arXiv subjects

Jiangang He

Publications and source records attributed to Jiangang He.

At least 19 recordsLinked to original sources

arXiv2026

Optical-Phonon-Enabled Large Lattice Thermal Conductivity Anisotropy in Hexagonal Perovskites Cs$BX_3$ ($B$ = Mg, Cd; $X$ = Cl, Br, I)

Materials exhibiting strongly anisotropic lattice thermal conductivity are desirable for thermal-management applications, yet such behavior is commonly associated with layered or quasi-one-dimensional van der Waals crystals and highly anisotropic elastic properties. Here, we investigate lattice thermal transport in the hexagonal perovskites Cs$BX_3$ ($B=$ Mg, Cd; $X=$ Cl, Br, I) using first-principles calculations. At 300~K, the calculated in-plane and out-of-plane lattice thermal conductivities range from 0.13--0.83 and 0.34--6.26~Wm$^{-1}$K$^{-1}$, respectively, corresponding to anisotropy ratios of 2.6--7.5. This pronounced anisotropy is remarkable given the relatively modest elastic anisotropy, characterized by $C_{33}/C_{11}$ = 0.994--1.842. Our analysis reveals that medium-frequency optical phonons provide an efficient out-of-plane heat-transport channel, contrary to the conventional picture in which heat transport is dominated by acoustic phonons. These findings identify face-sharing octahedral frameworks as a promising platform for engineering strong thermal-conductivity anisotropy in mechanically near-isotropic, non--van der Waals crystals.

cond-mat.mtrl-sci↗

arXiv2026

Autonomous Optimization of Complex Oxides for Thermochemical Fuel Production

Two-step thermochemical fuel production, including H2O and CO2 splitting, offers a promising route to sustainable fuel manufacturing, with performance governed by redox-active oxides that enable cyclic reduction-oxidation reactions. Maximizing thermal-to-fuel conversion efficiency demands materials that simultaneously satisfy multiple stringent thermodynamic and kinetic targets. Addressing these requirements has increasingly driven materials design toward complex, multi-cation oxides, such as mixed-cation fluorites, perovskites, and high-entropy oxides, wherein composition, defect chemistry, phase stability, and morphology should be co-optimized. This creates a challenging materials optimization problem that is poorly suited to traditional trial-and-error approaches. In this review, we argue that thermochemical fuel production provides a compelling frontier for autonomous materials design and optimization. We first examine why redox-active complex oxides are difficult to develop, owing to multidimensional phase spaces, harsh operating conditions, and competing functional targets. We then discuss how high-throughput computation, automated synthesis, characterization and testing, and machine learning can be integrated into closed-loop workflows to address these challenges. Building on broader oxide materials research, we organize recent progress into a capability roadmap for complex-oxide optimization, spanning compositionally diverse synthesis, operando characterization, robotic testing, operation-condition computation, and multi-objective optimization. Finally, we outline key experimental, computational, and data challenges for building self-improving materials development platforms for materials development in thermochemical fuel production.

cond-mat.mtrl-sci↗

arXiv2026

Net and Hidden Spin-Valley Locking Enable Ultrahigh Hole Mobility in Covalent Bulk WN$_2$

High carrier mobility at room temperature underpins high-performance electronics, yet high hole mobility remains rare in bulk semiconductors. Spin-valley locking can suppress intervalley scattering and enhance mobility, but it is limited to materials with broken inversion symmetry. Hidden spin polarization offers a possible route beyond this constraint, although whether its compensated spin textures could protect charge transport remains unclear. Using ab initio electron-phonon and transport calculations, we show that the two hexagonal phases of bulk WN$_2$ realize net and hidden spin-valley locking and exhibit ultrahigh room-temperature hole mobilities. In non-centrosymmetric $\alpha$-WN$_2$, a large valley spin splitting produces net spin-valley locking that nearly eliminates phonon-mediated intervalley scattering. In centrosymmetric $\beta$-WN$_2$, hidden Zeeman-type spin polarization yields a compensated, sector-resolved spin texture that reverses between valleys and suppresses intervalley scattering as effectively as the net locking does. The stiff W-N/N-N covalent network further keeps the remaining intravalley scattering weak. Our results establish hidden spin polarization as an effective transport-protection mechanism and extend spin-valley engineering to centrosymmetric bulk semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Ultrahigh Intrinsic Hole Mobilities in $M$N$_2$ ($M$= Mo and W) at Room Temperature

