Phonon Group Velocity Research Articles

A Theoretical Framework of Mechanical Properties of the Monolayer Graphene

For decades, scientists and researchers believed that two-dimensional (2D) crystals are thermodynamically unstable. Graphene was the first two dimensional material that has successfully been exfoliated from bulk graphite in 2004. We derive interatomic potentials for Graphene for two dimensional lattice structure and using Quasi-harmonic approximations, Mechanical Properties of monolayer Graphene were investigated. The compressibility, hardness, ductility, toughness, brittleness and bonding nature of the Graphene are too well connected with the SOECs. Thus, comprehensive studies on elastic properties are important to show the potential of Graphene in engineering applications. Present studies of monolayer Graphene have been carried out to investigate the elastic constants such as Young’s modulus, Poisson’s ratio, bulk modulus and shear modulus. With the help of elastic constants, the values longitudinal and transverse sound velocities have been computed. We, at present also find the phonon group velocities at Г points along symmetry directions by PYTHON Program. Mechanical Properties were calculated by PYTHON program is agreed very close with the result of other researchers.

Unexpected Enhanced Thermal Conductivity of GaxIn1–xSb Ternary Alloys

Alloying is one of the most common approaches to modulate the properties of semiconductors, such as indium antimonide (InSb) with practical applications in mid-infrared optoelectronics. Thermal conductivity plays a key role in governing the operating performance of devices, and the high-efficiency regulation is of great significance to desirable applications. In this study, we studied the thermal transport properties of InSb, GaSb, and their GaxIn1–xSb alloys by solving the Boltzmann transport equation based on first-principles calculations. The thermal conductivity of GaxIn1–xSb alloys (x = 0.25, κ = 9.21 Wm–1 K–1; x = 0.5, κ = 11.22 Wm–1 K–1; and x = 0.75, κ = 15.45 Wm–1 K–1) is enhanced compared with that of InSb (κ = 5.15 Wm–1 K–1), which is unlike the conventional belief that alloying usually reduces thermal conductivity. Through fundamental analysis of phonon transport properties, the higher phonon group velocity and phonon relaxation time caused by the weak phonon anharmonicity lead to higher thermal conductivity of the GaxIn1–xSb alloys. The weakened polarization of GaxIn1–xSb alloys occurring in the alloying process generates the increased thermal conductivity, which can be figured out by analyzing the orbital-projected electronic density of states and the electron localization functions. The underlying mechanism of unconventional high thermal conductivity of the GaxIn1–xSb alloys in this study would provide guidance for the device applications accompanied by thermal transport.

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 0.36W m-1 K-1 at 300K, 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 700K along the a-axis direction, indicating its great application potential in the thermoelectric field.

Enhancing phonon thermal transport in 2H-CrX2 (X = S and Se) monolayers through robust bonding interactions.

Inspired by the groundbreaking discovery of the 2H-MoS2 monolayer with outstanding physical properties, the electronic structure, structural stability, and thermal transport of 2H-CrX2 (X = S and Se) monolayers are theoretically evaluated using density functional theory (DFT) calculations and semiempirical Boltzmann transport theory. The 2H-CrX2 (X = S and Se) monolayers are direct semiconductors with the bandgaps of 0.91 and 0.69 eV. The elastic modulus and phonon dispersion curve analysis show that the 2H-CrX2 (X = S and Se) monolayers possess excellent mechanical and dynamic stabilities on account of elastic constants satisfying the Born-Huang criterion and the absence of negative frequencies. The thermal stabilities of the 2H-CrX2 (X = S and Se) monolayers at 300 K are proved by ab initio molecular dynamics (AIMD) simulations, as evidenced by the slight changes in the structural evolution and small fluctuation in total energy. High thermal conductivities of 131.7 and 88.6 W m-1 K-1 are discovered for 2H-CrS2 and 2H-CrSe2 monolayers at 300 K. Further analysis of the phonon group velocity, phonon relaxation time, and Grüneisen parameter shows that the high lattice thermal conductivities of 2H-CrX2 (X = S and Se) monolayers could be attributed to the great bond strength, large Young's modulus, relatively small atomic mass, high phonon group velocity, and long phonon relaxation time. In addition, the various scattering mechanisms are further considered in the calculations of phonon thermal transport to evaluate the effect of the scattering rates of the 2H-CrS2 and 2H-CrSe2 monolayers on the lattice thermal conductivity, and the determinative role is found for the phonon boundary scattering. Our present study would not only offer a fundamental understanding of the thermal transport properties of the 2H-CrX2 (X = S and Se) monolayers, but also provide theoretical guidelines for the experimental investigation of thermal management materials with 2H-phase.

