Large Anharmonicity Research Articles

Tuning the Lattice Thermal Conductivity in Bismuth Telluride via Cr Alloying

Decreasing thermal conductivity of a thermoelectric material is always a prerequisite for its potential application. Using first-principle calculations, we examine the magnetism induced change in lattice thermal transport in bismuth telluride. The source of magnetic moment, Cr in the doped system, weakly magnetizes the coordinated Te atoms to make the latter's phonon softer than that in the pure compound. Though the transition metal dopants do not participate directly in the heat conduction process, the anharmonicity induced by them favor in reducing the lattice thermal conductivity. Large anharmonicity in $(Bi_{0.67}Cr_{0.33})_2Te_3$ reduces the in-plane room temperature lattice thermal conductivity by $\sim 79\%$. The thermal conductivity, strictly, does not vary monotonically with doping concentration. Even, for any particular doping level, the thermal conductivity is different for different configurations which is related to the internal energy of the system. We found that the internal energy variance of 0.03 $eV$ would reduce the in-plane thermal conductivity of the room temperature lattice by at least 60$\%$ for 50$\%$ doping.

The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe

The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity κ. However, the κ of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in κ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating κ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates κ near the phase transition. Our findings elucidate the influence of structural phase transitions on κ and provide guidance for design of better thermoelectric materials.

Anharmonic phonons and anomalous thermal expansion of graphite

We have investigated the anisotropic thermal expansion of graphite using ab-initio calculations of the implicit and explicit anharmonicity of phonons, which occur due to change in volume and increase in the thermal amplitudes, respectively. We find that the negative thermal expansion (NTE) in the a-b plane below 600 K and very large positive thermal expansion along the c-axis up to high temperatures arise due to various phonons polarized along the c-axis. While the NTE arises from the anharmonicity of transverse phonons over a broad energy range up to 60 meV, the large positive expansion along the c-axis occurs largely due to the longitudinal optic phonon modes around 16 meV. We find that these phonons have large implicit anharmonicity, but very little explicit anharmonicity. We also find very significant increase in the linear compressibility along the c-axis with increase in volume, and this has important role to quantitatively explain the thermal expansion behavior along the c-axis. The hugely anisotropic bonding in graphite is found to be responsible for wide difference in the energy range of the transverse and longitudinal phonon modes polarized along the c-axis, which are responsible for the anomalous thermal expansion behavior. This behaviour is in contrast to other nearly isotropic hexagonal structures like water-ice, which show anomalous thermal expansion in a small temperature range arising from a narrow energy range of phonons.

High Room-Temperature Thermoelectric Performance of Honeycomb GaN Monolayer

Self-powered nanoscale devices that scavenge heat from the surrounding environment show great promise in applications as biosensors or environmental sensors. Recently, defect-free monolayer gallium nitride (GaN) with a honeycomb structure was realized experimentally. In this paper, based on first-principles calculations and Boltzmann transport theory, the thermoelectric properties of monolayer GaN are investigated. Its electronic structure characteristics approach the condition of Mahan–Sofo’s best thermoelectrics, which leads to outstanding room-temperature Seebeck coefficients, up to 310 μV·K−1 with a hole concentration of 5 × 1018cm−3. Combined with the intrinsic high electron mobility, it then boosts the power factor of the GaN monolayer system. Moreover, due to the strong in-plane polarization nature of Ga–N bonds, the in-plane lattice vibrations exhibit extremely large anharmonicity, resulting in low thermal conductivity (6.4 W·m−1·K−1 at 300K). These results immediately cause a room-temperature figure of merit ZT of 0.17, indicating the P-type monolayer GaN is a superior candidate for low-dimensional thermoelectrics.

Microscopic mechanism of unusual lattice thermal transport in TlInTe2

We investigate the microscopic mechanism of ultralow lattice thermal conductivity (κl) of TlInTe2 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 (κl(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 (κl(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 κl(PBTE) and κl(OD). With the combined inclusion of κl(PBTE), κl(OD), and additional grain-boundary scatterings, our calculations successfully reproduce the experimental results. Our analysis shows that large quartic anharmonicity of TlInTe2 (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 κl.

First-Principles Study of Anharmonic Lattice Dynamics in Low Thermal Conductivity AgCrSe_{2}: Evidence for a Large Resonant Four-Phonon Scattering.

