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We demonstrate the determination of anharmonic acoustic phonon properties via second-order Raman scattering exemplarily on copper iodide single crystals. The origin of multi-phonon features from the second-order Raman spectra was assigned by the support of the calculated 2-phonon density of states. In this way, the temperature dependence of acoustic phonons was determined down to 10 K. To determine independently the harmonic contributions of respective acoustic phonons, density functional theory in quasi-harmonic approximation was used. Finally, the anharmonic contributions were determined. The results are in agreement with earlier publications and extend CuI’s determined acoustic phonon properties to lower temperatures with higher accuracy. This approach demonstrates that it is possible to characterize the acoustic anharmonicities via Raman scattering down to zero-temperature renormalization constants of at least 0.1 cm−1.
Next-generation thermal management requires the development of low lattice thermal conductivity materials, as observed in ionic conductors. For example, thermoelectric efficiency is increased when thermal conductivity is decreased. Detrimentally, high ionic conductivity leads to thermoelectric device degradation. Battery safety and design also require an understanding of thermal transport in ionic conductors. Ion mobility, structural complexity, and anharmonicity have been used to explain the thermal transport properties of ionic conductors. However, thermal and ionic transport are rarely discussed in direct comparison. Herein, the ionic conductivity of Ag+ argyrodites is found to change by orders of magnitude without altering the thermal conductivity. Thermal conductivity measurements and two-channel lattice dynamics modeling reveal that the majority of Ag+ vibrations have a non-propagating diffuson-like character, similar to amorphous materials. It is found that high ionic mobility is not a requirement for diffuson-mediated transport. Instead, the same bonding and structural traits that can lead to fast ionic conduction also lead to diffuson-mediated transport. Bridging the fields of solid-state ionics and thermal transport, it is proposed that a vibrational perspective can lead to new design strategies for functional ionic conducting materials. As a first step, the authors relate the so-called Meyer-Neldel behavior in ionic conductors to phonon occupations.
The thermoelectric properties of Sr1-xBixTiO3-δ (0 ≤ x ≤ 0.07) have been investigated. Dense ceramics of Sr1-xBixTiO3-δ and Sr0.95TiO3-δ have been prepared by solid-state reaction and conventional sintering in air followed by annealing in a reducing atmosphere. XRD and SEM analyses show that the rutile TiO2 in Sr0.95TiO3 formed after sintering becomes Magnéli phase of TinO2n-1 after annealing. Moreover, Bi resolves from Sr1-xBixTiO3 after annealing, resulting in the formation of Sr1-xBixTiO3-δ/Bi/TinO2n-1 composites. With increasing Bi content in Sr1-xBixTiO3-δ, the electrical conductivity increases while the absolute values of the Seebeck coefficient decrease as a result of increasing carrier concentration. The thermal conductivity of SrTiO3-δ is reduced by doping Bi up to x = 0.07. Highest ZT ~ 0.13 is obtained in Sr0.93Bi0.07TiO3-δ at 1000 K.
The stability of the low thermal conductivity in Fe2TiO5 pseudobrookite ceramics has been studied. An increase in thermal diffusivity is observed after only three cycles of measurement. X-ray refraction shows an increase in the mean value of specific surface after the thermal diffusivity measurements. By using scanning electron microscopy and high‐angle annular dark‐field scanning transmission electron microscope equipped with energy dispersive X-ray spectroscopy, we observe a segregation of Ca- and F-rich nanocrystals at grain boundaries after three cycles of thermal diffusivity measurement. Therefore, impurities seem to be more efficient to scatter phonons as point defects in the pseudobrookite lattice rather than as nanocrystals at pseudobrookite grain boundaries. This emphasizes the importance of precursor purity and the influence of redistribution of impurities on thermoelectric properties: stability of micro-/nano-structures is a key point, and repeated thermoelectric measurements may allow detecting such metastable micro- /nano-structures and producing stable and reliable data.
The structural and thermoelectric properties of Na- and Ag-substituted CoO dense ceramics have been investigated. X-ray diffraction shows that pure phase and Ag/CoO composites have been obtained for Na-doped and Ag-doped CoO, respectively. Raman spectroscopy shows an effect of Na dopants on the lattice disorder of CoO. The chemical composition, element distribution, and valence states of the samples have been characterized by Auger electron microscopy and X-ray photoelectron spectroscopy. Substitution of Co by 5 at. % Na enhances the power factor to 250 μWm−1 K-2 at 1000 K, similar to that of Ca3Co4O9. The corresponding thermal conductivity is also reduced to 3.55 W.m−1 K−1 at 1000 K. Consequently, Co0.95Na0.05O exhibits the best thermoelectric figure of merit (ZT), which is 0.07 at 1000 K. On the other hand, the substitution of Ag into CoO leads to the formation of CoO/Ag composites and deteriorates ZT values.
Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.