Correlated missing linker defects increase thermal conductivity in metal-organic framework UiO-66.

Thermal transport in metal-organic frameworks (MOFs) is an essential but frequently overlooked property. Among the small number of existing studies on thermal transport in MOFs, even fewer have considered explicitly the influence of defects. However, defects naturally exist in MOF crystals and are known to influence many of their material properties. In this work, we investigate the influence of both randomly and symmetrically distributed defects on the thermal conductivity of the MOF UiO-66. Two types of defects were examined: missing linker and missing cluster defects. For symmetrically distributed (i.e., spatially correlated) defects, we considered three experimentally resolved defect nanodomains of UiO-66 with underlying topologies of bcu, reo, and scu. We observed that both randomly distributed missing linker and missing cluster defects typically decrease thermal conductivity, as expected. However, we found that the spatial arrangement of defects can significantly impact thermal conductivity. In particular, the spatially correlated missing linker defect nanodomain (bcu topology) displayed an intriguing anisotropy, with the thermal conductivity along a particular direction being higher than that of the defect-free UiO-66. We attribute this unusual defect-induced increase in thermal conductivity to the removal of the linkers perpendicular to the primary direction of heat transport. These perpendicular linkers act as phonon scattering sources such that removing them increases thermal conductivity in that direction. Moreover, we also observed an increase in phonon group velocity, which might also contribute to the unusual increase. Overall, we show that structural defects could be an additional lever to tune the thermal conductivity of MOFs.

The uniaxial zero thermal expansion and zero linear compressibility in distorted Prussian blue analogue RbCuCo(CN)6.

The uniaxial zero thermal expansion (ZTE) in distorted Prussian blue analogue (PBA) RbCuCo(CN)6 is reproduced by employing first-principles calculations, which agrees well with the experimental data. Also, the zero linear compressibility (ZLC) behavior in RbCuCo(CN)6 can be found. The special Jahn-Teller distortion introduced by Cu2+ in RbCuCo(CN)6 is noticed by investigating the change of the local structure with temperature and hydrostatic pressure. The lattice thermal conductivity (LTC) and phonon group velocity of RbCuCo(CN)6 are studied, where the LTC and phonon group velocity are significantly anisotropic. Especially, RbCuCo(CN)6 exhibits a quite low LTC, and its c-axis shows a characteristic of glasslike LTC at low temperatures. Our work facilitates a deep understanding of the coexistence mechanisms of uniaxial ZTE and ZLC properties in RbCuCo(CN)6.

Janus 2H-MXTe (M = Zr, Hf; X = S, Se) monolayers with outstanding thermoelectric properties and low lattice thermal conductivities.

Two-dimensional (2D) materials have been one of the most popular objects in the research field of thermoelectric (TE) materials and have attracted substantial attention in recent years. Inspired by the synthesized 2H-MoSSe and numerous theoretical studies, we systematically investigated the electronic, thermal, and TE properties of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers by using first-principles calculations. The phonon dispersion curves and AIMD simulations confirm the thermodynamic stabilities. Moreover, Janus 2H-MXTe were evaluated as indirect band-gap semiconductors with band gaps ranging from 0.56 to 0.90 eV using the HSE06 + SOC method. To evaluate the TE performance, firstly, we calculated the temperature-dependent carrier relaxation time with acoustic phonon scattering τac, impurity scattering τimp, and polarized scattering τpol. Secondly, the calculation of lattice thermal conductivity (κl) shows that these monolayers possess relatively poor κl with values of 3.4-5.4 W mK-1 at 300 K, which is caused by the low phonon lifetime and group velocity. After computing the electronic transport properties, we found that the n-type doped Janus 2H-MXTe monolayers exhibit a high Seebeck coefficient exceeding 200 μV K-1 at 300 K, resulting in a high TE power factor. Eventually, combining the electrical and thermal conductivities, the optimal dimensionless figure of merit (zT) at 300 K (900 K) can be obtained, which is 0.94 (3.63), 0.51 (2.57), 0.64 (2.72), and 0.50 (1.98) for n-type doping of ZrSeTe, HfSeTe, ZeSTe, and HfSTe monolayers. Particularly, the ZrSeTe monolayer shows the best TE performance with the maximal zT value. These results indicate the excellent application potential of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers in TE materials.

