ZT Value Research Articles

BAs/BlueP van der Waals heterostructures for photovoltaic and thermoelectric applications

The advancement of novel energy materials, encompassing photovoltaic and thermoelectric materials, assumes paramount significance in ameliorating the energy crisis and proactively combating climate change. The BAs/BlueP van der Waals heterostructure (vdWH), characterized by its outstanding optical absorption properties, high power conversion efficiency (PCE), and excellent thermoelectric performance, offers novel insights into the advancement of materials for photonic and thermoelectric applications. We have engineered a vdWH by combining BAs and BlueP, and conducted a comprehensive investigation of its electronic, optical, and thermoelectric properties. The BAs/BlueP vdWH demonstrates excellent thermodynamic and kinetic stability with type I band alignment, facilitating rapid interlayer electron-hole recombination. The remarkable optical absorption capability within the visible and ultraviolet (UV) spectral regions, coupled with an outstanding power conversion efficiency reaching up to 22.35 % under strain, firmly establishes the potential for utilization in photovoltaic conversion applications. At 1000 K, the significant ZT value (1.25) and high thermoelectric conversion efficiency (19.45 %) provide the theoretical foundation for its application in the field of thermoelectrics.

Optimization of Co4Sb11.5Te0.5 thermoelectric performance through Al filling under high temperature and high pressure

So far, the method of optimizing the thermoelectric properties of skutterudite CoSb3 is usually to achieve a breakthrough in high-properties thermoelectric merit by selecting excellent doping atoms and determining a reasonable percentage of atoms. In this study, we achieved the preparation of small atomic radius Al-filled Co4Sb11.5Te0.5 compounds by high-temperature and high-pressure (HTHP) synthesis methods. The thermoelectric properties of the samples AlxCo4Sb11.5Te0.5 were optimized from two dimensions: synthesis pressures (1.0 GPa, 1.5 GPa, 2.0 GPa) and Al filling concentration (x = 0.1, 0.2, 0.3, 0.5), ultimately achieving effective decoupling of electron-phonon transport properties. The experimental results show that Al doping can optimize the electrical transport properties of materials effectively, rapid synthesis method of HTHP can introduce abundant grain boundaries, diversification of grain sizes, a large number of dislocations and widely distributed nano second phase, etc. These microstructures in bulk materials form a multi-scale phonon scattering network in situ, which can effectively scatter phonons and reduce the lattice thermal conductivity effectively. After optimizing the synthesis pressure and Al filling concentration through orthogonal transformation, the sample Al0.3Co4Sb11.5Te0.5 prepared at a synthesis pressure of 1.5 GPa obtained a minimum lattice thermal conductivity value of 0.88 W m−1 K−1, a maximum power factor of 40.96 μ W cm−1 K−2 and a maximum zT value of 1.18 at 773.15 K.

Analytical study of the thermoelectric properties in silicene

Theoretically, we investigate the thermoelectric (TE) properties namely, electrical conductivity (σ), diffusion thermopower (S d), power factor (PF), electronic thermal conductivity (κ e) and thermoelectric figure of merit (ZT) for silicene on Al2O3 substrate. TE coefficients are obtained by solving the Boltzmann transport equation taking account of the electron scattering by all the relevant scattering mechanisms in silicene, namely charged impurity (CI), short-range disorder (SD), intra- and inter-valley acoustic (APs) and optical (OPs) phonons, and surface optical phonons (SOPs). The TE properties are numerically studied as a function of temperature T (2–400K) and electron concentration n s(0.1–10 × 1012 cm−2). The calculated σ and S dare found to be governed by CIs at low temperatures (T< ∼ 10 K), similar to that in graphene. At higher T, they are found to be mainly dominated by the intra- and inter-valley APs. The resultant σ (S d) is found to decrease (increase) with increasing T, whereas PF remains nearly constant for T> ∼ 100 K. On the other hand, n s dependence shows that σ (S d) increases (decreases) with increasing n s; with PF relatively constant at lower n s and then decreases with increasing n s. At room temperature, the calculated σ (S d) in silicene is closer to that in graphene and about an order of magnitude greater (less) than that in monolayer (ML) MoS2. The κ e is found to be weakly depending on T and Wiedemann–Franz law is shown to be violated. We have predicted a maximum PF ∼3.5 mW m−1 K−2, at 300 K for n s = 0.1 × 1012 cm−2 from which the estimated ZT = 0.11, taking a theoretically predicted lattice thermal conductivity κ l = 9.4 Wm−1 K−1, is a maximum. This ZT is much greater than that of graphene and ML MoS2. The ZT is found to decrease with the increasing n s. The ZT values for other values of n s in silicene, at 300 K, also show much superiority over graphene, thus making silicene a preferred thermoelectric material because of its relatively large σ and very small κ l.