We report a study of the anharmonic lattice dynamics in low lattice thermal conductivity (κ_{l}) material AgCrSe_{2} by many-body perturbation theory. We demonstrate surprisingly giant four-phonon scattering exclusive for the heat-carrying transverse acoustic phonons due to large quartic anharmonicity and nondispersive phonon band structure, which lead to four-phonon Fermi resonance and breaks the classical τ^{-1}∼ω^{m}T^{n} relation for phonon-phonon interactions. This strong resonant scattering extends over the Brillouin zone and substantially suppresses the thermal transport, even down to a low temperature of 100K. The present results provide fundamental insights into the four-phonon resonant dynamics in the low-κ_{l} system with flat phonon dispersions, i.e., cuprous halides and skutterudites.

Ultralow Thermal Conductivity in Cs–Sb–Se Compounds: Lattice Instability versus Lone-Pair Electrons

The presence of lone-pair electrons (LPE) is known to lead to large anharmonicities in materials and thus to low lattice thermal conductivity (κ). Materials with LPE typically have κ values that ar...

Vibrational Couplings in Hydridocarbonyl Complexes: A 2D-IR Perspective.

Hydridocarbonyl complexes, a class of industrially relevant catalysts, contain both the M-H and M-CO moieties. Here, using two-dimensional infrared spectroscopy, we examine the coupling of the typically weak M-H stretching mode and the intense M(C≡O) mode. By studying a series of Ir(I)- and Ir(III)-based hydridocarbonyl complexes, we show that the arrangement of the H and CO ligands in a trans configuration leads to strong vibrational coupling and mode delocalization. In contrast, a cis arrangement leads to no coupling, with the localized M-H mode having a much larger anharmonicity. These results highlight a promising strategy for enhancing the M-H vibration by intensity borrowing from the strong C≡O modes in a trans configuration, allowing for direct determination by infrared spectroscopy of both the oxidation (by frequency shifts) and the protonation state (via vibrational coupling) of the complex, in mechanistic studies of proton-coupled electron transfer reactions.

Giant low-temperature anharmonicity in silicon nanocrystals

The phonon density of states of silicon nanocrystals with size between 4 and 7.5 nm was measured by inelastic neutron scattering in the $5\ensuremath{\sim}600\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ temperature range. The narrow particle size distributions enable the study of size effects on phonon dynamics. Giant softening of phonon features below 30 meV, universal broadening of phonon features, and the disappearance of intermediate-energy phonons were observed with decreasing nanocrystals size. Such size effects are mostly attributed to the structure variations within the nanocrystals. The phonons below 30 meV in silicon nanocrystals show temperature dependence opposite to the bulk silicon, explained by the large anharmonicity of the under-constrained near-surface phonons. This is supported by the abnormal atomic mean-square-displacement, and low energy phonon population in small silicon nanocrystals. This work provides crucial information on the phonon dynamics in spatially confined materials.

Synthesis and Characterization of Two New Second Harmonic Generation Active Iodates: K3Sc(IO3)6 and KSc(IO3)3Cl.

Transparent single crystals of two new iodates K3Sc(IO3)6 and KSc(IO3)3Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K3Sc(IO3)6 crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO6] octahedra, [IO3] trigonal pyramids, and [KO8] distorted cubes. The compound KSc(IO3)3Cl crystallizes in the trigonal space group R3. The building blocks are [ScO6] octahedra, [KO12] polyhedra, and [IO3] trigonal pyramids. The Cl– ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K3Sc(IO3)6 and KSc(IO3)3Cl are around 2.8 and 2.5 times that of KH2PO4 (KDP), respectively. In addition, K3Sc(IO3)6 and KSc(IO3)3Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Ternary multicomponent Ba/Mg/Si compounds with inherent bonding hierarchy and rattling Ba atoms toward low lattice thermal conductivity.

Compositional tailoring externally enables the fine tuning of thermal transport parameters of materials using the dual modulation of electronic or thermal transport properties. Theoretically, we examined the lattice dynamics of three particularly ternary representatives with different stoichiometry, BaMgSi, Ba2Mg3Si4, and BaMg2Si2, and identified the inherent bonding hierarchy and rattling Ba atoms, which were responsible for reducing the lattice thermal conductivity. BaMgSi and Ba2Mg3Si4 exhibited inherently ultra-low lattice thermal conductivity of 1.27-0.37 W m-1 K-1 in the range of 300-1000 K due to the bonding hierarchy and rattling Ba atoms. The low-energy optical phonons are overlapping with the acoustic phonons. This is associated with the intrinsic rattler-like vibration of Ba cations and leads to the characteristic in the localization of the propagative phonons and large anharmonicity. Although BaMg2Si2 had a dumbbell-shaped Si-Si covalent and Ba-Si/Mg ionic bonding environment and intrinsic rattler-like vibration of Ba cations, the middle frequency optic phonon branches contribute considerably to the thermal conductivity of the lattice. At the same temperature, compared with BaMgSi and Ba2Mg3Si4, the lattice thermal conductivity of BaMg2Si2 almost doubles owing to the higher phonon lifetime and group velocities. Our findings highlight considerable potential for thermoelectric applications with a different stoichiometric ratio of Ba/Mg/Si systems due to their low lattice thermal conductivities via intrinsic modulating stoichiometry.