Thermoelectric properties of X3N2O2 (X = Hf, Zr) MXene monolayers: a first-principles study.

MXene monolayers have received increasing attention due to their unique properties, particularly their high conductivity, which shows great potential in thermoelectric materials. In this paper, we present a theoretical study of the thermoelectric properties of X3N2O2 (X = Hf, Zr) MXene monolayers, taking electron-phonon coupling into consideration. Owing to their similar geometrical structures, electronic band structures, and phonon dispersions, X3N2O2 MXene monolayers exhibit homogeneous electron and phonon transport properties. The conduction band shows multi-valley characteristics which leads to better n-type electron transport properties than p-type ones. The maximum values of the n-type power factor can reach 32 μW cm-1 K-2 for the Hf3N2O2 monolayer and 23 μW cm-1 K-2 for the Zr3N2O2 monolayer. In terms of phonon transport, the lattice thermal conductivity for the Zr3N2O2 monolayer is higher than that for the Hf3N2O2 monolayer, due to larger phonon group velocity. Our results show that the Hf3N2O2 monolayer is more suitable for thermoelectric materials than the Zr3N2O2 monolayer, with optimal n-type thermoelectric figure of merit (ZT) values of 0.36 and 0.15 at 700 K, respectively. These findings may be useful for the development of wearable thermoelectric devices and sensor applications based on X3N2O2 MXene monolayers.

Phonon interference effects in graphene nanomesh

Graphene nanomesh (GNM) is a single-layer graphene material that has a periodic distribution of nanoscale pores. GNM shows great potential applications in various fields such as thermoelectric energy conversion, energy storage, and field-effect transistors. In this study we utilize non-equilibrium molecular dynamics and lattice dynamics method to investigate the thermal transport mechanism of GNM. The thermal conductivity of GNM is mainly affected by the number of nanoscale pores and their horizontal and vertical spacing. Our study finds that as the number of nanoscale pores increases, the thermal conductivity of GNM decreases significantly. Additionally, the increase of the number of nanoscale pores causes phonon branch to be folded and confined, which results in a flatter dispersion curve, wider bandgap, and slower phonon group velocity. Moreover, the horizontal and vertical spacing of the nanoscale pores jointly affect the thermal transport process of GNM. When the horizontal spacing is small, the thermal conductivity of GNM decreases monotonically with the increase of vertical spacing, and increases monotonically with an increase of horizontal spacing. However, as the horizontal spacing increases, the interference effect caused jointly by phonon reflection and superposition leads to significant fluctuations in thermal conductivity. The analysis of the spectral heat flow, density of states, participation rate, and group velocity of GNM indicate that the variation in vertical spacing leads to different phonon contributions to heat flow, resulting in fluctuations in the thermal conductivity of GNM. These findings could serve as a reference for controlling the thermal transport of graphene nanomesh, and are of great significance in regulating the thermal conductivity and designing nanoscale pores in GNM.

Regulation of thermal conductivity of bilayer graphene nanoribbon through interlayer covalent bond and tensile strain

The interlayer bonding of graphene is a method of modifying graphene, which can change the mechanical property and conductivity of graphene, but also affect its thermal properties. In this paper, the non-equilibrium molecular dynamics method is used to study the thermal conductivity of bilayer graphene nanoribbon which is local carbon sp<sup>3</sup> hybridization (covalent bond formed between layers) under different concentration and angle of interlayer covalent bond chain and different tensile strain. The mechanism of the change of the thermal conductivity of bilayer graphene nanoribbon is analyzed through the density of phonon states. The results are as follows. The thermal conductivity of bilayer graphene nanoribbon decreases with the increase of the interlayer covalent bond concentration due to the intensification of phonon scattering and the reduction of phonon group velocities and effective phonon mean free path. Moreover, the decrease rate of thermal conductivity depends on the distribution angle of covalent bond chain. With the increase of interlayer covalent bond concentration, when the interlayer covalent bond chain is parallel to the direction of heat flow, the thermal conductivity decreases slowest because the heat transfer channel along the heat flow direction is gradually affected; when the interlayer covalent bond chain is at an angle with respect to the direction of heat flow, the thermal conductivity decreases more rapidly, and the larger the angle, the faster the thermal conductivity decreases. The rapid decline of thermal conductivity is due to the formation of interfacial thermal resistance at the interlayer covalent bond chain, where strong phonon-interface scattering occurs. In addition, it is found that the thermal conductivity of bilayer graphene nanoribbon with interlayer bonding will be further reduced by tensile strain due to the intensification of phonon scattering and the reduction of phonon group velocity. The results show that the thermal conductivity of bilayer graphene nanoribbon can be controlled by interlayer bonding and tensile strain. These conclusions are of great significance in designing and thermally controlling of graphene based nanodevices.

