Study on lattice dynamics and thermal conductivity of fluorite AF2 (A = Ca, Sr, Ba) based on first principles calculations.

Fluorite materials have received particular attention in electron optics due to their favorable optical properties. However, further exploration of these materials in the thermoelectric (TE) field is hampered by the lack of studies on their lattice thermal transport properties. In this work, we use first-principles calculations, combined with self-consistent phonon theory, compressive sensing lattice dynamics and the Boltzmann transport equation, to study the microscopic mechanism of lattice thermal transport properties in AF2 (A = Ca, Sr, Ba) with a fluorite structure. We investigate the effects of three-phonon and four-phonon scattering and quartic anharmonic renormalization of phonon frequencies on this system. The results show that the bonding strength of atoms A (Ca, Sr, and Ba) plays an important role in the thermal transport process, and the third-order anharmonicity also plays an important role in this system. Meanwhile, the role of the quartic anharmonicity cannot be ignored. Our findings not only fill in the gaps in the study of lattice thermal transport of fluorite materials, but also deepen the comprehensive understanding of the high κL value of fluorite materials.

Effects of Pd atom vibration in the Sr-Te octahedral interstitial space in Zintl compound SrPdTe on lattice anharmonicity and thermoelectric properties.

Based on first-principles calculations, the current study deeply explores the thermoelectric properties of the Zintl compound SrPdTe. We found that the anharmonic vibration of Pd atoms plays an important role in the quartic anharmonic effect and the temperature dependence of the thermal conductivity. In the crystalline structure, Sr atoms form octahedra with eight surrounding Te atoms, while Pd atoms are located in the gaps between the octahedra. This structure makes the strong atomic mean square displacement of Pd atoms the main factor leading to the ultralow thermal conductivity. The study also reveals the effects of phonon frequency renormalization and four-phonon scattering on heat transfer performance. Even considering the spin-orbit coupling effect, multiple secondary valence band tops maintain the power factor of the material at high temperatures, providing a potential opportunity for achieving excellent thermoelectric performance.

Correlation of rattlers with thermal transport and thermoelectric performance.

The presence of rattlers in the host-guest structure has sparked great interest in the field of thermoelectrics, as it allows for the suppression of thermal transport in materials through vigorous anharmonic vibrations. This work predicts a ternary half-Heusler compound, LiAgTe, with good thermoelectric properties and high-temperature stability, which possesses a host-guest structure. Furthermore, it provides a detailed analysis of the role of rattlers in the transport process. By microscopically exploring rattlers, we have revealed that rattlers (Ag atoms), while suppressing the thermal transport properties of the host framework, provide a significant enhancement of the electronic transport capability through the provision of nearly free weakly bound electrons. Using self-consistent phonon theory combined with compressive sensing lattice dynamics method, we captured the exact lattice thermal conductivity considering quartic anharmonicity and four-phonon scattering, and obtained the electronic transport parameters through the calculation of τe, which includes full anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering. We systematically dissected the role of rattlers in the host-guest structure by combining methods such as electron local function, Bader charge density, and Vibration visualization. The anharmonic vibrations of rattlers enhance the temperature response of scattering, resulting in rapid deterioration of thermal transport at high temperatures. Moreover, the extended d-orbital electrons of the rattlers, together with the p-orbital electrons of the Te atom in the host framework, result in the coexistence of maximum degeneracy and high dispersion bands in the valence band, which greatly enhances the electronic transport properties.

Strong anharmonicity and high thermoelectric performance of cubic thallium-based fluoride perovskites TlXF3 (X = Hg, Sn, Pb).

State-of-the-art first-principles calculations are performed to investigate the thermoelectric transport properties in thallium-based fluoride perovskites TlXF3 (X = Hg, Sn, Pb) by considering anharmonic renormalization of the phonon energy and capturing reasonable electron relaxation times. The lattice thermal conductivity, κL, of the three compounds is very low, among which TlPbF3 is only 0.42 W m-1 K-1 at 300 K, which is less than half of that of quartz glass. The low acoustic mode group velocity and strong three-phonon scattering caused by the strong anharmonicity of the Tl atom are the origin of the ultralow κL. Meanwhile, the strong ionic bonds between X (X = Hg, Sn, Pb) and F atoms provide good electrical transport properties. The results show that the ZT value of TlHgF3 at 900 K is 1.58, which is higher than the 1.5 value of FeNbSb at 1200 K. TlSnF3 and TlPbF3 also exceed 1, which is close to the classical thermoelectric material PbTe:Na. Furthermore, we present the methods and expected effects of improving the ZT value through nanostructures.

Thermoelectric transport properties of metal phosphide XLiP (X = Sr,Ba).