High-Coherence Fluxonium Qubit

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in one device). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit -- over 100 here -- must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.

Quantum effects in muon spin spectroscopy within the stochastic self-consistent harmonic approximation

In muon spin rotation experiments the positive implanted muon vibrates with large zero point amplitude by virtue of its light mass. Quantum mechanical calculations of the host material usually treat the muon as a point impurity, ignoring this large vibrational amplitude. As a first order correction, the muon zero point motion is usually described within the harmonic approximation, despite the large anharmonicity of the crystal potential. Here we apply the stochastic self-consistent harmonic approximation, a quantum variational method devised to include strong anharmonic effects in total energy and vibrational frequency calculations, in order to overcome these limitations and provide an accurate ab initio description of the quantum nature of the muon. We applied this full quantum treatment to the calculation of the muon contact hyperfine field in textbook-case metallic systems, such as Fe, Ni, Co including MnSi and MnGe, significantly improving agreement with experiments. Our results show that muon vibrational frequencies are strongly renormalized by anharmonicity. Finally, in contrast to the harmonic approximation, we show that including quantum anharmonic fluctuations, the muon stabilizes at the octahedral site in bcc Fe.

The nature of [formula omitted], [formula omitted] bands in 158Gd

The cross over of β-, and γ-band at N=94 in 158Gd signifies a major change in the nuclear structure of Gd isotopes. Using the microscopic dynamic pairing plus quadrupole (DPPQ) model, we reproduce this transition. The spectral features of the Kπ=03+, 21+ and 41+ bands besides the lower three: ground, β- and γ-bands have been analyzed. The B(E2) values from the Kπ=02+ and Kπ=03+ bands are analyzed to interpret the nature of these two bands. The Kπ=02+ band is confirmed as the axially symmetric β-vibration. The Kπ=03+ band is shown to be collective vibration, but instead of 2β vibration, it indicates very special features with large anharmonicity. The five excited 0+ states below 2.0 MeV identified in 158Gd from (p,t) reaction work are also predicted from the DPPQ model. The relative variation of β- and γ-band head energies and of Kπ=03+ bands, and of Kπ=4+ band with neutron number N in 152-160Gd isotopes is reproduced. But the vibrational energy scale of the excited bands remains expanded. The algebraic interacting sd-boson model is also applied for evaluating the energy spectrum and the B(E2) ratios in these K-bands and for prediction of the multi-phonon 0+ states.

Origin of Intrinsically Low Thermal Conductivity in Talnakhite Cu17.6Fe17.6S32 Thermoelectric Material: Correlations between Lattice Dynamics and Thermal Transport.

Understanding the nature of phonon transport in solids and the underlying mechanism linking lattice dynamics and thermal conductivity is important in many fields, including the development of efficient thermoelectric materials where a low lattice thermal conductivity is required. Herein, we choose the pair of synthetic chalcopyrite CuFeS2 and talnakhite Cu17.6Fe17.6S32 compounds, which possess the same elements and very similar crystal structures but very different phonon transport, as contrasting examples to study the influence of lattice dynamics and chemical bonding on the thermal transport properties. Chemically, talnakhite derives from chalcopyrite by inserting extra Cu and Fe atoms in the chalcopyrite lattice. The CuFeS2 compound has a lattice thermal conductivity of 2.37 W m-1 K-1 at 625 K, while Cu17.6Fe17.6S32 features Cu/Fe disorder and possesses an extremely low lattice thermal conductivity of merely 0.6 W m-1 K-1 at 625 K, approaching the amorphous limit κmin. Low-temperature heat capacity measurements and phonon calculations point to a large anharmonicity and low Debye temperature in Cu17.6Fe17.6S32, originating from weaker chemical bonds. Moreover, Mössbauer spectroscopy suggests that the state of Fe atoms in Cu17.6Fe17.6S32 is partially disordered, which induces the enhanced alloy scattering. All of the above peculiar features, absent in CuFeS2, contribute to the extremely low lattice thermal conductivity of the Cu17.6Fe17.6S32 compound.