The thermoelectric properties of the two-dimensional Nowotny-Juza material NaBeX (X = P, As, and Sb)

The two-dimensional β-type Nowotny-Juza phase-filled tetrahedral compound has excellent electrical conductivity, low thermal conductivity and outstanding thermoelectric properties. This study investigated the stability and electronic properties of NaBeX (X = P, As, and Sb) through first-principles calculations. The results demonstrate that these materials are structurally stable and are direct bandgap semiconductors with bandgaps of 0.6542 eV, 0.6549 eV, and 0.6686 eV, respectively. The electrical and thermal transport properties of NaBeX are investigated by combining the deformation potential theory and Boltzmann transport theory. Subsequently, the thermoelectric properties of NaBeX are evaluated at temperatures ranging from 400 to 800 K. The n-type NaBeX materials exhibit large ZT values of 2.48, 2.62, and 1.72. The results of the present study indicate that n-type NaBeX has the potential to be used as a thermoelectric candidate material at temperatures ranging from 400 to 800 K.

Enhanced thermoelectric performance of AgCuTe-doped polycrystalline SnSe by lattice plainification

Abstract Polycrystalline SnSe, renowned for its environmental sustainability, holds promise as a significant thermoelectric material, attracting considerable research attention. This study focuses on the thermoelectric properties of p-type polycrystalline SnSe doped with silver copper telluride (AgCuTe). Our experimental results conclusively show that the decomposition products of AgCuTe not only fill Sn vacancies but also act as acceptors, thereby introducing additional hole carriers. This leads to a notable improvement in both carrier mobility and concentration. Importantly, the thermal conductivity of the doped samples remains largely unchanged, as the lattice flattening strategy significantly boosts electrical performance without affecting lattice thermal conductivity. Ultimately, the doped sample Sn0.97Se-0.5%AgCuTe resulted in a power factor of 6.2 mW/mK and a peak ZT value of 1.4 at 798 K, representing a 109% improvement in ZT value. All samples exhibits superior stability and reproducibility, emphasizing its reliability for practical applications.

Comparison of ultra-low lattice thermal conductivity of the full-Heusler compound Li2Rb(Cs)Bi after considering strong quartic anharmonicity

This paper conducts a detailed study on the thermal transport and thermoelectric properties of Li2Rb(Cs)Bi and analyzes the optical phonon frequency shift caused by considering anharmonicity. We mainly focus on studying the microscopic mechanism of the difference in lattice thermal conductivity (κL) of the two materials. By calculating the group velocity, scattering rate, scattering phase space and scattering sub-process, it is concluded that κL is mainly dominated by the acoustic branch. Due to its small group velocity and large scattering rate, Li2CsBi has a low κL, which is 0.60 W m−1K−1 at 300 K. Research results show that n-type Li2CsBi has a higher ZT value of about 2.1 at T = 900 K, while p-type Li2RbBi has a higher ZT value of about 1.5 at the same temperature. These results provide an important theoretical basis for the application of Li2Rb(Cs)Bi in the field of thermoelectric conversion.

Microstructural Instability and Its Effects on Thermoelectric Properties of SnSe and Na-Doped SnSe.

