the HPT process, a significant effect on the shorter Sb–Sb bond was observed as compared with the longer Sb–Sb bond.     

Point Defect Engineering of High‐Performance Bismuth‐Telluride‐Based Thermoelectric Materials

Developing high‐performance thermoelectric materials is one of the crucial aspects for direct thermal‐to‐electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)₂(Te,Se)₃ thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor‐like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n‐type polycrystalline Bi₂Te_2.3Se_0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p‐type polycrystalline Bi_0.3Sb_1.7Te₃ alloys, both values being higher than those of commercial zone‐melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.     

The Enabling Electronic Motif for Topological Insulation in ABO3 Perovskites

Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO₃ as a lone‐pair, electron‐rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO₃ compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy‐lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO₃ and RbIO₃). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion‐energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI‐removing atomic displacements can be effectively screened out.     

Impact of Bonding on the Stacking Defects in Layered Chalcogenides

Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge₄Se₃Te, GaSe, and Sb₂Te₃ are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.