Lattice Dynamics Study of Thermoelectric Oxychalcogenide BiCuChO (Ch = Se, S)

The BiCuSeO-based compounds with very low thermal conductivity are among the most promising thermoelectric materials recently discovered. Their lattice dynamics, which remains mostly unexplored experimentally, is investigated in this paper. We report Raman experiments on BiCuSe1–xSxO solid solutions and infrared experiments on BiCuSeO and BiCuSO alloys coupled to first-principles-based calculations. We have observed that the high-energy A1g Raman-active mode strongly depends on the chalcogen content. We also report the density functional theory calculations of the dielectric and elastic constants, the phonons in the whole Brillouin zone, and the thermodynamic properties. We have determined the thermal expansion of BiCuSeO at room temperature from neutron diffraction experiments and evaluated its thermodynamic Gruneisen parameter and found a quite large value compared to other thermoelectric materials, which confirms the large anharmonicity of BiCuSeO.

First-principles study of the layered thermoelectric material TiNBr.

Layer-structured materials are often considered to be good candidates for thermoelectric materials, because they tend to exhibit intrinsically low thermal conductivity as a result of atomic interlayer interactions. The electrical properties of layer-structured materials can be easily tuned using various methods, such as band modification and intercalation. We report TiNBr, as a member of the layer-structured metal nitride halide system MNX (M = Ti, Zr, Hf; X = Cl, Br, I), and it exhibits an ultrahigh Seebeck coefficient of 2215 μV K−1 at 300 K. The value of the dimensionless figure of merit, ZT, along the A axis can be as high as 0.661 at 800 K, corresponding to a lattice thermal conductivity as low as 1.34 W (m K)−1. The low κl of TiNBr is associated with a collectively low phonon group velocity (2.05 × 103 m s−1 on average) and large phonon anharmonicity that can be quantified using the Grüneisen parameter and three-phonon processes. Animation of the atomic motion in highly anharmonic modes mainly involves the motion of N atoms, and the charge density difference reveals that the N atoms become polarized with the merging of anharmonicity. Moreover, the fitting procedure of the energy–displacement curve verifies that in addition to the three-phonon processes, the fourth-order anharmonic effect is also important in the integral anharmonicity of TiNBr. Our work is the first study of the thermoelectric properties of TiNBr and may help establish a connection between the low lattice thermal conductivity and the behavior of phonon vibrational modes.

Ultralow thermal conductivity in a two-dimensional material due to surface-enhanced resonant bonding

Crystalline materials with ultralow thermal conductivity are highly desirable for thermoelectric applications. Many known crystalline materials with low thermal conductivity, including PbTe and Bi2Te3, possess a special kind of chemical bond called ‘resonant bond’. Resonant bonds consist of superposition of degenerate bonding configurations that leads to structural instability, anomalous long-range interatomic interaction, and soft optical phonons. These factors contribute to large lattice anharmonicity and strong phonon-phonon scattering, which result in low thermal conductivity. In this work, we use first principles simulation to investigate the effect of resonant bonding in two dimensions (2D), where resonant bonds are in proximity to the surface. We find that the long-range interatomic interaction due to resonant bonding becomes more prominent in 2D because of reduced screening of the atomic displacement–induced charge density distortion. To demonstrate this effect, we analyze the phonon properties of quasi-2D Bi2PbTe4 with an ultralow thermal conductivity of 0.74 W/mK at 300 K. By comparing the interatomic force constants of quasi-2D Bi2PbTe4 and its bulk counterpart and the properties of resonant bonds near the surface and in the bulk, we conclude that resonant bonds are significantly enhanced in reduced dimensions and are more effective in reducing the lattice thermal conductivity. Our results will provide new clues to search for thermal insulators in low-dimensional materials.

Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy

In this study we use ultrafast infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of two intramolecular hydrogen bonded complexes, 10-hydroxybenzo[h]quinoline (HBQ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) dissolved in carbon tetrachloride. We pump the OH stretching (νOH) mode and probe across 1800 cm−1–3300 cm−1 using a broadband IR probe pulse. Both systems exhibit large anharmonicities (>250 cm−1), which are characteristic of medium strong hydrogen bonded systems. Additionally, we observe the coherent beating of low-frequency modes of 245 cm−1 and 118 cm−1 across the νOH stretch of HBQ and HBT, respectively. Anharmonic frequency calculations at the DFT level identify these low-frequency structural modes as in-plane bending modes, which modulate the intramolecular hydrogen bonding distance. These findings provide evidence that the anharmonic nature of the fundamental νOH stretch is primarily dictated by the coupling to low-frequency structural modes and these motions play an important role in the hydrogen bonding dynamics.