High-mobility $p$-type semiconductors are essential for advanced electronic devices but remain scarce. Here, using a hierarchical screening framework that combines first-principles calculations with Boltzmann transport theory, we identify $M$N$_2$ ($M$= Mo and W) family as polar semiconductors with exceptionally high intrinsic hole mobilities. In particular, 1H-WN$_2$ exhibits a room-temperature hole mobility exceeding $10^{4}$~$\mathrm{cm^2\,V^{-1}\,s^{-1}}$. This exceptional transport performance arises from the synergistic suppression of polar-optical-phonon and acoustic-phonon scattering, together with a reduced intervalley-scattering phase space induced by spin--valley locking. These effects arise from anomalously small Born effective charges, strong covalent N--N bonds, and orbital hybridization between N-$2p_x$/$2p_y$ and W-$5d_{xy}$/$5d_{x^2-y^2}$ in the N$_2$-dimer-based structure. Our results establish MoN$_2$ and WN$_2$ as a promising class of high-mobility polar semiconductors and introduce a crystal-structure-based strategy for concurrently suppressing multiple electron--phonon scattering channels, thereby revising design principles for high-mobility materials.

cond-mat.mtrl-sci↗

arXiv2026

Mechanism-Dependent Descriptors Enable Predictive Design of Oxygen Capacity in Perovskite Oxides

Perovskite oxides can reversibly accommodate substantial changes in oxygen stoichiometry, making them attractive for clean-energy technologies including chemical looping and oxygen storage. Despite extensive efforts to optimize their redox properties, predictive descriptors capable of assessing oxygen capacity across diverse compositions remain under development. Here, we combine experiments and first-principles calculations to establish composition and oxygen-capacity relationships in the model perovskite series LnxSr1-xCoO3. We confirm that increasing Sr2+ content promotes the formation of high-valence Co4+, expanding the cationic redox reservoir available during oxygen release and thereby enhancing oxygen capacity. In this regime, oxygen-vacancy formation energy captures the observed trend because oxygen release is primarily compensated by Co4+/Co3+/Co2+ redox. Across the rare-earth series, however, oxygen capacity decreases from La to Lu despite progressively lower oxygen-vacancy formation energies. We reveal that this counterintuitive behavior originates from an alternative charge-compensation pathway, in which lattice oxygen is partially oxidized to O1- -like species during oxygen removal. Heavy rare-earth compositions (Tb-Lu) preferentially stabilize these oxygen-hole species through distinct local bonding environments, with charge compensation involving both oxidized lattice oxygen and reduced rare-earth and cobalt cations, thereby suppressing net oxygen release despite favorable vacancy thermodynamics. We further identify average metal-oxygen bond strength, quantified by integrated crystal orbital Hamilton population, as a physically meaningful descriptor for oxygen capacity when anionic redox becomes dominant.

cond-mat.mtrl-sci↗

arXiv2026

Polarization Rotation Drives a Spin-Topological Transition in Ferroelectric Bismuth Monolayer

Bismuth monolayer is the first two-dimensional elemental ferroelectric and an appealing platform for coupling polar order to spin-orbit-driven topology. However, its microscopic switching mechanism remains elusive. Here, using first-principles lattice dynamics and symmetry-adapted mode analysis, we identify a previously overlooked rotational pathway for in-plane polarization switching. Its energy barrier is more than four times lower than that of direct reversal, naturally explaining the vortexlike domain textures observed in molecular dynamics simulations. Remarkably, this polarization rotation also drives a spin-topological transition, changing the spin Chern number from $C_s=-2$ to $0$. Directional uniaxial strain further steers the polarization orientation and tunes the associated topological transition. These results establish polarization rotation as the switching mechanism of ferroelectric Bi monolayer and as an efficient route to electrically and mechanically programmable topology in two-dimensional ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Stacking-dependent thermoelectric transport in layered Sc_2Si_2Te_6 from first principles