Metal phosphides are stable and have excellent electrical characteristics, their high thermal conductivity has prevented them from being used as thermoelectric materials. In this paper, the thermoelectric transport properties of XLiP (X = Sr Ba) are investigated on the basis of first-principles calculations, Boltzmann transport equation and self-consistent phonon theory. In addition, we also consider the effect of quartic anharmonicity on the thermal transport properties and lattice dynamics of SrLiP and BaLiP. The strong anharmonicity of the two compounds make the lattice thermal conductivity decrease rapidly with the increase of temperature. At 300 K, the lattice thermal conductivity of SrLiP and BaLiP on thea(b)-axis is only 2.98 and 2.93 Wm-1K-1, respectively. Due to its excellent electron transport properties, it has greater conductivity in thea(b) axis. Finally, due to the energy pocket and anisotropy at the bottom of the conduction band, the n-type maximum ZT values of trapped SrLiP and BaLiP on thea(b) axis are 0.87 and 0.94 at 900 K, respectively. The high thermoelectric performance of both compounds encourages further studies on the thermoelectric properties of metal phosphides.

Thermoelectric transport properties of orthorhombic RbBaX (X = Sb, Bi) with strong anharmonicity.

The thermoelectric properties of RbBaX (X = Sb, Bi), an anisotropic material with strong anharmonicity, are systematically studied by first-principles calculations, combined with the self-consistent phonon theory and the Boltzmann transport equation. A reasonable lattice thermal conductivity κL is captured by fully handling the phonon frequency shift and four-phonon scattering caused by the quartic anharmonicity. The κL of RbBaSb and RbBaBi along the a-axis is only 0.60 and 0.36 W m-1 K-1 at 300 K, respectively, which is much lower than that of most thermoelectric materials. The low phonon group velocity resulting from the unusually weak atomic bonding strengths along the a-axis is the origin of the ultralow κL. Furthermore, the high dispersion near the conduction band minimum enables n-type doping with a higher electrical conductivity. The results show that orthorhombic RbbaBi has a ZT as high as 1.04 at 700 K along the a-axis direction, indicating its great application potential in the thermoelectric field.

Ultra-low lattice thermal conductivity and anisotropic thermoelectric transport properties in Zintl compound ß-K2Te2.

A good thermoelectric (TE) performance is usually the result of the coexistence of an ultralow thermal conductivity and a high TE power factor in the same material. In this paper, we investigate the thermal transport and TE properties of the Zintl compound ß-K2Te2 based on a combination of first-principles calculations and the Boltzmann transport equation. Remarkably, the calculated lattice thermal conductivity κL in hexagonal ß-K2Te2 is ultralow with a value of 0.19 (0.30) W m-1 K-1 along the c (a and b) axis at 300 K due to the small phonon group velocity and phonon lifetime, which is comparable to the κL for wood and promises possible good TE performance. By taking the fully anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering into account, the rational electron scattering and transport properties are captured, which indicates a power factor exceeding 2.0 mW m-1 K-2. As a result, the anomalously high n-type ZT of 2.62 and p-type ZT of 3.82 at 650 K along the c axis are obtained in the hexagonal ß-K2Te2, breaking the long-term record of ZT < 3.5 in the majority of the reported TE materials until now. These findings support that hexagonal ß-K2Te2 is a potential candidate for high-efficiency TE applications.

Two-Dimensional Conductive Metal-Organic Frameworks as Highly Efficient Electrocatalysts for Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LiSBs) which are expected to fulfill the increasing demands of high-density energy storage have been under intensive investigation. However, the development of LiSBs is facing many obstacles, such as the poor electronic conductivity of sulfur, shuttling effects of lithium polysulfides (LiPSs), sluggish Li2S decomposition, and low discharging/charging efficiency. Suitable electrocatalysts that can solve the above problems are promising in the development of LiSBs. Herein, 13 two-dimensional (2D) metal-organic frameworks (MOFs) of nitrogen-, sulfur-, and oxygen-coordinated transition-metal (TM) atoms (Co, Ni, Cu, and Zn) are selected and constructed to reveal the structure-activity relationship of 2D MOFs in terms of the electrocatalytic performance. Among all the 2D MOFs investigated, Cu3(HITP)2, Zn3(HITP)2, and Cu3(C18H9O3N3)2 offer moderate binding strength to LiPSs, which effectively suppresses Li2Sn dissolution and shuttling. Cu3(HITP)2 exhibits good electrical conductivity, a low Gibbs free energy barrier, effective electrocatalytic ability for Li2S decomposition, and a high sulfur loading amount. A descriptor φ is proposed to correlate the binding energies of the 2D MOFs to the coordination environment and the electronegativity of the TM atoms in the LiPSs via an implicit volcano plot. These findings are helpful for understanding the electrocatalytic effect of 2D MOFs in LiSBs and represent a promising approach for the development of future LiSBs.