First-principles study on phonon transport properties of MoTe2 and WTe2 monolayers in different phases

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDC) monolayer is a key method for manipulating the lattice thermal conductivity. In this paper, we investigate the phonon thermal transport properties of MoTe2 and WTe2 monolayers with H and T′ phases (H-TMDC and T′-TMDC) via first-principles calculations. H-MoTe2 and H-WTe2 monolayers have isotropic lattice thermal conductivities, whereas the lattice thermal conductivities of T′-MoTe2 and T′-WTe2 monolayers are highly anisotropic. Meanwhile, in comparison with the H phase, the maximal lattice thermal conductivity in T′ phase has a reduction of 83.85% and 77.92% at 300 K for MoTe2 and WTe2 monolayers, respectively. Furthermore, we observe that phonon heat capacities and phonon lifetimes of T′-TMDC monolayers are lower than those of H-TMDC monolayers, while the phonon group velocities of T′-TMDC monolayers are larger. Hence, the competitive relationship among these parameters leads to a lower lattice thermal conductivity of the T′-TMDC monolayer. In addition, lower critical phonon MFPs (mean free path) of T′-TMDC monolayers prove that the lattice thermal conductivity has a weaker size dependence. These findings demonstrate that MoTe2 and WTe2 monolayers with different phases have a great potential in 2D nanoelectronic devices.

Vacancy Manipulation Induced Optimal Carrier Concentration, Band Convergence and Low Lattice Thermal Conductivity in Nano‐Crystalline SnTe Yielding Superior Thermoelectric Performance

AbstractSynergetic optimization of electrical and thermal transport properties is achieved for SnTe‐based nano‐crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m−1 K−2 for Sn0.96Gd0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd‐doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano‐pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe‐based nanoscale polycrystalline materials.

Phonon transport in two-dimensional carbon-boron material and heterointerfaces

Developing semiconducting materials with suitable band gap has attracted increasing attention for next-generation intelligent electronic devices. Recently, the newly synthesized monolayer carbon boron (C3B) material exhibits outstanding electronic properties and processes with indirect bandgap, which is considered as the promising alternatives of graphene. The heat conduction capability plays a critical role in the performance of C3B-based electronic devices and thermocatalysis applications. Herein, the thermal conductivity of monolayer C3B and graphene, and interfacial phonon transport across heterointerface are systematically investigated utilizing non-equilibrium molecular dynamics simulation. Compared with graphene, the reduced thermal conductivity of C3B sheet mainly stems from the decreased phonon group velocity and phonon relaxation time when the periodical boron atoms are introduced in C3B sheet based on the lattice dynamics and spectral phonon transmission analysis. Meanwhile, the influences of temperature and strain on the thermal conductivity of C3B are also discussed. Moreover, the thermal transport across the graphene|C3B heterointerface with different temperatures is analyzed. Interestingly, it is found that the larger interfacial thermal conductance of zigzag heterointerfaces mainly originates from the stronger phonon-phonon coupling between the out-of-plane vibrational modes of C3B and graphene. Our work would provide more insights for fundamental understanding the thermal transport properties of two-dimensional materials and be beneficial to the design of thermal management and thermoelectric materials.

Atomic site-targeted doping in Ti2FeNiSb2 double half-Heusler alloys: zT improvement via selective band engineering and point defect scattering

Ti2FeNiSb2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti2FeNiSb2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti2Fe0.9Co0.1NiSb2 is achieved, which is approximately six times higher than that of the pristine Ti2FeNiSb2. The thermoelectric performance of Ti2FeNiSb2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping.