Stacking polymorphism is a common characteristic of van der Waals layered materials and can substantially modify their physical properties. Here, based on first-principles calculations combined with electron and phonon transport theories, we systematically investigate the thermodynamic stability, electronic structure, lattice dynamics, and thermoelectric performance of Sc_2Si_2Te_6 with three high-symmetry stacking sequences, namely, AA, AB, and ABC. We find that the AA- and AB-stacked structures are nearly degenerate in energy with the experimentally reported ABC phase, and that the maximum sliding barrier among these stacking sequences is only about 10~meV/atom, thereby accounting for the stacking faults observed experimentally. These three stacking sequences exhibit distinct electronic structures, with the conduction-band minimum being highly sensitive to the stacking sequence. As a consequence, the conduction-band degeneracies are 12, 2, and 8 for the ABC, AA, and AB stackings, respectively, leading to markedly different electronic transport properties near the band edge. The lattice thermal conductivity is governed primarily by three-phonon scattering, whereas four-phonon scattering provides an additional reduction, particularly in the ABC stacking. Among the three structures, the AB stacking exhibits the lowest lattice thermal conductivity owing to its stronger three-phonon scattering and lower phonon group velocity. As a result, the maximum thermoelectric figure of merit, ZT, is achieved in the ABC structure, followed closely by the AB structure, whereas the AA structure shows a substantially reduced value. These results demonstrate that the stacking sequence exerts a non-negligible influence on the thermoelectric performance of Sc_2Si_2Te_6 and suggest that suppressing the formation of the AA stacking is important for achieving high thermoelectric performance.

cond-mat.mtrl-sci↗

arXiv2026

Weak Polar Optical Phonon Scattering Decouples Electron and Phonon Transport in Layered Thermoelectric Materials

High-performance thermoelectric (TE) materials are crucial for efficient waste-heat recovery and solid-state cooling technologies. A persistent challenge in TE materials design arises from the strong interdependence among the electrical conductivity ($\sigma$), Seebeck coefficient ($S$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$). Layered compounds can effectively suppress $\kappa_{\mathrm{L}}$ along the cross-plane direction owing to weak interlayer interactions; however, they often suffer from low carrier mobility ($\mu$) caused by limited band dispersion and strong polar optical phonon (POP) scattering. Here, we perform high-throughput density functional theory calculations to screen 236 layered semiconductors and identify candidates with low effective mass ($m^{*}$) and weak POP scattering. We identify 23 compounds with high cross-plane $\mu$, among which 14 exhibit large power factors ($S^{2}\sigma$). Notably, GaGe$_{2}$Te stands out with exceptionally high cross-plane $\sigma$ and power factor, enabled by a favorable combination of small $m^{*}$ and a small ionic dielectric constant. Simultaneously, GaGe$_{2}$Te exhibits an ultralow cross-plane $\kappa_{\mathrm{L}}$ of 0.57~W~m$^{-1}$~K$^{-1}$ at 300~K, originating from weak interlayer bonding and pronounced phonon anharmonicity. These results demonstrate an effective strategy to decouple electron and phonon transport in layered materials by mitigating POP scattering, thereby providing a promising pathway toward high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2026

Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

AgNbO$_3$ is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group $R3$, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses--including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics--can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO$_3$ but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic Origin of the Ultralow Lattice Thermal Conductivity in Vacancy-Ordered Halide Double Perovskites Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$= Cl, Br, and I)

