Phonon Velocity Research Articles

Phonon thermal transport in Bi2Te3/Sb2Te3 monolayer superlattices: a neural network potential study.

Superlattices are significant means to reduce the lattice thermal conductivity of thermoelectric materials and optimize their performance. In this work, using high-precision first-principles based neural network potentials combined with non-equilibrium molecular dynamics simulations and the phonon Boltzmann transport equation, the lattice thermal conductivities of Bi2Te3 monolayer and lateral Bi2Te3/Sb2Te3 monolayer superlattices are thoroughly investigated. As the period length increases, the thermal conductivity shows a trend of an initial decrease followed by an increase, which aligns with conventional observations. However, it is found that the decrease of thermal conductivity in coherent short-period superlattices is attributed to the variation of phonon scattering behaviors, including the increased phonon anharmonicity due to enhanced structural complexity and symmetry breaking, as well as intensified scattering between acoustic and optical phonons caused by the downward shift of low-frequency optical branches. This is contrary to expectations that the phonon velocity is the primary driver of reduced cross-plane lattice thermal conductivity in coherent short-period superlattices. In addition, it is found that the four-phonon (4ph) scattering process plays a significant role in the thermal conductivities of the Bi2Te3 monolayer and 1-1 and 3-3 Bi2Te3/Sb2Te3 monolayer superlattices. After the 4ph scattering process is considered, the a-axis thermal conductivities of these three structures decrease by 27.8%, 26.4%, and 31.1%, respectively. This study not only advances our knowledge of phonon thermal transport behavior in superlattices, but also serves as a good research case for investigating the thermal conductivity of complex superlattice structures using accurate and efficient neural network potentials.

Dual Crystal-Liquid Thermal Transport Behavior in MAPbCl3.

Phonon dynamics in organic-inorganic hybrid perovskites (OIHPs) exhibit inherent complexity driven by the intricate interactions between rotatable organic cations and dynamically disordered inorganic octahedra, mediated by hydrogen bonding. This study aims to address this complexity by investigating the thermal transport behavior of MAPbCl3 as a gateway to the OIHPs family. The results reveal that the ultralow thermal conductivity of MAPbCl3 arises from a synergistic interplay of exceptionally low phonon velocities, short phonon lifetimes, and phonon mean free paths approaching the Regel-Ioffe limit. Additionally, the thermal conductivity of MAPbCl3 approaches its theoretical amorphous limit across a broad temperature range, with its thermal transport behavior transitioning from crystal-like to more liquid-like during the orthorhombic-to-cubic phase transitions. Furthermore, a phonon drag effect is observed at 17K, with Umklapp scattering serving as the predominant phonon resistive mechanism in the orthorhombic phase. In contrast, dynamic lattice distortions caused by the jumping rotation of MA+ cations become the primary factors influencing thermal transport in the cubic phase.

Thermal characteristics of CsPbX3 (X =Cl/Br/I) halide perovskites

Halide perovskites represent a remarkably promising category of materials for the development of novel solar cells and opto-electronic devices. In this study, we used the neuroevolution machine learning potential to examine the thermal properties of CsPbX3 perovskites (X = Cl / Br / I). In alignment with previous research, our results disclose a pronounced correlation between lattice dynamics and thermal conductivity. Regardless of the specific perovskite variant, phonon band structure analysis revealed dynamical instability, characterized by overlapping phonon branches and densely packed optical branches beginning at imaginary low frequencies around 20i cm−1. We observed a significantly low lattice thermal conductivity of approximately 0.20W/(mK). This is due to the displacement of Cs atoms from their central positions and the deformation of the PbX6 octahedra, which results in lower phonon velocities and shorter phonon lifetimes.

Propagation velocity measurements of substrate phonon bursts using MKIDs for superconducting circuits

High-energy bursts in superconducting quantum circuits from various radiation sources have recently become a practical concern due to induced errors and their propagation in the chip. The speed and distance of these disturbances have practical implications. We used a linear array of multiplexed MKIDs on a single silicon chip to measure the propagation velocity of a localized high-energy burst, introduced by driving a normal metal-insulator-superconductor (NIS) junction. We observed a reduction in the apparent propagation velocity with NIS power, which is due to the combined effect of reduced phonon flux with distance and the existence of a minimum detectable QP density in the MKIDs. A simple theoretical model is fitted to extract the longitudinal phonon velocity in the substrate and the conversion efficiency of phonons to QPs in the superconductor.