Effects of thermal cycling on the microstructure and thermoelectric properties are studied for the undoped and Na-doped SnSe samples using X-ray computed tomography and property measurements. It is observed that thermal cycling causes significant cracks to develop, which decrease both the electrical and lattice thermal conductivities but do not affect the thermopower. The zT values are drastically reduced after the repeated heat treatment. It is important to account for density changes during cycling to obtain accurate values of the thermal conductivity. Even before thermal cycling, the spark-plasma sintered (SPS) samples have a significant number of microcracks. The orientation of cracks within the SPS pellets and their effect on the microstructure are influenced by the presence of a Na-rich impurity. The SnSe and Sn0.995Na0.005Se samples without the impurity develop cracks and exhibit grain growth parallel to the pellet surface, which is also the plane of the 2D SnSe layers. The Sn0.97Na0.03Se sample containing the impurity develops cracks that are orthogonal to the pellet surface. Such an orientation of cracks in Sn0.97Na0.03Se inhibits grain growth. All samples appear mechanically unstable after thermal cycling.

Anti-Wilson mechanism for adjusting bandgap and electronic transport property of a half-Heusler material LiCdP

Using first-principles calculations, we have found that LiCdP, an existing half-Heusler material, exhibits an anti-Wilson mechanism for adjusting the bandgap. Specifically, instead of widening according to the conventional Wilson mechanism, the bandgap shows a significant decrease in response to lattice strain, eventually closing at 5.02% tensile strain. This anti-Wilson mechanism is attributed to the weakened repulsion between the 3s and 5s orbitals of the P and Cd atoms, respectively, as observed by the analysis of the neighbouring atomic orbital coupling. In addition, we have found that the ZT value, which measures the thermoelectric efficiency of this material, can reach 1.33 at a temperature of T = 1200 K when lattice strain is induced by thermal expansion. This result suggests that LiCdP is an excellent thermoelectric material in a high operating temperature range. From the point of view of actual applications, such an interesting tunability of the bandgap of LiCdP provides a novel alternative for designing electronic or optoelectronic devices in a controllable way.

Prediction of X2AuYZ6 (X = Cs, Rb; Z = Cl, Br, I) double halide perovskites for photovoltaic and wasted heat management device applications

We used density functional theory to study the phase stability, the opto-electronic properties, and the thermo-electric behavior of X2AuYZ6 (where X = Cs, Rb, and Z = Cl/Br/I) double halide perovskites. The compounds belong to the cubic perovskite arrangement and are verified by the tolerance and octahedral factor. Formation enthalpy and binding energy primarily meet the requirements for structural stability. The positive frequency of phonon dispersion shows that all compounds are dynamically stable except for Rb2AuYI6. The negative formation energy considering the competing phase also confirmed that all compounds are thermodynamically stable. The Pugh's ratio, Poisson's ratio, and Cauchy pressure validated their ductile nature in the analysis of thermo-mechanical behavior. The electronic characteristics of Cs2AuYZ6 (Rb2AuYZ6) [Z = Cl, Br, I] double halide perovskites (DHP) have been investigated using the TB-mBJ technique, yielding band gap values of 2.85 (2.91), 2.35 (2.40), and 1.74 (1.78) eV, in that order. The optical properties are calculated, revealing the maximum absorption coefficients with respect to wavelength in the visible range of the titled compounds are 0.23 (0.35) × 105 cm−1, 1.01 (1.06) × 105 cm−1, and 1.42 (1.48) × 105 cm−1, respectively, indicating their potential for photovoltaic applications. The thermo-electric transport properties were also studied. The investigated compounds [Cs (Rb)-based] exhibit ZT values of 0.51 (0.55), 0.53 (0.62), and 0.58 (0.75) at room temperature with Cl, Br, and I, respectively. As a result, the studied compounds, particularly those Rb-based with halides, have significantly greater potential than Cs in thermoelectric fields. So, among the compounds, X2AuYI6 (X = Cs, Rb) shows the most promise for the technological applications mentioned above due to its lower band gap, high absorption coefficient, and high ZT value.

