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Journal of materials chemistry / A 10(3), 1473 - 1480 (2022). doi:10.1039/D1TA09489K, In recent years hierarchical thermoelectric materials have been engineered to reach record breaking thermoelectric figures-of-merit (zT) making them attractive in green transition energy conversion applications. PbTe constitutes an archetypical example, where PbS nanoinclusions presumably scatter phonons, while leaving the electrical properties intact. For widespread use of such materials, their structural integrity and chemical stability are equally important to the thermoelectric performance. Here, we show by operando X-ray diffraction and physical property measurements that PbS nanoinclusions in PbTe are not stable at operating conditions. The PbS nanoinclusions grow in size, but surprisingly this does not diminish the thermoelectric performance. Comparison with samples without nanostructures reveals that the PbS inclusions primarily affect the electrical properties at elevated temperatures, and to a lesser degree the lattice thermal conductivity. Nanostructured materials have significantly moved thermoelectric performance limits, but if we are to develop materials by design rather than trial-and-error, it is important that the nature, effect and stability of nanoinclusions are characterized under operating conditions., Published by RSC, London [u.a.]

The discovery of novel materials for thermoelectric energy conversion has potential to be accelerated by data-driven screening combined with high-throughput calculations. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. Using a data-driven screening, we selected 12 potential candidates in the trigonal ABX2 family, followed by charge transport property simulations from first principles. The results suggest that carrier scattering processes in these materials are dominated by ionised impurities and polar optical phonons, contrary to the oft-assumed acoustic-phonon-dominated scattering. Using these data, we further derive ground-state transport descriptors for the carrier mobility and the thermoelectric powerfactor. In addition to low carrier mass, high dielectric constant was found to be an important factor towards high carrier mobility. A quadratic correlation between dielectric constant and transport performance was established and further validated with literature. Looking ahead, dielectric constant can potentially be exploited as an independent criterion towards improved thermoelectric performance. Combined with calculations of thermal conductivity including Peierls and inter-branch coherent contributions, we conclude that the trigonal ABX2 family has potential as high performance thermoelectrics in the intermediate temperature range for low grade waste heat harvesting. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) T. D., M. B. S., G. W., and S.-W. Y. acknowledge the support of Agency for Science, Technology and Research (A*STAR) of Singapore (Grant No. 1527200024). J. R.-G., D. V. M. R., P. K., A. A., and K. H. would like to acknowledge the Accelerated Materials Development for Manufacturing Program at A*STAR via the AME Programmatic Fund by the Agency for Science, Technology and Research under Grant No. A1898b0043. K. H. would like to acknowledge funding via the NRF Fellowship NRF-NRFF13-2021. J. R.-G. and I. N. would like to thank A*STAR Graduate Academy's ARAP programme for funding J. R.-G.'s graduate studies in IMRE, A*STAR. M. O. and J. S. would like to acknowledge JSPS KAKENHI (19H00744, 20K14661) and JST CREST (JPMJCR20Q3).

The heat flux across the core-mantle boundary (Q(CMB)) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the Q(CMB) require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (k(rad)) of a realistic lower mantle (pyrolite) in situusing an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to similar to 3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from similar to 0.8 W/m/K at 1000 km to similar to 0.35 W/m/K at the CMB, the latter being similar to 30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of similar to 8.5 TW,on the lower end of previous Q(CMB) estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo. (C) 2020 Elsevier B.V. All rights reserved.

GeTe-Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial GeTe-Sb2Te3 superlattices have been grown with different techniques and were thoroughly investigated to unravel the structure of their crystalline state with particular focus on atomic stacking and van-der-Waals bonding. It is found that, due to the bonding anisotropy of GeTe and Sb2Te3, the materials intermix to form van-der-Waals heterostructures of Sb2Te3 and stable GeSbTe. Moreover, it is found through annealing experiments that intermixing is stronger for higher temperatures. The resulting ground state structure contradicts the dominant ab-initio results in the literature, requiring revisions of the proposed switching mechanisms. Overall, these findings shed light on the bonding nature of GeTe-Sb2Te3 superlattices and open a way to the understanding of their functionality.

The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV–VI, V and related materials, these interactions are strong, and the materials exist near electronic and structural phase transitions. The prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity and thermoelectricity. Here we show that it is primarily electron-phonon coupling involving electron states near the band edges that leads to the ferroelectric instability in PbTe. Using a combination of nonequilibrium lattice dynamics measurements and first principles calculations, we find that photoexcitation reduces the Peierls-like electronic instability and reinforces the paraelectric state. This weakens the long-range forces along the cubic direction tied to resonant bonding and low lattice thermal conductivity. Our results demonstrate how free-electron-laser-based ultrafast X-ray scattering can be utilized to shed light on the microscopic mechanisms that determine materials properties., Group IV–VI materials often exist in a state near an electronic or structural phase transition. Here, the authors use ultrafast X-ray scattering to show that coupling of band-edge electrons and phonons causes the ferroelectric instability observed in lead telluride.

Inspired by recent research results that have demonstrated appealing thermoelectric performance of A-site cation-deficient titanates, this work focuses on detailed analysis of the changes in performance promoted by altering the defect chemistry mechanisms. The series of cation-stoichiometric SrTi1-xTaxO3 +/-delta and A-site deficient Sr1-x/2Ti1-xTaxO3-delta compositions (0.05

The Lorentz oscillator model is well-known for its effectiveness to describe the far-infrared optical properties of polar materials. The oscillator strength and damping factor in this model are usually obtained by fitting to experimental data. In this work, a method based on first-principles simulations is developed to parameterize the Lorentz oscillator model without any fitting parameters. The high frequency dielectric constant is obtained from density functional perturbation theory, while the optical phonon frequencies and damping factors are calculated using an analysis of ab initio molecular dynamics trajectories. This method is then used to predict the far-infrared properties of GaAs, and the results are in good agreement with experimental data. (c) 2012 Elsevier Ltd. All rights reserved.

A reduction in phonon thermal conductivity enhances thermoelectric performance. One method to accomplish this is by applying mechanical stress to a nanostructure. We consider an initially twisted 20 nm polyvinyl acetate nanofiber undergoing torsion. Our analytical method uses the continuum approach of Mooney's model. Torsion modifies the phonon dispersion. Angles of twist between 45 degrees and 70 degrees reduce the averaged phonon group velocity and the phonon thermal conductivity but when these angles are increased further, this conductivity increases. This suggests a phonon engineering approach to tune the thermal conductivity of nanomaterials. (C) 2013 AIP Publishing LLC. National Nanotechnology Center (NANOTEC)

We have performed molecular dynamics simulations with phonon spectral analysis to predict the mode-wise phonon relaxation times (RT) of suspended and supported graphene at room temperature, and the findings are consistent with recent optical measurements. For acoustic phonons, RTs reduce from up to 50 ps to less than 5 ps when graphene is put on silicon dioxide substrate. Similarly, optical phonon RTs reduce by half when supported. Stronger interfacial bonding is found to result in more RT reduction. Our results provide a fundamental understanding at the spectral phonon property level for the observed thermal conductivity reduction in supported graphene. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4712041]

We investigate vibrational behaviors in misch-metal filled antimony skutterudites in the time domain. At higher filling ratios of guest atoms, the vibration frequency approaches that of the cage structure and the amplitude becomes stronger. Furthermore, the reduction of lattice thermal conductivity over a wide temperature range can be explained using the measured resonant vibrational frequency. These findings reveal that the reduction of the lattice thermal conductivity is a result of scattering of acoustic phonons due to the resonant interaction between guest atoms and lattice phonons.