Dual-phase zirconate/tantalate high-entropy ceramics boost thermal properties and fracture toughness for thermal barrier coating materials

Working temperatures, thermal insulation performance, and life span of thermal barrier coatings (TBCs) are primarily influenced by their high-temperature stability, thermal expansion coefficients (TECs), thermal conductivity, and fracture toughness. To address the limitations of current zirconate- and tantalate-based oxides, dual-phase zirconate/tantalate high-entropy ceramics (HECs) are designed and synthesized to improve their thermal and mechanical properties. The combined effects of high entropy, high concentrations of oxygen vacancies, and relatively low phonon velocity result in glass-like thermal conductivity, with a minimum value of 1.55 W m−1 K−1 at 1200 °C. The high TECs (10.6–10.9 × 10−6 K−1 at 1400 °C) and exceptional high-temperature stability demonstrate that these materials can withstand 1300 °C for more than 300 h, significantly surpassing the performance of traditional yttria-stabilized zirconia (YSZ). Compared with YSZ (3.6 MPa m1/2) and YTaO4 (2.5 MPa m1/2), the increments in fracture toughness (4.4 MPa m1/2) of dual-phase zirconate/tantalate HECs are as high as 22.2% and 76.0%, respectively. It is evident that the designed dual-phase zirconate/tantalate HECs can effectively promote thermal properties and fracture toughness, positioning them as the next-generation TBCs with high operating temperatures and outstanding thermal insulation performance.

Ultralow thermal conductivity of 2D Dion-Jacobson (PDA)(FA)n - 1PbnI3n + 1 perovskite films.

Two-dimensional (2D) perovskites exhibit enhanced thermal stability compared to three-dimensional perovskites, especially the emerging 2D Dion-Jacobson (DJ) phase perovskite. However, the heat transfer mechanisms in DJ phase perovskites are rarely reported. Herein, we determine thermal conductivities of (PDA)(FA)n - 1PbnI3n + 1 films with n = 1-6 by time-domain thermoreflectance. The measured results indicate that the thermal conductivities of these films are extremely low, showing a trend from decline to rise with increasing n values, and reaching to the lowest when n = 2. We measure the propagation of acoustic phonons in films with n = 1-3 by time-domain Brillouin scattering and find phonon velocity plays a key role in the thermal conductivity, which can be explained by the mismatch of spring constants between the inorganic layer and the organic layer using the bead-spring model. The gradually increasing thermal conductivity for larger n values is attributed to the gradual transformation of the grain orientation from horizontal to vertical, which is demonstrated by the grazing-incidence wide-angle x ray scattering (GIWAXS) results. Our work deepens the understanding of the thermal transport process in 2D DJ phase perovskite films and provides insights into thermal management solutions for their devices.

Excellent thermoelectric performance of Fe2NbAl alloy induced by strong crystal anharmonicity and high band degeneracy

Full-Heusler alloys with earth-abundant elements exhibit high mechanical strength and favorable electrical transport behavior, but their high intrinsic lattice thermal conductivity limits potential thermoelectric application. Here, the thermoelectric transport properties of Fe-based Full-Heusler Fe2MAl (M = V, Nb, Ta) alloys are comprehensively investigated utilizing density functional theory. The results suggest that Fe2NbAl exhibits exceptionally low lattice thermal conductivity due to low phonon velocities and weakly bound Nb atoms. In Fe2NbAl, the underbonding of the Nb atoms leads large Grüneisen parameters and high anharmonic scattering rates of low-frequency acoustic phonon. Meanwhile, the high band degeneracy and large electrical conductivity lead to a maximum p-type power factor of 255.6 μW·K−2·cm−1 at 900 K. The combination of low lattice thermal conductivity and favorable electrical transport properties leads a maximum p-type dimensionless figure of merit of 1.7. Our work indicates Fe2NbAl, as a low-cost, environmentally friendly, is a potential high-performance p-type thermoelectric material.

Insights into enhanced thermoelectric performance of the n-type Mg3Sb2-based materials by amphoteric Al doping

Doping has the potential to alter the levels of anharmonicity in compounds by attenuating bonding strength. In this study, we explore the efficacy of amphoteric Al doping for stimulating anharmonicity in n-type Mg3.25AlxSb1.48Bi0.48Te0.04 to attain enhanced phonon scattering and thermoelectric performance. First-principles calculations and experimental data reveal the occupation of both Sb and Mg2 sites by amphoteric Al atoms in the anionic framework of Mg3.25AlxSb1.48Bi0.48Te0.04. A marginal variation in both carrier concentration and mobility sustains the high power factor without affecting the Seebeck coefficient, implying amphoteric doping induced charge compensation. While phonon velocity, Grüneisen parameter, and crystal orbital Hamilton population calculations results indicate that phonon softening and bond weakening are realized via Al doping, leading to an enhanced lattice anharmonicity and a reduced lattice thermal conductivity. A remarkable enhancement ∼16% in the peak figure of merit ZTpeak and the average ZTavg, was attained for the x = 0.015 sample, when compared with the un-doped sample. Hence, the amphoteric doping can serve as an effective means to optimize ZT values by decoupling the intertwined thermoelectric transport properties.