Vacancy-ordered halide double perovskites Cs$_2BX_6$ have recently attracted significant attention due to their intrinsically ultralow lattice thermal conductivity ($\kappa_{\mathrm{L}}$), which is highly desirable for thermal insulation and thermoelectric applications. In this work, we systematically investigate the anharmonic lattice dynamics and thermal transport properties of Cs$_2BX_6$ ($B$ = Zr, Pd, Sn, Te, Hf, and Pt; $X$ = Cl, Br, and I) using state-of-the-art first-principles calculations, based on a unified theory of thermal transport for crystals and glasses. All studied compounds are found to exhibit ultralow $\kappa_{\mathrm{L}}$ below 1.0~W\,m$^{-1}$\,K$^{-1}$ at room temperature and large derivation from the conventional $T^{-1}$ temperature dependence. Our analysis combining with machine-learning approach show that low sound velocities (1100 -- 1600~m\,s$^{-1}$), which originates from the intrinsically weak chemical bonding, play a crucial role in suppressing heat transport of the most compounds, instead of the strong scattering of rattling phonon modes expected from the large void in the structure. Furthermore, the influence of $B$ and $X$-site elements on phonon dispersion, anharmonicity, and scattering phase space is clarified. Our results provide microscopic insights into the origin of ultralow $\kappa_{\mathrm{L}}$ in Cs$_2BX_6$ and offer guiding principles for the rational design of halide-based materials with tailored thermal transport properties.

cond-mat.mtrl-sci↗

arXiv2025

Strong Intra- and Interchain Orbital Coupling Leads to Multiband and High Thermoelectric Performance in Na$_2$Au$X$ ($X$ = P, As, Sb, and Bi)

The intrinsic coupling among electrical conductivity ($\sigma$), Seebeck coefficient ($S$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$) imposes a fundamental limit on the dimensionless figure of merit $ZT$ in thermoelectric (TE) materials. Increasing band degeneracy can effectively balance $\sigma$ and $S$, enabling a high power factor (PF, $S^{2}\sigma$). However, compounds with intrinsically large band degeneracy are scarce. Here, we present an unconventional strategy to realize elevated band degeneracy in zigzag-chain Na$_2$Au$X$ ($X$ = P, As, Sb, Bi) compounds by harnessing strong intra- and interchain orbital coupling. Pronounced hybridization between Au-$d_{z^{2}}$ and $X$-$p_{z}$ orbitals along the Au--$X$ zigzag chains, together with unexpectedly strong interchain $X$-$p_{x}/p_{y}$ coupling, produces a highly dispersive, multivalley valence band structure that supports an exceptional PF. Concurrently, the intrinsically weak interchain interactions arising from the quasi-one-dimensional framework, together with the weakened Au--$X$ and Au--Au bonds within the chains due to filling of $p$-$d^{*}$ antibonding states, result in an ultralow $\kappa_{\mathrm{L}}$. First-principles calculations combined with Boltzmann transport theory predict that $p$-type Na$_2$AuBi achieves a PF of $63.9\,\mu\mathrm{W}\,\mathrm{cm}^{-1}\,\mathrm{K}^{-2}$, an ultralow $\kappa_{\mathrm{L}}$ of $0.49\,\mathrm{W}\,\mathrm{m}^{-1}\,\mathrm{K}^{-1}$, and a maximum $ZT$ of $4.7$ along the zigzag-chain direction at $800\,\mathrm{K}$. This work establishes a new design paradigm for high-efficiency TE materials by exploiting substantial orbital overlap in structurally weakly bonded, quasi-one-dimensional systems, opening promising avenues for the discovery and engineering of next-generation high-performance TE materials.

cond-mat.mtrl-sci↗

arXiv2025

Synergetic Enhancement of Power Factors and Suppression of Lattice Thermal Conductivities via Biaxial Strain in ScAgSe$_2$ and TmAgTe$_2$