Understanding the origins of low lattice thermal conductivity in a novel two-dimensional monolayer NaCuS for achieving medium-temperature thermoelectric applications

Understanding the lattice dynamics from the perspective of chemical bonds and phonon transport is essential for finding and designing high-efficiency thermoelectric (TE) materials and achieving applications. The coexistence of weak chemical bonding and strong phonon anharmonicity is elucidated as the origin of the intrinsically low lattice thermal conductivity by combining the first principles with the Boltzmann transport theory. By utilizing the crystal orbital Hamilton population (COHP) analysis, the weaker Cu-S chemical bonding originating from the filled antibonding orbitals gives rise to the low average phonon group velocity (va). Here, this research also investigates the scattering processes (the out-of-plan acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D NaCuS possesses strong phonon anharmonicity. The low va and strong phonon scattering enable 2D NaCuS to exhibit a low room temperature lattice thermal conductivity (klat) of 1.95 W/mK. Furthermore, our studies demonstrate 2D NaCuS presents a high ZT of 1.44 at 500 K due to the low klat and excellent electronic transport performance. This study demonstrates that 2D NaCuS is identified as a new TE material for promising medium-temperature applications.

Low lattice thermal conductivity in alkali metal based Heusler alloys

On the basis of purely their electronic properties, semiconducting alkali and alkaline earth metal based Heusler alloys have been identified as potentially promising thermoelectrics. Their thermal transport properties are decisive and help to further gauge this potential. This motivates us to systematically investigate lattice properties of three eight valence electron Heusler alloys, namely, ${\mathrm{Li}}_{2}\mathrm{NaSb}, {\mathrm{Na}}_{2}\mathrm{KSb}$, and ${\mathrm{K}}_{2}\mathrm{CsSb}$, using first-principles approaches. On the basis of both qualitative and quantitative descriptors (assessing covalent bonding, the atomic displacement parameter, the phonon density of states, the Gr\"uneisen parameter, the phonon group velocity, and the phonon lifetime) we analyze in detail the lattice degrees of freedom and their contribution to thermal transport. Out of the three materials, ${\mathrm{Na}}_{2}\mathrm{KSb}$ and ${\mathrm{K}}_{2}\mathrm{CsSb}$ exhibit surprisingly low lattice thermal conductivity, which we can attribute to the anharmonic rattling of the loosely bound alkali atom in the crystal lattice. ${\mathrm{K}}_{2}\mathrm{CsSb}$ has a lattice thermal conductivity two times lower than that of SnSe, a material with a record figure of merit.

Stress effect on lattice thermal conductivity of anode material NiNb2O6 for lithium-ion batteries

The thermal transport properties of NiNb2O6 as anode material for lithium-ion battery and the effect of strain were studied by machine learning interatomic potential combined with Boltzmann transport equation. The results show that the lattice thermal conductivity of NiNb2O6 along the three crystal directions [100], [010], and [001] are 0.947 W⋅m−1⋅K−1, 0.727 W⋅m−1⋅K−1, and 0.465 W⋅m−1⋅K−1, respectively, indicating the anisotropy of the lattice thermal conductivity of NiNb2O6. This anisotropy of the lattice thermal conductivity stems from the significant difference of phonon group velocities in different crystal directions of NiNb2O6. When the tensile strain is applied along the [001] crystal direction, the lattice thermal conductivity in all three directions decreases. However, when the compressive strain is applied, the lattice thermal conductivity in the [100] and [010] crystal directions is increased, while the lattice thermal conductivity in the [001] crystal direction is abnormally reduced due to the significant inhibition of compressive strain on the group velocity. These indicate that the anisotropy of thermal conductivity of NiNb2O6 can be enhanced by the compressive strain, and reduced by the tensile strain.

First-principles investigation on the novel half-Heusler VXTe (X=Cr, Mn, Fe, and Co) alloys for spintronic and thermoelectric applications