Raman spectrum and phonon thermal transport in van der Waals semiconductor GaPS4

The emergent van der Waals semiconductor GaPS4 is heralding frontiers for gallium-based semiconductors. Despite its potential, the intricacies of its Raman spectrum and phonon heat transport remain elusive. In this research, experimental and theoretical methods are employed to give a comprehensive portrayal. The Raman spectra and phonon calculations obtained were cross-validated, affirming the study's credibility. A total of 28 Raman peaks were identified, with all phonon irreducible representations delineated. Advanced calculations unveiled notable shifts in the transition of GaPS4 from bulk to monolayer. During this process, phonons undergo a red shift, and the vibration contributions of different atoms change. The lifetime and group velocity of low wavenumber phonons are markedly reduced, suppressing the thermal conductivity in the monolayer. The thermal conductivity of GaPS4 bulk at 300 K is 0.5 W/m K, and 0.13 W/m K for monolayer, while the thermal conductivity in the cleavage direction is lower. These findings offer a detailed account of the complex Raman spectra and phonon thermal transport properties of GaPS4, setting the stage for its subsequent exploration and prospective applications in electronic and thermal devices, and contributing to enriching condensed matter theory of phonon thermal transport in van der Waals materials.

In-Plane Overdamping and Out-Plane Localized Vibration Contribute to Ultralow Lattice Thermal Conductivity of Zintl Phase KCdSb.

Zintl phases typically exhibit low lattice thermal conductivity, which are extensively investigated as promising thermoelectric candidates. While the significance of Zintl anionic frameworks in electronic transport properties is widely recognized, their roles in thermal transport properties have often been overlooked. This study delves into KCdSb as a representative case, where the [CdSb4/4]- tetrahedrons not only impact charge transfer but also phonon transport. The phonon velocity and mean free path, are heavily influenced by the bonding distance and strength of the Zintl anions Cd and Sb, considering the three acoustic branches arising from their vibrations. Furthermore, the weakly bound Zintl cation K exhibits localized vibration behaviors, resulting in strong coupling between the high-lying acoustic branch and the low-lying optical branch, further impeding phonon diffusion. The calculations reveal that grain boundaries also contribute to the low lattice thermal conductivity of KCdSb through medium-frequency phonon scattering. These combined factors create a glass-like thermal transport behavior, which is advantageous for improving the thermoelectric merit of zT. Notably, a maximum zT of 0.6 is achieved for K0.84Na0.16CdSb at 712 K. The study offers both intrinsic and extrinsic strategies for developing high-efficiency thermoelectric Zintl materials with extremely low lattice thermal conductivity.

Structural, electronic, phonon, and elastic properties of zigzag single-walled and double-walled boron nitride nanotubes

The study used Density Functional Theory to simulate the properties of single-walled and double-walled boron nitride nanotubes. The cohesive energy calculations suggest that double-walled nanotubes are more stable than single-walled nanotubes. Double-walled nanotubes have a narrower bandgap than single-walled nanotubes. The smaller diameter of the nanotubes reduces available energy for vibrational modes. Double-walled nanotubes have larger Young’s modulus, shear modulus, and bulk modulus than single-walled nanotubes. Both single-walled and double-walled nanotubes perform like auxetic materials in some directions. The group velocity of acoustic phonons in SWBNNTs (6, 0) is higher than in SWBNNTs (12, 0).