Boosting thermoelectric performance of polycrystalline SnSe by controlled in-situ Ag2Se precipitates in grain boundaries

Boundary engineering has proven effective in enhancing the thermoelectric performance of materials. SnSe, known for its low thermal conductivity, has garnered significant interest; however, its application is hindered by poor electrical conductivity. Herein, the Ag8GeSe6 is introduced into the p-type polycrystalline SnSe matrix to optimize the thermoelectric performance, and the in-situ Ag2Se precipitates are formed in grain boundaries, which play dual roles, acting as an electron attraction center for improving hole concentration and a phonon scattering center for reducing lattice thermal conductivity. It effectively decouples the thermal and electrical transport properties to optimize the thermoelectric performance. Importantly, the amount of Ag2Se can be controlled by adjusting the amount of Ag8GeSe6 added to the SnSe matrix. The introduction of Ag8GeSe6 enhances electrical conductivity due to the increased hole carrier caused by the introduced Ag+ and the formed electron attraction center (in-situ Ag2Se precipitates). Based on the DFT calculations, the band gap of the Ag8GeSe6-doped samples is considerably decreased, facilitating carrier transport. As a result, the electrical transport properties increase to 808 μW m−1 K−2 at 823 K for SnSe + 0.5 wt% Ag8GeSe6. In addition, in-situ Ag2Se precipitates in grain boundaries strongly enhance phonon scattering, causing a decrease in lattice thermal conductivity. Furthermore, the presence of defects contributes to a reduction in lattice thermal conductivity. Specifically, the thermal conductivity of SnSe + 1.0 wt% Ag8GeSe6 decreases to 0.29 W m−1 K−1 at 823 K. Consequently, SnSe + 0.5 wt% Ag8GeSe6 obtains a high ZT value of 1.7 at 823 K and maintains a high average ZT value of 0.57 over the temperature range of 323−773 K. Additionally, the mechanical properties of Ag8GeSe6-doped also show an improvement. These advancements can be applied to energy supply applications during deep space exploration.

Advanced GeSe-based thermoelectric materials: Progress and future challenge

GeSe, composed of ecofriendly and earth-abundant elements, presents a promising alternative to conventional toxic lead-chalcogenides and earth-scarce tellurides as mid-temperature thermoelectric applications. This review comprehensively examines recent advancements in GeSe-based thermoelectric materials, focusing on their crystal structure, chemical bond, phase transition, and the correlations between chemical bonding mechanism and crystal structure. Additionally, the band structure and phonon dispersion of these materials are also explored. These unique features of GeSe provide diverse avenues for tuning the transport properties of both electrons and phonons. To optimize electrical transport properties, the strategies of carrier concentration engineering, multi-valence band convergence, and band degeneracy established on the phase modulation are underscored. To reduce the lattice thermal conductivity, emphasis is placed on intrinsic weak chemical bonds and anharmonicity related to chemical bonding mechanisms. Furthermore, extra-phonon scattering mechanisms, such as the point defects, ferroelectric domains, boundaries, nano-precipitates, and the phonon mismatch originating from the composite engineering, are highlighted. Additionally, an analysis of mechanical properties is performed to assess the long-term service of thermoelectric devices based on GeSe-based compounds, and correspondingly, the theoretical energy-conversion efficiency is discussed based on the present zT values of GeSe. This review provides an in-depth insight into GeSe by retrospectively examining the development process and proposing future research directions, which could accelerate the exploitation of GeSe and elucidate the development of broader thermoelectric materials.

Machine Learning for Predicting Ultralow Thermal Conductivity and High ZT in Complex Thermoelectric Materials.

