Origin Of Low Thermal Conductivity Research Articles

Origin of low lattice thermal conductivity and mobility of lead-free halide double perovskites

Lead-free halide double perovskites of Cs2BiAgBr6 and Cs2BiAgCl6 have attracted extensive attention due to potential application in optoelectronic field. The transport behavior plays an important role in electronic devices based on semiconductors, which includes lattice thermal conductivity and carier mobility. Since impurities and defects may exist in the samples, it is difficult to investigate the intrinsic transport behavior by experimental measurements. The theoretical prediction of their transport behaviors can provide a guidance for the application of materials. In this work, we first develop machine learning interatomic potential (MLIP) to explore their lattice thermal conductivities. They exhibit extremely low lattice thermal conductivity at 300 K with the values of κx = 0.122 ± 0.009∕κz = 0.103 ± 0.004 Wm−1K−1 for Cs2BiAgBr6, and κx = 0.352 ± 0.014∕κz = 0.337 ± 0.012 Wm−1K−1 for Cs2BiAgCl6, respectively. It is revealed that the thermal conductivity is mainly controlled by low-frequency phonons. Subsequently, we apply semi-empirical models to predict their mobilities. The mobility is mainly determined by the scattering from longitudinal optical phonon with the values of μe,x= 22.0/μh,x= 18.8cm2V−1s−1 for Cs2BiAgBr6, and μe,x= 19.2/μh,x= 19.5cm2V−1s−1 for Cs2BiAgCl6, respectively. The stretching between Bi and Br/Cl atoms limits the mobility. Finally, the origin of low thermal conductivity and mobility on double perovskites is revealed.

Metavalent Bonding-Mediated Dual 6s2 Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe2.

Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p-bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe2 that held prospects for 6s2 dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe2 to dual 6s2 lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1-0.4 Wm-1 K-1 between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe2 exists not in a monomorphous R-3m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe2, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe2. With additional point-defect scattering, a κlatt of 0.3 Wm-1 K-1 was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit (zT) of ∼0.8 at 715 K.

Short-range order and origin of the low thermal conductivity in compositionally complex rare-earth niobates and tantalates

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and “diffusons” give rise to the observed amorphous-like thermal conductivity.

Radial and axial vibration modes of graphyne nanotubes

The radial breathing modes (RBM) of carbon nanotubes (CNTs) arise from the collective inward and outward vibration along the radial direction of tubes, becoming characteristic Raman vibrational modes widely used for determining the radii of CNTs. The graphyne nanotubes (GNTs) can form by rolling graphyne sheets in a similar way as the CNT formation from rolled graphene sheets. In this work the atomic configuration and vibration modes of the GNTs with varied chirality and radii were investigated by classical molecular dynamics. The GNTs exhibit RBMs with an inversed linear relationship between the RBM vibration frequencies and radii, similar with those of CNTs. More importantly, we reported an axial vibrational mode (AVM) of the GNTs where the vibration waves propagate along the axial direction of the GNTs. The AVM is a unique vibration mode of GNTs due to the more flexible sp bonded carbon chain atoms arranged along the tube axis, providing additional axial vibration degree of freedom. The features of phase difference of the standing axial waves depend on the structure types of GNTs: The armchair GNTs exhibit zigzag-like sawtooth waves, while the zigzag GNTs have armchair-like square waves. The AVMs of GNTs can be regarded as transverse acoustic phonon modes of carbon atom chains at low frequency ranges. Moreover, the AVMs of GNTs generally have higher frequencies than the RBMs of CNTs with similar diameters. Therefore, our simulation suggests that AVM may be used as a fingerprint vibrational mode of GNTs for the refined structure characterization in experiments. The AVM may also be the origin of low thermal conductivity of GNTs with implications as thermoelectric materials.

Low intrinsic thermal conductivity of Spark Plasma Sintered dense KNbO3 and NaNbO3 perovskite ceramics

The origin of low thermal conductivity in niobium-containing perovskites was investigated. Therefore, dense NaNbO3 and KNbO3 ceramics were sintered by spark plasma sintering. The grain size was in the 1–10 μm range. NaNbO3 ceramic has shown a stable thermal conductivity circa 2.6 W m−1 K−1 from 373 to 1000 K and KNbO3, values from 2.5 to a plateau at 1.2 W m−1 K−1 since 673 K. This value is the lowest thermal conductivity reported for an oxide presenting a perovskite structure. The origin of such low thermal conductivities is indicated to be related with cation deficiency on the A-site of the niobium-containing perovskites.

Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2

Superionic crystals exhibit ionic mobilities comparable to liquids while maintaining a periodic crystalline lattice. The atomic dynamics leading to large ionic mobility have long been debated. A central question is whether phonon quasiparticles—which conduct heat in regular solids—survive in the superionic state, where a large fraction of the system exhibits liquid-like behaviour. Here we present the results of energy- and momentum-resolved scattering studies combined with first-principles calculations and show that in the superionic phase of CuCrSe2, long-wavelength acoustic phonons capable of heat conduction remain largely intact, whereas specific phonon quasiparticles dominated by the Cu ions break down as a result of anharmonicity and disorder. The weak bonding and large anharmonicity of the Cu sublattice are present already in the normal ordered state, resulting in low thermal conductivity even below the superionic transition. These results demonstrate that anharmonic phonon dynamics are at the origin of low thermal conductivity and superionicity in this class of materials. Neutron and X-ray scattering studies combined with first-principles calculations suggest that the large, liquid-like ionic mobility in the canonical superionic crystal CuCrSe2 is due to anharmonic phonon dynamics.