First-principles calculation of P-type Mg3Sb2 thermoelectric performance modification by Ge and Si doping

Mg3Sb2 has been considered a highly promising thermoelectric material for mid-temperature applications. Optimizing the properties of the material is crucial for accelerating its commercial use. In this work, first-principles molecular simulations of P-type Mg3Sb2 doped with the carbon group elements Ge and Si have been carried out. Results indicate that doping with Ge and Si enhances the thermodynamic stability and electrical conductivity of the material. This improvement is achieved by decreasing the bandgap, increasing the local and peak density of states, flattening the band structure, and elevating the relative mass of carriers. Additionally, doping with Ge and Si decreases the phonon velocity and Debye temperature, which weakens the thermal transport properties of the material. These findings suggest that Ge and Si doping is an effective method for improving the thermoelectric properties of the material. At the same doping concentration, the Si single-doped system possesses the smallest bandgap value with the highest peak density of states and forms an indirect bandgap, leading to the best electrical transport properties; the Ge single-doped system has the lowest phonon velocity and Debye temperature, which has the most significant effect in attenuating the thermal transport properties of the material; and the Ge–Si co-doped system has the highest relative mass of carriers, which is conducive to the enhancement of Seebeck coefficient. The results offer theoretical guidance for experimentally analyzing the effects of Ge and Si doping on the thermoelectric properties of Mg3Sb2.

Understanding the anomalously low thermal properties of Zr3Ni3-xCoxSb4 thermoelectric material

By controlling the SPS temperature, we prepared Zintl Zr3Ni2.9Co0.1Sb4 samples, namely Co doping in Ni sites of Zr3Ni3Sb4, and investigated the corresponding thermoelectric properties. It was found that a significant reduction of the lattice thermal conductivity occurred due to Co-doping while tuning the SPS temperature slightly affected ZT values. The influence of aliovalent element Co doping on the phonon structure and transport properties of Zr3Ni3Sb4 was conducted through first-principles calculations. Co-doping enhanced phonon anharmonicity and induced a completely avoided crossing between acoustic and optical phonons, which resulted in the suppressed phonon velocity and lifetime, leading to a remarkable reduction in lattice thermal conductivity. Our finding enriches the understanding of aliovalent doping in thermal transport theories which should be applicable to other thermoelectric systems.

Lattice thermal conductivity in half-Heusler compounds XNiSn (X=Ti, Zr, Hf) using Slack's model

Reducing lattice thermal conductivity (κl) that reflects the material’s heat-carrying capacity through lattice phonon vibrations is crucial for optimizing the figure of merit (zT). Using density functional theory (DFT) and density functional perturbation theory (DFPT), present work considers the structural, electronic, magnetic, and phonon properties of the XNiSn (X=Ti, Zr, Hf ) half Heusler (hH) compounds. TiNiSn, ZrNiSn, and HfNiSn are identified as semiconductors with indirect band gaps of 0.43 eV, 0.47 eV and 0.39 eV, respectively, exhibiting dynamical stability. The temperature- dependent κl of hH XNiSn are compared using Slack’s model with two approaches for analyzing phonon velocity: elastic constants from quasi- harmonic approximation (QHA) as implemented in the Thermo_pw code and slope of phonon bands based on DFPT. At room temperature, TiNiSn, ZrNiSn and HfNiSn have κl values of 28.61 Wm−1K−1, 34.61 Wm−1K−1 and 29.85 Wm−1K−1 respectively using phonon velocity from slopes of phonon bands based on DFPT. These values show deviation of 1.48%, 6.29%, and 5.82% to those κl values 29.01 Wm−1K−1’, 36.79 Wm−1K−1 and 31.59 Wm−1K−1 for TiNiSn, ZrNiSn and HfNiSn respectively using QHA.

Realizing High Thermoelectric Performance in GeTe‐Based Supersaturated Solid Solutions

AbstractAs a highly promising thermoelectric material in the mid‐temperature region, pristine GeTe shows deteriorated thermoelectric properties due to the low Seebeck coefficient and the intrinsic high thermal conductivity. Herein, it is discovered that the introduction of strongly correlated d, f electrons by alloying rare‐earth atoms at Ge sites will increase the band effective mass and promote band convergency, which can largely improve the electric transport property. Moreover, the heavy atoms (Pb, Bi) doping induces optical phonon softening and the avoided crossing between acoustic and optical phonon branches, decelerating both optical and acoustic phonon velocities. The modulated composition wave from the fluctuated multi‐component distribution introduces a complicated hierarchical structure including point defects and nanoprecipitates, which provides all‐scale scattering sources for heat‐carrying phonons and results in low lattice thermal conductivity. Consequently, a high zT of 2.4 at 800 K can be obtained in the Ge0.86Pb0.09Bi0.03Ce0.005Te sample due to the combination of strong correlation for d, f electrons and the full‐spectrum scattering for phonons. This work provides an effective strategy to increase the thermoelectric performance of IV–VI compounds by combining the modulated band structure and the softening phonon dispersion.