The challenge of achieving high thermoelectric (TE) performance is mainly from the entanglement among Seebeck coefficient ($S$), electrical conductivity ($\sigma$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$). In this work, we propose a synergetic strategy of enhancing power factor (PF, $S^2\sigma$) and suppressing $\kappa_{\mathrm{L}}$ by applying a biaxial tensile strain in two silver chalcogenides ScAgSe$_2$ and TmAgTe$_2$ with TlCdS$_2$-type structure. The forbidden $p$-$d$ orbital coupling at the $\Gamma$ point and allowed $p$-$d$ orbital coupling at the A point and the middle of $\Lambda$ line leads to high electronic band dispersion along the $\Gamma$-A direction and a high-degeneracy valence band valley ($\Lambda_2$). The elongation of the Ag-Se bond under tensile strain weakens the orbital coupling between Ag-$d$ and Se/Te-$p$ orbitals and reduces the band energy at the A point, which aligns the valence band and achieving a high band degeneracy. Concurrently, the weaker Ag-Se/Ag-Te bond under a small tensile strain leads to lower phonon group velocity and strong three- and four phonon scatterings, leading to lower $\kappa_{\mathrm{L}}$. Our first-principles calculations combined with electron-phonon coupling analysis as well as phonon and electron Boltzmann transport equations show that applying a 3\% (2\%) tensile strain can enhance the PF along the $c$-axis of ScAgSe$_2$ (TmAgTe$_2$) by 243\% (246\%) at a carrier concentration of 3$\times$10$^{20}$ cm$^{-3}$ and reduce the $\kappa_{\mathrm{L}}$ by 37\% (26\%) at 300 K. Consequently, 2 $\sim$ 4 times of $ZT$ enhancement is obtained by 3\% or 1\% tensile strain in ScAgSe$_2$ (TmAgTe$_2$) at 300 K, achieving a maximum $ZT$ of 3.10 (3.62) at 800 K. Our material design strategy based on molecular orbital analysis reveals an effective route to boosting TE performance, and can be extended to other systems as well.

cond-mat.mtrl-sci↗

arXiv2025

Thermoelectric Properties of Copper-based Chalcopyrite Semiconductors Cu$MX_2$ ($M$ = Al, Ga, and In; $X$ = S, Se, and Te) from First-Principles Calculations

Copper-based chalcopyrite semiconductors have attracted sustained interest owing to their promising thermoelectric (TE) performance, yet the microscopic origins of their TE behavior remain incompletely understood. Here, we systematically investigate the TE properties of Cu$MX_2$ ($M=$ Al, Ga, and In; $X=$ S, Se, and Te) using first-principles calculations. For $p$-type doping, the calculated electrical conductivities ($\sigma$), hole mobilities ($\mu$), Seebeck coefficients ($S$), and power factors (PFs) of CuGaTe$_2$ and CuInTe$_2$ show excellent agreement with experimental data. At fixed temperature and hole concentration, as $X$ varies from S to Te, the hole mobility increases markedly due to progressively weaker polar--optical--phonon scattering, reflecting the reduced ionic contribution to the dielectric response in compounds with heavier chalcogens. Combined with smaller transport effective masses, Cu$M$Te$_2$ compounds therefore exhibit high $\sigma$ and large PFs. Across the Cu$MX_2$ family, the anomalously lower $\kappa_{\mathrm{L}}$ of Cu$M$Se$_2$ relative to Cu$M$Te$_2$ arises primarily from enhanced three-phonon scattering at low-frequency region. For a given $M$, Cu$M$S$_2$ displays the steepest temperature-induced decrease in $\kappa_{\mathrm{L}}$ and attains a smaller $\kappa_{\mathrm{L}}$ than Cu$M$Se$_2$ and Cu$M$Te$_2$ at 800~K. Given the low band degeneracy and comparatively modest hole mobilities of Cu$MX_2$ compounds, the most effective routes to further improve their TE performance are to enhance $\sigma$ and reduce $\kappa_{\mathrm{L}}$ through doping.

cond-mat.mtrl-sci↗

arXiv2025

Data-driven Discovery of Novel High-performance Quaternary Chalcogenide Photovoltaics

