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.

Strong electron-phonon coupling and multigap superconductivity in 2H/1T Janus MoSLi monolayer.

Two-dimensional (2D) Janus transition metal dichalcogenides MXY manifest novel physical properties owing to the breaking of out-of-plane mirror symmetry. Recently, the 2H phase of MoSH has been demonstrated to possess intrinsic superconductivity, whereas the 1T phase exhibits a charge density waves state. In this paper, we have systematically studied the stability and electron-phonon interaction characteristics of MoSLi. Our results have shown that both the 2H and 1T phases of MoSLi are stable, as indicated by the phonon spectrum and the ab initio molecular dynamics. However, the 1T phase exhibits an electron-phonon coupling constant that is twice as large as that of the 2H phase. In contrast to MoSH, the 1T phase of MoSLi exhibits intrinsic superconductivity. By employing the ab initio anisotropic Migdal-Eliashberg formalism, we have revealed the two-gap superconducting nature of 1T-MoSLi, with a transition temperature (Tc) of 14.8 K. The detailed analysis indicates that the superconductivity in 1T-MoSLi primarily originates from the interplay between the vibration of the phonon modes in the low-frequency region and the dz2 orbital. These findings provide a fresh perspective on superconductivity within Janus structures.

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.

Two-gap-like anisotropic superconductivity in a bulk boron kagome lattice.

Since a report of superconductivity in elemental boron at high pressure [M. I. Eremets et al., Science, 2001, 293, 272-274], many efforts have been devoted to the search for superconductivity in diverse boron allotropes. However, there are few superconducting phenomena to be discovered theoretically and experimentally in elemental bulk boron crystals at normal pressure to date. In this paper, we propose a metastable but dynamically stable metallic bulk boron phase within the kagome lattice, and demonstrate from first principles good superconductivity with a high superconducting critical temperature Tc, e.g., ∼34-39 K, in the elemental bulk boron at ambient pressure. Our calculations indicate that such a high-Tc superconductivity is closely related to the Fermi surface displaying strong electron-phonon coupling with a two-region-like distribution feature, which resulted from two different types of covalent bonding crossing the Fermi level and also gives rise to a two-gap-like superconducting nature in the system. We uncover that the strong electron-lattice coupling is dominated by the transversal acoustic phonon modes around a degenerate softening kink that places the system on the verge of a latent charge density wave. The present findings shed light on a study of the high-Tc superconductivity of the elemental bulk boron phase at normal pressure.

Prediction of superconductivity in sandwich XB4 (X = Li, Be, Zn and Ga) films.

Borophenes and 2D boron allotropes are metallic and exhibit a BCS superconducting state, unlike graphene. In-plane stretching vibrational modes in bulk MgB2 boron layers induce phonon-mediated superconductivity. However, the effect of stretching vibrational phonon modes on transition temperature (Tc) still requires further investigations. Here, we use first-principles calculations combined with conventional BCS theory to explore the superconducting properties in a series of dynamically stable boron-based sandwich films that have not been realized experimentally. The sandwich films of XB4 (where X = Li, Be, Zn, Ga) are predicted to exhibit good phonon-mediated superconductivity with high Tc values of 25.1 K, 28.7 K, 38.7 K, and 36.2 K, respectively. The origin of the superconducting states is mainly caused by the high metallicity and strong electron-phonon coupling (EPC), which can be attributed to the presence of intercalated atoms within the borophene layers. It is further demonstrated in the XB4 compounds (where X = Li, Be, Zn, Ga) that the pronounced EPC is not solely attributable to the in-plane vibrations of B atoms, but it is also influenced significantly by the out-of-plane vibrations of B atoms. Sandwich (Li,Be,Zn,Ga)B4 films may be a great choice for nanoscale superconductors as the electron-phonon coupling parameter becomes greater than unity, thereby providing a powerful approach for investigating these systems with high critical temperatures.

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.

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.

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.

Ultralow lattice thermal conductivities and excellent thermoelectric properties of hypervalent triiodides XI3 (X = Rb, Cs) discovered by machine learning method.