Half-Heusler (HH) alloys are a well-known and extensively researched family of thermoelectric (TE), magnetic, and spintronic materials. Doping may significantly increase the thermoelectric conversion efficiency of these materials; nevertheless, practical applications remain far. As a result, the hunt for superior parent TE alloys is critical. Using extensive first-principles density functional calculations, we predicted a novel class of vanadium-based four HH VXTe alloys, where X is one of the four elements: Cr, Mn, Fe, and Co. Their TE properties, as well as their mechanical, magnetic, electrical, and structural stability, have been studied in depth. Their mechanical and thermodynamic stability is confirmed using the predicted elastic constants, formation and cohesive energies, and phonon spectra. A comprehensive analysis of elastic constants and moduli demonstrates that HH VXTe alloys possess elastic anisotropy with reasonably good machinability, higher melting and Debye temperatures, mixed bonding characteristics with ionic and covalent contributions, and brittle nature, except for VCoTe, which is ductile. We find that the ground state of HH VCrTe and VFeTe is ferrimagnetic, while HH VCoTe is ferromagnetic and HH VMnTe is non-magnetic. Three VXTe (X = Cr, Fe, & Co) alloys demonstrated half-metallicity with 100% spin-polarization, whereas HH VMnTe is an 18-electron indirect semiconductor. In the studied HH VXTe alloys, most of the heat flow is caused by the phonon-group velocity of the acoustic phonons. Further studies on the relationship between carrier concentration and temperature dependence of TE properties reveal that the high ZT ∼1.2 at 1000 K of the pristine HH VMnTe alloy is obtained due to its high-power factor of 249.4×1012Wm−1K−2 and this value is greater than the values of some known pristine and doped HH TE materials. Our findings pave the way for further investigation into the HH VXTe alloys in the quest for improved TE and spintronic materials for use in domains that necessitate high thermoelectricity and spintronic performance.

The thermoelectric properties of XTe (X = Ge, Sn and Pb) monolayers from first-principles calculations

Thermoelectric (TE) materials are increasingly attracting the attention of researchers as new energy materials that are capable of converting thermal energy into electrical energy. In this work, combining first-principles calculations and the Boltzmann transport equation, the TE related properties of XTe (X = Ge, Sn and Pb) monolayers have been thoroughly studied. The calculated results show that XTe monolayers are indirect band gap semiconductors, and they possess small effective masses which lead to large carrier mobilities and high electrical conductivities. Except for p-type PbTe, the other XTe monolayers share extremely high PF, thanks to the high Seebeck coefficients and large electrical conductivity. Furthermore, owing to the low phonon group velocity and strong anharmonicity, the lattice thermal conductivities of SnTe and PbTe are quite low. At 500 K, the optimum figure of merit (ZT) values are calculated to be 1.26, 2.61 and 5.91 for GeTe, SnTe and PbTe respectively. The obtained ZT values of the XTe monolayers are larger than these of their bulk counterparts. These results qualify XTe monolayers as promising candidates for building outstanding TE devices.

Comprehensive investigation of the extremely low lattice thermal conductivity and thermoelectric properties of BaIn2Te4

Recently, an extremely low lattice thermal conductivity value has been reported for the alkali-based telluride material ${\mathrm{BaIn}}_{2}{\mathrm{Te}}_{4}$. The value is comparable with low-thermal conductivity metal chalcogenides, and the glass limit is highly intriguing. Therefore, to shed light on this issue, we performed first-principles phonon thermal transport calculations. We predicted highly anisotropic lattice thermal conductivity along different directions via the solution of the linearized phonon Boltzmann transport equation. More importantly, we determined several different factors as the main sources of the predicted ultralow lattice thermal conductivity of this crystal, such as the strong interactions between low-frequency optical phonons and acoustic phonons, small phonon group velocities, and lattice anharmonicity indicated by large negative mode Gr\"uneisen parameters. Along with thermal transport calculations, we also investigated the electronic transport properties by accurately calculating the scattering mechanisms, namely the acoustic deformation potential, ionized impurity, and polar optical scatterings. The inclusion of spin-orbit coupling (SOC) for electronic structure is found to strongly affect the $p$-type Seebeck coefficients. Finally, we calculated the thermoelectric properties accurately, and the optimal $ZT$ value of $p$-type doping, which originated from high Seebeck coefficients, was predicted to exceed unity after 700 K and have a direction averaged value of 1.63 (1.76 in the $y$-direction) at 1000 K around $2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ hole concentration. For $n$-type doping, a $ZT$ around $3.2\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ concentration was predicted to be a direction-averaged value of 1.40 (1.76 in the $z$-direction) at 1000 K, mostly originating from its high electron mobility. With the experimental evidence of high thermal stability, we showed that the ${\mathrm{BaIn}}_{2}{\mathrm{Te}}_{4}$ compound has the potential to be a promising mid- to high-temperature thermoelectric material for both $p$-type and $n$-type systems with appropriate doping.