Achieving Superior Thermoelectric Performance in Ge4Se3Te via Symmetry Manipulation with I–V–VI2 Alloying

AbstractAlthough orthorhombic GeSe is predicted to have an ultrahigh figure of merit, ZT ≈ 2.5, up to now, the highest experimental value is ≈0.2 due to the low carrier concentration (nH ≈ 1018 cm−3). Improving symmetry is an effective approach for enhancing the ZT of GeSe‐based materials. With Te‐alloying, Ge4Se3Te displays the two‐dimensional hexagonal structure and high nH ≈ 1.23 × 1021 cm−3. Interestingly, Ge4Se3Te transformed from the hexagonal into the rhombohedral phase with only ≈2% I–V–VI2‐alloying (I = Li, Na, K, Cu, Ag; V = Sb, Bi; VI = Se, Te). According to the calculated results of Ge0.82Ag0.09Bi0.09Se0.614Te0.386 single‐crystal grown via AgBiTe2‐alloying, it exhibits a higher valley degeneracy than the hexagonal Ge4Se3Te. For instance, AgBiTe2‐alloying induces a strong band convergence and band inversion effect, resulting in a significantly enhanced Seebeck coefficient and power factor with a similar nH from 17 µV K−1 and 0.63 µW cm−1 K−2 for pristine Ge4Se3Te to 124 µV K−1 and 5.97 µW cm−1 K−2 for 12%AgBiTe2‐alloyed sample, respectively. Moreover, the sharply reduced phonon velocity, nano‐domain wall structure, and strong anharmonicity lead to low lattice thermal conductivity. As a result, a record‐high average ZT ≈0.95 over 323–773 K with an excellent ZT ≈ 1.30 is achieved at 723 K.

Thermoelectric properties of the low-spin lanthanide cobalate perovskites LaCoO3, PrCoO3, and NdCoO3 from first-principles calculations.

An ab initio modelling workflow is used to predict the thermoelectric properties and figure of merit ZT of the lanthanide cobalates LaCoO3, PrCoO3 and NdCoO3 in the orthorhombic Pnma phase with the low-spin magnetic configuration. The LnCoO3 show significantly lower lattice thermal conductivity than the widely-studied SrTiO3, due to lower phonon velocities, with a large component of the heat transport through an intraband tunnelling mechanism characteristic of amorphous materials. Comparison of the calculations to experimental measurements suggests the p-type electrical properties are significantly degraded by the thermal spin crossover, and materials-engineering strategies to suppress this could yield improved ZT. We also predict that n-doped LnCoO3 could show larger Seebeck coefficients, superior power factors, lower thermal conductivity, and higher ZT than SrTiO3. Our results highlight the exploration of a wider range of perovskite chemistries as a facile route to high-performance oxide thermoelectrics, and identify descriptors that could be used as part of a modelling-based screening approach.

Anomalous mass dependence of phonon thermal transport in lanthanum monopnictides and its origin in the nature of chemical bonding

In certain compounds of binary lanthanum monopnictides exhibiting a rocksalt crystal structure, a phenomenon arises: larger average atomic mass, smaller acoustic phonon velocities, and smaller acoustic optical frequency gap do...

Inverse-Perovskite Ba3 BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity.

High energy-conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic-element free Ba3 BO (B = Si and Ge) withinverse-perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p-type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat ) 1.0-0.4Wm-1 K-1 at T = 300-600 K are observed in Ba3 BO. Highly distorted O-Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3 BO. As a consequence of high PF and low κlat , Ba3 SiO (Ba3 GeO) exhibits rather high ZT = 0.16-0.84 (0.35-0.65) at T = 300-623 K (300-523 K). Finally, based on first-principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3 SiO and 1.21 for Ba3 GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse-perovskites would be a new platform of environmentally-benign high-ZT thermoelectric materials.

Lenaite (AgFeS2): A dynamically stable mineral with excellent thermoelectric performance

Diamond-like chalcopyrites have gained superior attention in thermoelectrics due to their attractive thermal transportation. On the mark of exploring environmental-friendly thermoelectric (TE) materials, this work unveils the suitability of lenaite (AgFeS2) through first-principles calculations. Owing to the beneficial band features leading to favourable effective mass, AgFeS2 exhibits good electron transport characteristics. Concurrently, the ultra-low lattice thermal conductivity of 0.47 W m−1 K−1 is realized at 500 K; ascribed to the large Grüneisen parameter, moderate phonon velocity, and Debye temperature. The combinations of these transport properties yield significant figure of merit of 0.87 at 500 K for 1.39 × 1020 holes per cm3. This study not only emphasis the TE properties of lenaite but also facilitates to understand its fundamental properties, including structural, electron and phonon-related properties.