Photovoltaic materials facilitate the conversion of sunlight into electricity by harnessing the interaction between light and matter, offering an eco-friendly and cost-efficient energy solution. Combining data-driven approaches with static and time-dependent density functional theories and nonadiabatic molecular dynamics simulations, we predict 14 high-performance photoabsorber materials from a family of known quaternary semiconductors. Among these, we investigate four compounds - SrCuGdSe3, SrCuDyTe3, BaCuLaSe3, and BaCuLaTe3 in greater detail. Hybrid density functional theory calculations including spin-orbit coupling reveal that SrCuGdSe3, SrCuDyTe3, BaCuLaSe3 and BaCuLaTe3 possess direct band gaps of 1.65, 1.79, 1.05, and 1.01 eV, respectively. These band gap values lie close to an optimal range ideal for visible-light absorption. Consequently, the calculated optical absorption coefficient and spectroscopic limited maximum efficiency for these compounds become comparable or larger than crystalline silicon, GaAs, and methylammonium lead iodide. Calculated exciton binding energies for these compounds are relatively small (30-32 meV), signifying easy separation of the electron-hole pairs, and hence enhanced power conversion efficiencies. Investigations of photoexcited carrier dynamics reveal a relatively long carrier lifetime (~ 30-40 ns), suggesting suppressed nonradiative recombination and enhanced photo-conversion efficiencies. We further determined the defect formation energies in these compounds, which showed that despite the likely formation of cation vacancies and interstitial defects, midgap states remain absent making these defects non-detrimental to carrier recombination. Our theoretical predictions invite experimental verification and encourage further investigations of these and similar compounds in this quaternary semiconductor family.

cond-mat.mtrl-sci↗

arXiv2025

Forbidden p-d Orbital Coupling Accelerates High-Power-Factor Materials Discovery

The intrinsic entanglement between electrical conductivity ($\sigma$) and the Seebeck coefficient ($S$) significantly constrains power factor (PF) enhancement in thermoelectric (TE) materials. While high valley degeneracy ($N_{\mathrm{vk}}$) effectively balances $\sigma$ and $S$ to improve PF, identifying compounds with high $N_{\mathrm{vk}}$ remains challenging. In this study, we develop an effective approach to rapid discover $p$-type semiconductors with high $N_{\mathrm{vk}}$ through manipulating anion-$p$ and cation-$d$ orbital coupling. By prohibiting $p$-$d$ orbital coupling at the $\Gamma$ point, the valence band maximum shifts away from the $\Gamma$ point (where $N_{\mathrm{vk}}$=1), thereby increasing $N_{\mathrm{vk}}$. Through the examination of the common irreducible representations of anion-$p$ and cation-$d$ orbitals at the $\Gamma$ point, we identify 7 compounds with $N_{\mathrm{vk}}$ $\ge$ 6 from 921 binary and ternary semiconductors. First-principles calculations with electron-phonon coupling demonstrate that PtP$_2$, PtAs$_2$, and PtS$_2$ exhibit exceptionally high PFs of 130, 127, and 82 $\mu$Wcm$^{-1}$K$^{-2}$ at 300K, respectively, which are three to five times higher than those of the well-studied TE materials. This work not only elucidates the underlying mechanism of high $N_{\mathrm{vk}}$ formation through group theory, but also establishes an efficient high-PF material discovery paradigm, extended to more complex systems.

cond-mat.mtrl-sci↗

arXiv2025

Stacking effects on magnetic, vibrational, and optical properties of CrSBr bilayers

