Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. ...Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earth’s crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi2) sandwiched between two Bi2Se3 quintuple layers Se–Bi–Se–Bi-Se, resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κlat of ∼0.6 W/mK at 300 K), which is significantly lower than that of Bi2Se3 (κlat of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi2) m (Bi2Se3) n . Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κlat. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κlat. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.
Understanding the origin of intrinsically low thermal conductivity is fundamentally important to the development of high‐performance thermoelectric materials, which can convert waste‐heat into ...electricity. Herein, we report an ultralow lattice thermal conductivity (ca. 0.4 W m−1 K−1) in mixed valent InTe (that is, In+In3+Te2), which exhibits an intrinsic bonding asymmetry with coexistent covalent and ionic substructures. The phonon dispersion of InTe exhibits, along with low‐energy flat branches, weak instabilities associated with the rattling vibrations of In+ atoms along the columnar ionic substructure. These weakly unstable phonons originate from the 5s2 lone pair of the In+ atom and are strongly anharmonic, which scatter the heat‐carrying acoustic phonons through strong anharmonic phonon–phonon interactions, as evident in anomalously high mode Grüneisen parameters. A maximum thermoelectric figure of merit (z T) of about 0.9 is achieved at 600 K for the 0.3 mol % In‐deficient sample, making InTe a promising material for mid‐temperature thermoelectric applications.
It's in the rattle: An ultralow lattice thermal conductivity (ca. 0.4 W m−1 K−1) is found in mixed‐valent InTe, which has an intrinsic bonding asymmetry with coexistent covalent and ionic substructures. The phonon dispersion of InTe reveals the rattling vibrations of In+ cations along the columnar ionic substructure, which are strongly anharmonic and scatter the heat‐carrying acoustic phonons through strong anharmonic phonon–phonon interactions.
Abstract
We investigate the microscopic mechanism of ultralow lattice thermal conductivity (
κ
l
) of TlInTe
2
and its weak temperature dependence using a unified theory of lattice heat transport, ...that considers contributions arising from the particle-like propagation as well as wave-like tunneling of phonons. While we use the Peierls–Boltzmann transport equation (PBTE) to calculate the particle-like contributions (
κ
l
(PBTE)), we explicitly calculate the off-diagonal (OD) components of the heat-flux operator within a first-principles density functional theory framework to determine the contributions (
κ
l
(OD)) arising from the wave-like tunneling of phonons. At each temperature,
T
, we anharmonically renormalize the phonon frequencies using the self-consistent phonon theory including quartic anharmonicity, and utilize them to calculate
κ
l
(PBTE) and
κ
l
(OD). With the combined inclusion of
κ
l
(PBTE),
κ
l
(OD), and additional grain-boundary scatterings, our calculations successfully reproduce the experimental results. Our analysis shows that large quartic anharmonicity of TlInTe
2
(a) strongly hardens the low-energy phonon branches, (b) diminishes the three-phonon scattering processes at finite T, and (c) recovers the weaker than T
−1
decay of the measured
κ
l
.
Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal ...conductivity for various applications including thermoelectrics. TlInTe2, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe2 exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe2.
Crystalline semiconductors exhibiting innate low lattice thermal conductivity (κl) are technologically very important for the development of thermal barrier coatings, thermal data-storage devices, ...and high-performance thermoelectrics. Here, using first-principles calculations based on density functional theory and anharmonic lattice dynamics, we predict intrinsically low κl (<1 W/m K along the stacking direction for T ≥ 400 K) in many known layered semiconductors, AMM′Q3 (A = Na, K, Cs, Tl; M = Cu; M′ = Zr, Hf; Q = S, Se), that possess chemical bonding heterogeneity. We show that low κl in this class of materials arises from (a) the rattling vibrations of the weakly bonded A atoms, characterized by low-frequency localized phonon branches with very small dispersion that give rise to numerous additional scattering channels and (b) strong lattice anharmonicity which is manifested in the large-mode Gruneisen parameters that increase the phonon scattering rates. Our work uncovers inherent low κl in this previously unexplored class of metal chalcogenides which should open up opportunities for applications of these compounds in various thermal energy management devices.