The microscopic origin of low thermal conductivity for enhanced thermoelectric performance of Yb doped MgAgSb

An in-depth understanding of the origin of low lattice thermal conductivity κlat is considered to be the prerequisite for developing and designing high thermoelectric performance materials. However, the microscopic mechanism of low κlat for α-MgAgSb has not been fully elaborated. Here we first thoroughly unveil the underlying mechanism using density functional theory (DFT) calculations. It is demonstrated that the unique lattice dynamic properties of α-MgAgSb contribute to the low κlat, including a large number of optical branches, weak chemical bonding, the strong anharmonic coupling between longitudinal acoustic (LA) and longitudinal optical (LO) branches, high anharmonic behavior of LO branch. Nanostructuring has an effect in decreasing the lattice thermal conductivity due to the high density of boundaries and defects. Most importantly, Yb doping on the Mg site is chosen to optimize carrier concentration and simultaneously create strong point-defect phonon scattering. Collectively, a peak ZT ∼1.4 at 550 K and average ZT ∼1.2 from 300 K to 550 K have been achieved, which further demonstrates the realistic prospect of MgAgSb based thermoelectric materials for low-temperature waste heat harvesting.

Origin of low thermal conductivity in SnSe

We provide direct evidence to understand the origin of low thermal conductivity of SnSe using elastic measurements. Compared to state-of-the-art lead chalcogenides $\mathrm{Pb}Q(Q=\mathrm{Te}$, Se, S), SnSe exhibits low values of sound velocity $(\ensuremath{\sim}1420\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{s})$, Young's modulus $(E\ensuremath{\sim}27.7\phantom{\rule{0.28em}{0ex}}\mathrm{GPa})$, and shear modulus $(G\ensuremath{\sim}9.6\phantom{\rule{0.28em}{0ex}}\mathrm{GPa})$, which are ascribed to the extremely weak Sn-Se atomic interactions (or bonds between layers); meanwhile, the deduced average Gr\uneisen parameter \ensuremath{\gamma} of SnSe is as large as \ensuremath{\sim}3.13, originating from the strong anharmonicity of the bonding arrangement. The calculated phonon mean free path (l \ensuremath{\sim} 0.84 nm) at 300 K is comparable to the lattice parameters of SnSe, indicating little room is left for further reduction of the thermal conductivity through introducing nanoscale microstructures and microscale grain boundaries. The low elastic properties indicate that the weak chemical bonding stiffness of SnSe generally causes phonon modes softening which eventually slows down phonon propagation. This work provides insightful data to understand the low lattice thermal conductivity of SnSe.

Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity

The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9--400 K) and powder x-ray diffraction (300--725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below $60\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{D}}=195\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{\ensuremath{\Theta}}}_{\mathrm{E}1}=78\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{E}2}=277\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as ${\mathrm{Zn}}_{4}{\mathrm{Sb}}_{3}$, CdSb, ${\mathrm{Cd}}_{4}{\mathrm{Sb}}_{3}, {\mathrm{Cd}}_{13\ensuremath{-}x}{\mathrm{In}}_{y}{\mathrm{Zn}}_{10}$, and ${\mathrm{Zn}}_{5}{\mathrm{Sb}}_{4}{\mathrm{In}}_{2\ensuremath{-}\ensuremath{\delta}}$.

From Bonding Asymmetry to Anharmonic Rattling in Cu12Sb4S13Tetrahedrites: When Lone‐Pair Electrons Are Not So Lonely

Some of the best thermoelectrics are complex materials with rattling guests inside oversized atomic cages. Understanding the chemical and structural origins of the rattling behavior is essential to the design of thermoelectric materials. In this work, a clear connection is established between the local bonding asymmetry and anharmonic rattling modes in tetrahedrite thermoelectrics, enabled by the chemically active electron lone pairs. The studies reveal a five‐atom atomic cage Sb[CuS3]Sb in Cu12Sb4S13tetrahedrites that exhibits strong local bonding asymmetry: covalent bonding inside the CuS3trigonal plane and weak out‐of‐plane bonding induced by the lone‐pair electrons of Sb. This bonding asymmetry leads to out‐of‐plane rattling modes that are quasilocalized and anharmonic with low frequency and large amplitude, and are likely the origin of low thermal conductivity in tetrahedrites. Such knowledge highlights the importance of local structure asymmetry and lone‐pair atoms in driving anharmonic rattling, providing a stepping stone to the discovery and design of next‐generation thermoelectrics.

On the Origin of Low Thermal Conductivity in High Thermoelectric Performance in n-type BiAgSeS

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Origin of Low Thermal Conductivity in Nuclear Fuels

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grüneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.