The van der Waals layered semiconductor CrSBr, which exhibits A-type antiferromagnetism and a relatively high N\'{e}el temperature, has been successfully exfoliated into atomically thin sheets. In this study, we investigate the structural, lattice dynamical, electronic, magnetic, and optical properties of four distinct stacking structures of CrSBr bilayers using first-principles calculations and Monte Carlo simulations. Our findings show that though the most energetically favorable bilayer structure retains the stacking pattern of the bulk counterpart, three other high-symmetry stacking structures can be achieved by sliding one of the layers along three distinct directions, with energy costs comparable to that observed in MoS$_2$ bilayer. All these four bilayers exhibit semiconductor behavior with A-type antiferromagnetic ordering, similar to the bulk material, and demonstrate closely aligned N\'{e}el temperatures. Moreover, these bilayers exhibit relatively low lattice thermal conductivities, pronounced anisotropy, and a strong dependence on stacking patterns. This behavior is attributed to significant phonon-phonon scattering arising from avoided crossings between acoustic and optical phonons, as well as the presence of flat optical phonon bands in the low-frequency region. While the electronic structures and optical properties of these bilayers show weak dependence on the stacking pattern for antiferromagnetic ordering, they undergo significant changes for ferromagnetic ordering, influencing the band gap, valence and conduction band splitting, and effective mass. Furthermore, we found that antiferromagnetic ordering can transition to ferromagnetic under intense visible light illumination. Thus, the integration of layer stacking and visible light illumination offers an effective means to control the heat transfer, magnetic, and optical properties of CrSBr bilayers.

cond-mat.mtrl-sci↗

arXiv2024

Realizing Intrinsically Glass-like Thermal Transport via Weakening the Ag-Ag Bonds in Ag$_{6}$ Octahedra

Crystals exhibiting glass-like and low lattice thermal conductivity ($\kappa_{\rm L}$) are not only scientifically intriguing but also practically valuable in various applications, including thermal barrier coatings, thermoelectric energy conversion, and thermal management. However, such unusual $\kappa_{\rm L}$ are typically observed only in compounds containing heavy elements, with large unit cells, or at high temperatures, primarily due to significant anharmonicity. In this study, we utilize chemical bonding principles to weaken the Ag-Ag bonds within the Ag$_6$ octahedron by introducing a ligand in the bridge position. Additionally, the weak Ag-chalcogen bonds, arising from fully filled $p$-$d$ antibonding orbitals, provide an avenue to further enhance lattice anharmonicity. We propose the incorporation of a chalcogen anion as a bridge ligand to promote phonon rattling in Ag$_6$-octahedron-based compounds. Guided by this design strategy, we theoretically identified five Ag$_6$ octahedron-based compounds, $A$Ag$_3X_2$ ($A$ = Li, Na, and K; $X$ = S and Se), which are characterized by low average atomic masses and exhibit exceptionally strong four-phonon scattering. Consequently, these compounds demonstrate ultralow thermal conductivities (0.3 $\sim$ 0.6 Wm$^{-1}$K$^{-1}$) with minimal temperature dependence (T$^{-0.1}$) across a wide temperature range. Experimental validation confirmed that the $\kappa_{\rm L}$ of NaAg$_3$S$_2$ is 0.45 Wm$^{-1}$K$^{-1}$ within the temperature range of 200 to 550 K. Our results clearly demonstrate that weak chemical bonding plays a crucial role in designing compounds with glass-like $\kappa_{\rm L}$, highlighting the effectiveness of chemical bonding engineering in achieving desired thermal transport properties.

cond-mat.mtrl-sci↗

arXiv2024

Bonding Hierarchy and Coordination Interaction Leading to High Thermoelectricity in Wide Bandgap TlAgI2

High thermoelectric properties are associated with the phonon-glass electron-crystal paradigm. Conventional wisdom suggests that the optimal bandgap of semiconductor to achieve the largest power factor should be between 6 and 10 kbT. To address challenges related to the bipolar effect and temperature limitations, we present findings on Zintl-type TlAgI2, which demonstrates an exceptionally low lattice thermal conductivity of 0.3 W m-1 K-1 at 300 K. The achieved figure of merit (ZT) for TlAgI2, featuring a 1.55 eV bandgap, reaches a value of 2.20 for p-type semiconductor. This remarkable ZT is attributed to the existence of extended antibonding states Ag-I in the valence band. Furthermore, the bonding hierarchy, influencing phonon anharmonicity, and coordination bonds, facilitating electron transfer between the ligand and the central metal ion, significantly contribute to electronic transport. This finding serves as a promising avenue for the development of high ZT materials with wide bandgaps at elevated temperatures.

cond-mat.mtrl-sci↗