Efficient and precise calculations of thermal transport properties and figures of merit, alongside a deep comprehension of thermal transport mechanisms, are essential for the practical utilization of advanced thermoelectric materials. In this study, we explore the microscopic processes governing thermal transport in the distinguished crystalline material Tl9SbTe6 by integrating a unified thermal transport theory with machine learning-assisted self-consistent phonon calculations. Leveraging machine learning potentials, we expedite the analysis of phonon energy shifts, higher-order scattering mechanisms, and thermal conductivity arising from various contributing factors, such as population and coherence channels. Our finding unveils an exceptionally low thermal conductivity of 0.31 W m-1 K-1 at room temperature, a result that closely correlates with experimental observations. Notably, we observe that the off-diagonal terms of heat flux operators play a significant role in shaping the overall lattice thermal conductivity of Tl9SbTe6, where the ultralow thermal conductivity resembles that of glass due to limited group velocities. Furthermore, we achieve a maximum ZT value of 3.17 in the c-axis orientation for p-type Tl9SbTe6 at 600 K and an optimal ZT value of 2.26 in the a-axis and b-axis direction for n-type Tl9SbTe6 at 500 K. The crystalline Tl9SbTe6 not only showcases remarkable thermal insulation but also demonstrates impressive electrical properties owing to the dual-degeneracy phenomenon within its valence band. These results not only elucidate the underlying reasons for the exceptional thermoelectric performance of Tl9SbTe6 but also suggest potential avenues for further experimental exploration.

Enhancement of thermoelectric performance by Ag/Bi/Fe co-doping into Cu3SbSe4 ceramics for green thermoelectric applications

Cu3SbSe4 is recognized for its abundant elemental availability, cost-effectiveness, and non-toxic properties, making it a promising candidate for thermoelectric applications at medium temperatures. This study explored the preparation of Bi/Fe, Ag/Fe, and Ag/Bi double-doped Cu3SbSe4 ceramics using vacuum melting and cold isostatic pressing techniques. The focus was on examining the influence of these dopants on the microstructural and thermoelectric performances of Cu3SbSe4. The analysis revealed the presence of Cu3SbSe4 and Cu2−xSe phases in the doped samples. Double doping optimized the carrier concentration, increased the effective mass, and significantly enhanced the electrical conductivity from 12.45 to 64.01 S cm−1. Notably, the power factor of the Cu2.85Ag0.15Sb0.985Bi0.015Se4 sample reached 649.1 μWm−1K−2 at 573 K. Furthermore, the thermal conductivity was significantly reduced to 0.73 W/mK. The maximum ZT value of the Cu2.85Ag0.15Sb0.985Bi0.015Se4 sample was 0.47 at 573 K. These breakthrough results demonstrate that dual doping markedly enhances the thermoelectric properties of the Cu3SbSe4 system.

High thermoelectric performance of n–type SrTiO3 by Dy and Nb co–doping

The effects of Dy/Nb co–doping on the structural and thermoelectric properties of Sr1−xDyxTi1−yNbyO3−δ (0 ≤ x, y ≤ 0.10) are systematically investigated. Single–doped Sr0.95Dy0.05TiO3−δ and SrTi0.95Nb0.05O3−δ consist of tetragonal (I4/mcm space group) and cubic (Pm3¯m space group) perovskite phases. On the other hand, Dy/Nb co–doped SrTiO3, Sr1−xDyxTi1−yNbyO3−δ, has a tetragonal perovskite phase (I4/mcm space group) since the Dy/Nb co–doping induces a distortion of the TiO6 octahedra and lowers the crystal symmetry. The co–doping effectively increases the electrical conductivity and reduces the thermal conductivity, thus noticeably enhancing the dimensionless figure–of–merit (ZT). The highest ZT value (0.24) is achieved for Sr0.95Dy0.05Ti0.95Nb0.05O3−δ at 600 °C. This ZT value is 41 % and 50 % larger than those of single–doped Sr0.95Dy0.05TiO3−δ (0.17) and SrTi0.95Nb0.05O3−δ (0.16), respectively, at 600 °C.

Demonstrating the Simplicity and In Situ Temperature Monitoring of the Mechanochemical Synthesis of Metal Chalcogenides Suitable for Thermoelectrics.