The search for new thermoelectric materials has gained rapid progress in recent years as thermoelectric technology offers the potential for environmentally friendly and sustainable energy conversion ...methods from waste heat to electricity. In this work, we use first-principles calculations based on density functional theory to predict high thermoelectric performance in BaAu2P4, a layered Zintl compound with a small band gap. BaAu2P4 exhibits crystallographic heterogeneity in which rigid Au2P42– units are separated by layers of Ba2+ cations, which are bonded relatively weakly to the lattice through electrostatic interactions. The phosphorus atoms are covalently bonded to each other and form infinite chains within the crystal. While the phosphorus chains facilitate large electrical conductivity, the presence of multiple bands near the Fermi level gives rise to an enhanced Seebeck coefficient. On the other hand, the loosely bound Ba along with Au strongly scatter the heat carrying acoustic phonons, significantly reducing the lattice thermal conduction along the stacking direction. As a consequence of this bonding hierarchy (i.e., coexisting rigid and fluctuating sublattices), BaAu2P4 exhibits a large power factor and low lattice thermal conductivity, which results in a high thermoelectric figure of merit (zT). Thus, our findings should encourage the exploration of new thermoelectric materials in the family of layered compounds with small band gaps and crystallographic heterogeneity.
Efficiency in generation and utilization of energy is highly dependent on materials that have the ability to amplify or hinder thermal conduction processes. A comprehensive understanding of the ...relationship between chemical bonding and structure impacting lattice waves (phonons) is essential to furnish compounds with ultralow lattice thermal conductivity (κlat) for important applications such as thermoelectrics. Here, we demonstrate that the n-type rock-salt AgPbBiSe3 exhibits an ultra-low κlat of 0.5–0.4 W m−1 K−1 in the 290–820 K temperature range. We present detailed analysis to uncover the fundamental origin of such a low κlat. First-principles calculations augmented with low temperature heat capacity measurements and the experimentally determined synchrotron X-ray pair distribution function (PDF) reveal bonding heterogeneity within the lattice and lone pair induced lattice anharmonicity. Both of these factors enhance the phonon–phonon scattering, and are thereby responsible for the suppressed κlat. Further optimization of the thermoelectric properties was performed by aliovalent halide doping, and a thermoelectric figure of merit (zT) of 0.8 at 814 K was achieved for AgPbBiSe2.97I0.03 which is remarkable among n-type Te free thermoelectrics.
Fundamental understanding of the correlation between chemical bonding and lattice dynamics in intrinsically low thermal conductive crystalline solids is important to thermoelectrics, thermal barrier ...coating, and more recently to photovoltaics. Two-dimensional (2D) layered halide perovskites have recently attracted widespread attention in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κL) in the single crystal of all-inorganic layered Ruddlesden–Popper (RP) perovskite, Cs2PbI2Cl2, synthesized by the Bridgman method. We have measured the anisotropic κL value of the Cs2PbI2Cl2 single crystal and observed an ultralow κL value of ∼0.37–0.28 W/mK in the temperature range of 295–523 K when measured along the crystallographic c-axis. First-principles density functional theory (DFT) analysis of the phonon spectrum uncovers the presence of soft (frequency ∼18–55 cm–1) optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm–1 that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s2 lone pair. We provide experimental evidence for such low energy optical phonon modes with low-temperature heat capacity and temperature-dependent Raman spectroscopic measurements. The strong anharmonic coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons to ultrasoft frequency (maximum ∼37 cm–1). The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic–optical phonon coupling results in an intrinsically ultralow κL value in the all-inorganic layered RP perovskite Cs2PbI2Cl2.
Abstract
The development of efficient thermal energy management devices such as thermoelectrics and barrier coatings often relies on compounds having low lattice thermal conductivity (
κ
l
). Here, ...we present the computational discovery of a large family of 628 thermodynamically stable quaternary chalcogenides, AMM′Q
3
(A = alkali/alkaline earth/post-transition metals; M/M′ = transition metals, lanthanides; Q = chalcogens) using high-throughput density functional theory (DFT) calculations. We validate the presence of low
κ
l
in these materials by calculating
κ
l
of several predicted stable compounds using the Peierls–Boltzmann transport equation. Our analysis reveals that the low
κ
l
originates from the presence of either a strong lattice anharmonicity that enhances the phonon-scatterings or rattler cations that lead to multiple scattering channels in their crystal structures. Our thermoelectric calculations indicate that some of the predicted semiconductors may possess high energy conversion efficiency with their figure-of-merits exceeding 1 near 600 K. Our predictions suggest experimental research opportunities in the synthesis and characterization of these stable, low
κ
l
compounds.
Abstract
As the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κ
L
, decreases, and its ...fundamental characteristics changes. The realization of ultralow and unusual glass-like κ
L
in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2–400 K) of κ
L
in a single crystal of layered halide perovskite, Cs
3
Bi
2
I
6
Cl
3
. Acoustic phonons with low cut-off frequency (20 cm
−1
) are responsible for the low sound velocity in Cs
3
Bi
2
I
6
Cl
3
and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κ
L
is intriguing from lattice dynamical perspective as well as have potential applications.
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