Expression and interactions of stereochemically active lone pairs and their relation to structural distortions and thermal conductivity

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Implications and Optimization of Domain Structures in IV-VI High-Entropy Thermoelectric Materials

High-entropy semiconductors are now an important class of materials widely investigated for thermoelectric applications. Understanding the impact of chemical and structural heterogeneity on transport properties in these compositionally complex systems is essential for thermoelectric design. In this work, we uncover the polar domain structures in the high-entropy PbGeSnSe1.5Te1.5 system and assess their impact on thermoelectric properties. We found that polar domains induced by crystal symmetry breaking give rise to well-structured alternating strain fields. These fields effectively disrupt phonon propagation and suppress the thermal conductivity. We demonstrate that the polar domain structures can be modulated by tuning crystal symmetry through entropy engineering in PbGeSnAgxSbxSe1.5+xTe1.5+x. Incremental increases in the entropy enhance the crystal symmetry of the system, which suppresses domain formation and loses its efficacy in suppressing phonon propagation. As a result, the room-temperature lattice thermal conductivity increases from κL = 0.63 Wm-1 K-1 (x = 0) to 0.79 Wm-1 K-1 (x = 0.10). In the meantime, the increase in crystal symmetry, however, leads to enhanced valley degeneracy and improves the weighted mobility from μw = 29.6 cm2 V-1 s-1 (x = 0) to 35.8 cm2 V-1 s-1 (x = 0.10). As such, optimal thermoelectric performance can be achieved through entropy engineering by balancing weighted mobility and lattice thermal conductivity. This work, for the first time, studies the impact of polar domain structures on thermoelectric properties, and the developed understanding of the intricate interplay between crystal symmetry, polar domains, and transport properties, along with the impact of entropy control, provides valuable insights into designing GeTe-based high-entropy thermoelectrics.

Preparation of Mixed Few-Layer GeSe Nanosheets with High Efficiency by the Thermal Sublimation Method

Two-dimensional (2D) GeSe has been proven promising in fast and broadband optoelectronic applications for its complicated band structure, inert surface property, and excellent stability. The major challenge is the deficiency of the effective technique for controllably prepared large-scale few-to-monolayer GeSe films. For this purpose, a layer-by-layer thinning method by thermal sublimation for manufacturing large-scale mixed few-layer GeSe with direct bandgaps is proposed, and an optimized sublimation temperature of 300 °C in vacuum is evaluated by atomic force microscopy. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectra, and fluorescence mapping measurements are performed on the thinned GeSe layers, and results are well-indexed to the orthorhombic lattice structure with direct bandgaps with an atomic ratio of Ge/Se ≈ 5:4. Raman and fluorescence spectra show an α-type crystalline structure of the thinned GeSe films, indicating the pure physical process of the sublimation thinning. Both the bulk and few-layer GeSe films demonstrate broadband absorption. Conductivity of the few-layer GeSe device indicates the overall crystalline integrity of the film after thermal thinning. Given the convenience and efficiency, we provide an effective approach for fabrication of large-scale 2D materials that are difficult to be prepared by traditional methods.

Insights into Low Thermal Conductivity in Inorganic Materials for Thermoelectrics

Efficient manipulation of thermal conductivity and fundamental understanding of the microscopic mechanisms of phonon scattering in crystalline solids are crucial to achieve high thermoelectric performance. Thermoelectric energy conversion directly and reversibly converts between heat and electricity and is a promising renewable technology to generate electricity by recovering waste heat and improve solid-state refrigeration. However, a unique challenge in thermal transport needs to be addressed to achieve high thermoelectric performance: the requirement of crystalline materials with ultralow lattice thermal conductivity (κ_L). A plethora of strategies have been developed to lower κ_L in crystalline solids by means of nanostructural modifications, introduction of intrinsic or extrinsic phonon scattering centers with tailored shape and dimension, and manipulation of defects and disorder. Recently, intrinsic local lattice distortion and lattice anharmonicity originating from various mechanisms such as rattling, bonding heterogeneity, and ferroelectric instability have found popularity. In this Perspective, we outline the role of manipulation of chemical bonding and structural chemistry on thermal transport in various high-performance thermoelectric materials. We first briefly outline the fundamental aspects of κ_L and discuss the current status of the popular phonon scattering mechanisms in brief. Then we discuss emerging new ideas with examples of crystal structure and lattice dynamics in exemplary materials. Finally, we present an outlook for focus areas of experimental and theoretical challenges, possible new directions, and integrations of novel techniques to achieve low κ_L in order to realize high-performance thermoelectric materials.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen

Chemical reactivity between As and N₂, leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single‐crystal synchrotron X‐ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P2₁3 space group, in which each of the two elements is single‐bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non‐bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Investigation of the sublimation mechanism of GeSe and GeS

GeSe and GeS have emerged as promising light-harvesting materials for photovoltaics due to their attractive optoelectronic properties, non-toxic and earth-abundant constituents, and excellent stability. Here we unveil the diatomic molecule sublimation mechanism of GeSe and GeS that directly guides the optimization of GeSe and GeS solar-cell fabricated via the close-space sublimation method.

Ψ-Polyhedral symbols for coordination geometries of lead(II) with a stereochemically active lone pair

Because an IUCr/IUPAC-designated set of letters/numbers identifies the configuration of the atoms linked to the Pb^II atom in its coordination compounds, a Ψ prefix before such as a polyhedral symbol provides useful information when its lone pair is stereochemically active. Such notation is especially relevant when the metal atom is connected to eight or more atoms regardless of whether the lone pair is active or inert. The polyhedral symbols for the crystal structures in some 50 articles published after 2000 are reported here as the original studies did not expressly identify coordination geometries.

Intrinsically ultralow thermal conductive inorganic solids for high thermoelectric performance

Thermoelectric materials which can convert heat energy to electricity rely on crystalline inorganic solid state compounds exhibiting low phonon transport (i.e. low thermal conductivity) without much inhibiting the electrical transport. Suppression of phonons traditionally has been carried out via extrinsic pathways, involving formation of point defects, foreign nanostructures, and meso-scale grains, but the incorporation of extrinsic substituents also influences the electrical properties. Crystalline materials with intrinsically low lattice thermal conductivity (κlat) provide an attractive paradigm as it helps in simplifying the complex interrelated thermoelectric parameters and allows us to focus largely on improving the electronic properties. In this feature article, we have discussed the chemical bonding and structural aspects in determining phonon transport through a crystalline material. We have outlined how the inherent material properties like lone pair, bonding anharmonicity, presence of intrinsic rattlers, ferroelectric instability, weak and rigid substructures, etc. influence in effectively suppressing the heat transport. The strategies summarized in this feature article should serve as a general guide to rationally design and predict materials with low κlat for potential thermoelectric applications.

Inverse Design of Ultralow Lattice Thermal Conductivity Materials via Materials Database Screening of Lone Pair Cation Coordination Environment

The presence of lone pair (LP) electrons is strongly associated with the disruption of lattice heat transport, which is a critical component of strategies to achieve efficient thermoelectric energy conversion. By exploiting an empirical relationship between lattice thermal conductivity, κ_L, and the bond angles of pnictogen group LP cation coordination environments, we develop an inverse design strategy based on a materials database screening to identify chalcogenide materials with ultralow κ_L for thermoelectrics. Screening the ∼635 000 real and hypothetical inorganic crystals of the Open Quantum Materials Database based on the constituent elements, nominal electron counting, LP cation coordination environment, and synthesizability, we identify 189 compounds expected to exhibit ultralow κ_L. As a validation, we explicitly compute the lattice dynamical properties of two of the compounds (Cu₂AgBiPbS₄ and MnTl₂As₂S₅) using first-principles calculations and successfully find both achieve ultralow κ_L values at room temperature of ∼0.3-0.4 W/(m·K) corresponding to the amorphous limit. Our data-driven approach provides promising candidates for thermoelectric materials and opens new avenues for the design of phononic properties of materials.

Chemical Bonding in Colossal Thermopower FeSb2

FeSb₂ exhibits a colossal Seebeck coefficient ( S ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb₂ are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb₂ . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.