Mechanochemical synthesis is an extremely useful strategy to reach thermoelectric materials due to its solvent-free one-step character, as the targeted thermoelectricity (TE) materials in a nanocrystalline format can be prepared by mere high-energy milling of elemental precursors. Nevertheless, the subsequent densification method (e.g., spark plasma sintering or hot pressing) is required afterward, similarly to other synthetic methodologies. In this study, the simplicity of mechanochemical synthesis ispresented for two selected metal chalcogenides, namely copper sulfide (Cu1.8S, digenite) and tin selenide (SnSe, svetlanaite), which are known for high ZT values. These compounds can be prepared via a mechanically induced self-propagating reaction (MSR), which is a combustion-like process instantly yielding the products in a very short timeframe (within 1 min). The occurrence of MSR can be well-tracked by in situ temperature monitoring since an abrupt temperature increase occurs at the moment of MSR. We have developed a device which is capable of monitoring the temperature inside the milling jar every 80 ms during planetary ball milling, and it is therefore possible to very precisely track the moment of MSR ignition. The developed device presents an improvement in the monitoring capabilities in comparison with commercially available analogs. This contribution aims to provide a visual insight into all steps, with simple high-energy ball milling of elements to reach TE materials and in situ temperature monitoring being the central points.

Electrical and thermal transport properties of Cr2Se3-Cr2Te3 solid-solution alloy system and estimation of optimal thermoelectric properties

Cr2Se3 is a layered narrow-bandgap semiconductor with a low lattice thermal conductivity of ∼1 W/mK, which can be considered as a potential thermoelectric material. A solid-solution alloy system with Cr2Te3 can be designed to manipulate the electrical and thermal transport properties. In this study, a series of Cr2Se3-Cr2Te3 solid-solution alloys (Cr2(Se1-xTex)3, x = 0, 0.33, 0.5, 0.67, 0.83, and 1) were synthesized, and their electrical and thermal transport properties were investigated. Regarding the electrical transport properties, the power factor (∼0.35 mW/mK2) of Cr2Se3 gradually and significantly decreased as the Te content (x) increased owing to the significant decrease in the Seebeck coefficient that resulted from the large increase in the carrier concentration. Regarding the thermal transport properties, the total thermal conductivity increased with x as a result of the large increase in the electrical conductivity, despite the continuous decrease in the lattice thermal conductivity. Consequently, the thermoelectric figure of merit (zT ∼ 0.20) of Cr2Se3 gradually and significantly decreased to 0.009 for Cr2Te3 at 700 K; thus, no enhancement of zT was observed from the experimental results of simple solid-solution alloying. On the other hand, the thermoelectric quality factors of the solid-solution compositions for x = 0.33–0.83 were found to be enhanced compared to that of the Cr2Se3 sample, implying that zT could be further enhanced by optimizing the carrier concentration. Indeed, enhanced zT values were estimated based on a single parabolic band model for the solid-solution compositions with x = 0.33–0.83 if the carrier concentration was optimized to ∼1019 cm−3, which was found to be owing to the increased nondegenerate and weighted mobility (inferring weaker carrier–phonon interaction). Therefore, it is suggested that the solid-solution alloying approach could provide a feasible strategy to enhance the thermoelectric performance by reducing carrier-phonon interaction when further combined with carrier concentration optimization.

Lattice Softening and Band Convergence in GeTe-Based Alloys for High Thermoelectric Performance.

GeTe-based alloys have been studied as promising TE materials in the midtemperature range as a lead-free alternate to PbTe due to their nontoxicity. Our previous study on GeTe1-xIx revealed that I-doping increases lattice anharmonicity and decreases the structural phase transition temperature, consequently enhancing the thermoelectric performance. Our current work elucidates the synergistic interplay between band convergence and lattice softening, resulting in an enhanced thermoelectric performance for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16). Sb doping in GeTe0.9I0.1 serves a double role: first, it leads to lattice softening, thereby reducing lattice thermal conductivity; second, it promotes a band convergence, thus a higher valley degeneracy. The presence of lattice softening is corroborated by an increase in the internal strain ratio observed in X-ray diffraction patterns. Doping also introduces phonon scattering centers, further diminishing lattice thermal conductivity. Additionally, variations in the electronic band structure are indicated by an increase in density of state effective mass and a decrease in carrier mobility with Sb concentration. Besides, Sb doping optimizes the carrier concentration efficiently. Through a two-band modeling and electronic band structure calculations, the valence band convergence due to Sb doping can be confirmed. Specifically, the energy difference between valence bands progressively narrows upon Sb doping in Ge1-ySbyTe0.9I0.1 (y = 0, 0.02, 0.05, 0.10, 0.12, 0.14, and 0.16). As a culmination of these effects, we have achieved a significant enhancement in zT for Ge1-ySbyTe0.9I0.1 (y = 0.10, 0.12, 0.14, and 0.16) across the entire range of measured temperatures. Notably, the sample with y = 0.12 exhibits the highest zT value of 1.70 at 723 K.