The excellent TE performance of photoelectric material CdSe along with a study of Zn(Cd)Se and Zn(Cd)Te based on first-principles.

Zn(Cd)Se and Zn(Cd)Te are well known for their excellent photoelectric performance, however, their thermoelectric (TE) properties are usually ignored. By taking advantage of first-principles calculations, the Boltzmann transport equation and semiclassical analysis, we executed a series of thermal and electronic transport investigations on these materials. Our results show that CdSe has the lowest anisotropic thermal conductivity, κ L, of the four materials, at 4.70 W m-1 K-1 (c axis) and 3.85 W m-1 K-1 (a axis) at a temperature of 300 K. Inspired by the very low lattice conductivity, other thermoelectric parameters were calculated in the following research. At a temperature of 1200 K we obtained a pretty large power factor, S 2 σ, of 4.39 × 10-3 W m-1 K-2, and based it on the fact that the corresponding figure of merit ZT can reach 1.8 and 1.6 along the a axis and c axis, respectively. We revealed the neglected thermoelectric potential of CdSe by means of systematic studies and demonstrated that it is a promising material with both excellent photoelectric performance and thermoelectric performance.

Pressure induced excellent thermoelectric behavior in skutterudites CoSb3 and IrSb3.

Utilizing the first-principle calculations combined with Boltzmann transport equation (BTE) and semiclassical analysis, we have systematically investigated the electronic structure, lattice thermal conductivity κL, Seebeck coefficient S, and the dimensionless figure of merit zT as a function of hydrostatic pressure P in crystalline skutterudites CoSb3 and IrSb3. Interestingly, as the pressure increases, the band gap and κL show an approximate parabolic trend, which results in extraordinarily high S and excellent thermoelectric properties, and zT even exceeds 1.4(1.09) in IrSb3(CoSb3) at 54(58) GPa. This anomalous behavior arises from the electron distribution and intrinsic scattering processes. Further analyses indicate that (i) nonbonding electron pairs of Sb atoms are gradually transferred to the region between Co(Ir) and Sb atoms as the pressure increases, which leads to the formation of a partial metallic bond and thus the band gap first expands and then shrinks; (ii) the change of the strength of the anharmonic phonon scattering process results in the variation of κL. As a result, these behaviors cause excellent thermoelectric properties. Our results provide insight into the thermal transport properties of skutterudites, meanwhile, forecast potential high pressure applications for thermoelectric materials.

Low lattice thermal conductivity and excellent thermoelectric behavior in Li3Sb and Li3Bi.

Utilizing the first-principle calculations combined with Boltzmann transport equation and semiclassical analysis, we present a systematic investigation of the electron structure, lattice thermal conductivity [Formula: see text], Seebeck coefficient S, and the dimensionless figure of merit ZT of crystal Li3Sb and Li3Bi. The [Formula: see text] of 2.2 and 2.09 W m-1 K-1 are obtained at room temperature in Li3Sb and Li3Bi systems with the band gap of [Formula: see text] eV, respectively. The low [Formula: see text] can induce excellent thermoelectric properties. Thus the effect of doping on the transport properties has been judiciously researched and the maximum ZT of 2.42, 1.54 is obtained at 900 K in the p-type doped Li3Sb and p-type doped Li3Bi with the stable structures. Up to date, experimental finding of the maximum ZT is 2.6 at 850 K in the Cu2Se sample with 1 mol indium, our results are very close to this value. This letter provides insight into the thermal transport properties of Li3Sb and Li3Bi, meanwhile, supports that crystalline Li3Sb and Li3Bi may be promising materials for thermoelectric devices and application.

Phonon thermal transport in a class of graphene allotropes from first principles.

Utilizing first principle calculations combined with the phonon Boltzman transport equation (PBTE), we systematically investigate the phonon thermal transport properties of α, ß and γ graphyne, a class of graphene allotropes. Strikingly, at room temperature, a low lattice thermal conductivity κL of 21.11, 22.3, and 106.24 W m-1 K-1 is obtained in α, ß and γ graphyne, respectively, which are much lower than that of graphene. We observe contributions from the phonon modes below the specified frequency and find that many optical phonon modes play critical roles in the phonon thermal transport. These optic modes participate in thermal transport, enhancing the phonon scattering process, thus leading to the low κL value. Our results provide insights into the thermal transport of graphyne, and forecast its potential applications for thermoelectric and thermal barrier coatings.