Thermal conductivity and power factor are key factors in evaluating heat transfer performance and designing thermoelectric conversion devices. To search for materials with ultralow thermal conductivity and a high power factor, we proposed a set of universal statistical interaction descriptors (SIDs) and developed accurate machine learning models for the prediction of thermoelectric properties. For lattice thermal conductivity prediction, the SID-based model achieved the state-of-the-art results with an average absolute error of 1.76 W m-1 K-1. The well-performing models predicted that hypervalent triiodides XI3 (X = Rb, Cs) have ultralow thermal conductivities and high power factors. Combining first-principles calculations, the self-consistent phonon theory, and the Boltzmann transport equation, we obtained the anharmonic lattice thermal conductivities of 0.10 and 0.13 W m-1 K-1 for CsI3 and RbI3 in the c-axis direction at 300 K, respectively. Further studies show that the ultralow thermal conductivity of XI3 arises from the competition of vibrations between alkali metal atoms and halogen atoms. In addition, at 700 K, the thermoelectric figure of merit ZT values of CsI3 and RbI3 are 4.10 and 1.52, respectively, at the optimal hole doping level, which indicates hypervalent triiodides are potential high performance thermoelectric materials.

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.

Motif based high-throughput structure prediction of superconducting monolayer titanium boride.

Two-dimensional boron structures, due to their diverse properties, have attracted great attention because of their potential applications in nanoelectronic devices. A series of TiBn (2 ≤ n ≤ 13) monolayers are efficiently constructed through our motif based method and theoretically investigated through high-throughput first-principles calculations. The configurations are generated based on the motifs of boron dimeric/triangular/quadrilateral fragments and multi-coordinate titanium-centered boron molecular wheels. Besides previously reported TiB4 and TiB9 which were discovered by the global search method, we predict that high symmetry monolayer TiB7 (Cmmm), which is octa-coordinate titanium boride, is dynamically stable. The TiB7 monolayer is a BCS superconductor with a transition temperature Tc of up to 8.3 K. The motif based approach is proved to be efficient in searching stable structures with prior knowledge so that the potentially stable transition metal monolayers can be quickly constructed by using basic cluster motifs. As an efficient way of discovering materials, the method is easily extended to predict other types of materials which have common characteristic patterns in the structure.

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.

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.

Phonon-mediated superconductivity in Mg intercalated bilayer borophenes.

Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes BxMgBx (x = 2-5). Remarkably, B2MgB2 and B4MgB4 are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (Tc) of 23.2 K and 13.3 K, respectively, while B4MgB4 is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg-B systems with high Tcs.

Spin-semiconducting properties in silicene nanoribbons.

We have investigated the relative stabilities and electronic properties of silicene nanoribbons with sawtooth edges (SSiNRs) by first-principles calculations. The SSiNR is more stable than the zigzag silicene nanoribbon (ZSiNR) and has a ferromagnetic ground state with an intrinsic energy gap between majority and minority spin-polarized bands, which shows that SSiNR is a spin-semiconductor. Under an external transverse electric field, the energy gap decreases and even vanishes. Meanwhile, the charge densities of the two edge bands near the Fermi level become spatially separated at different edges. We find also that the electric field-induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. At last, the electronic structures of SSiNRs tuned by electric field and strain together are studied, showing that a small tensile strain makes the SSiNRs more sensitive to the electric field. These results suggest that the electric field or/and strain modulated SSiNRs have potential applications in silicon-based spintronic nanodevices.

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.

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.

Intrinsic Thermal conductivities of monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te).

The successful synthesis of the single to few layer transition metal dichalcogenides has opened a new era in the nanoelectronics. For their efficient implementations in the electronic devices while taking care of their overheating issues, the characterization of their thermal transport properties is extremely vital. So, we have systematically investigated the thermal transport properties of monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) by combining the first-principles calculations with Boltzmann transport equation. We find that monolayer WTe2 possesses the lowest lattice thermal conductivity κL (33:66 Wm-1K-1 at 300 K) among these six semiconducting materials, in contrast to the highest κL (113:97 Wm-1K-1 at 300 K) of WS2 among them. Further analyses reveal that the higher (lower) anharmonic and isotopic scatterings together with the lower (higher) phonon group velocities lead to the lowest (highest) value of κL in WTe2 (WS2) monolayer. In addition, we have also calculated the cumulative thermal conductivity κC as a function of mean free path, which indicates that the nanostructures with the length of about 400 nm would reduce κL drastically. These results offer important understanding from thermal conductivity point of view to design the 2D transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) electronics.

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.