Thermoelectric properties of Sn2SSe via band engineering with Ge alloying

The thermoelectric properties of Sn2SSe are investigated via band engineering using Ge alloying. In this work, the electronic and thermoelectric properties of Sn2SSe doped with Ge at different concentrations (x=0, 0.25, 0.5, 0.75, and 1) are investigated using density functional theory and Boltzmann transport theory. At 300K, the Seebeck coefficient and electrical conductivity are enhanced with Ge alloying from −960μV/K to −1535 μV/K and from 3.4 × 105 S/cm to 4.1 × 105 S/cm respectively. However, the lowest value of lattice thermal conductivity is observed at 700 K which is 2.7W/mK. At x = 1, A remarkably high ZT value 1.7 is achieved at 700 K for Sn2(1−x)Ge2(x)SSe . The high ZT value is 1.8 times greater than pure compound.

Excellent thermoelectric properties and significant anisotropy for filled-tetragonal NaZnAs caused by strong anharmonicity

We have theoretically investigated the transport properties of the filled-tetrahedral NaZnAs using first-principles calculations in combination with self-consistent phonon theory and the Boltzmann transport equation. The results show that the transport behavior of NaZnAs exhibits obvious anisotropy, and the influence of quartic anharmonicity on the lattice thermal conductivity cannot be neglected as shown by the thermal conductivity results obtained from different computational models. Both the increase in scattering rates(SRs) and the decrease in group velocity signify a strong anharmonicity. We further find that the special vibrational modes due to the weak bonding of Zn atoms are the main source of anharmonicity. We find that considering the bubble diagram for the phonon self-energy correction reduces the κL by up to 28.04% at 700 K, which favors the capture of large ZT values. In addition, The ZT value of NaZnAs can reach 3.07 at 700 K, which shows great potential for thermoelectric applications.

High thermoelectric and opto-electronic properties of Ba3NX3 (X=F, Cl) perovskite: Insights from DFT computation

Researchers are working for discovering smart and efficient materials to cope with energy crises. Amongst them, Perovskite are explored extensively by researchers owing to their role in energy harvesting. In this study, structural, elastic, electronic, optical and thermoelectric properties of halide perovskites Ba3NX3 (X = F, Cl) have been explored by using density functional theory techniques. The permissible values of tolerance factor and formation energy ensure the structural and thermodynamic stability of these compounds. Band structure calculations revealed that both materials have semiconducting nature, with the band gap of ∼2.1 eV and 1.9 eV for Ba3NF3 and Ba3NCl3 respectively. The elastic constants suggest both compounds are mechanically stable and exhibit ductile nature. Optical response is studied in the energy range of 0–40 eV of photons. The absorption peaks are observed from 2 eV to 25 eV. Furthermore, it is observed that the relaxation time for Ba3NF3 is longer than Ba3NCl3. Thermoelectric properties such as Seebeck coefficient, electrical conductivity electronic thermal conductivity and figure of merit (ZT) are obtained using BoltzTrap2 code. It is observed that for both materials Seebeck coefficient has almost same maximum value of 1.54 × 10−3 V/K at 300K. The ZT values are found to be 1.23 for Ba3NF3 while 0.9 for Ba3NCl3 at 700K. This work demonstrates that these halides perovskites have a lot of potentiality to prove themselves as best candidates for solar cells and for thermoelectric devices.