Phonon Dispersion Relations in NaCl

Raunio, G.; Almqvist, L.; Stedman, R.

doi:10.1103/physrev.178.1496

Cited by 120 publications

(48 citation statements)

References 25 publications

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“…In addition, the highly delocalized p‐electrons in a metavalent configuration produce a large electron‐TO phonon coupling, causing optical phonon softening. In contrast, the octahedral coordinated NaCl shows very flat TO dispersions due to ionic bonding and localized electrons . Even though the optical phonon modes are inefficient for heat transport due to their low group velocity, they can couple with the heat‐carrying acoustic phonons by strong anharmonic scattering and, hence, lower the group velocity of acoustic phonons as well as an increase of the phase space for Umklapp phonon scattering processes.…”

Section: Intrinsically Low Lattice Thermal Conductivitymentioning

confidence: 99%

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Cagnoni

Cojocaru‐Mirédin

et al. 2019

Adv Funct Materials

179

214

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Thermoelectric materials have attracted significant research interest in recent decades due to their promising application potential in interconverting heat and electricity. Unfortunately, the strong coupling between the material parameters that determine thermoelectric efficiency, i.e., the Seebeck coefficient, electrical conductivity, and thermal conductivity, complicates the optimization of thermoelectric energy converters. Main‐group chalcogenides provide a rich playground to alleviate the interdependence of these parameters. Interestingly, only a subgroup of octahedrally coordinated chalcogenides possesses good thermoelectric properties. This subgroup is also characterized by other outstanding characteristics suggestive of an exceptional bonding mechanism, which has been coined metavalent bonding. This conclusion is further supported by a map that separates different bonding mechanisms. In this map, all octahedrally coordinated chalcogenides with good performance as thermoelectrics are located in a well‐defined region, implying that the map can be utilized to identify novel thermoelectrics. To unravel the correlation between chemical bonding mechanism and good thermoelectric properties, the consequences of this unusual bonding mechanism on the band structure are analyzed. It is shown that features such as band degeneracy and band anisotropy are typical for this bonding mechanism, as is the low lattice thermal conductivity. This fundamental understanding, in turn, guides the rational materials design for improved thermoelectric performance by tailoring the chemical bonding mechanism.

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Section: Intrinsically Low Lattice Thermal Conductivitymentioning

confidence: 99%

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Cagnoni

Cojocaru‐Mirédin

et al. 2019

Adv Funct Materials

179

214

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“…In NaCl at 10.6 /im the lowest-order process involves four phonons. 19 The experimental data of HH can be explained in terms of Eq. (3) if an additional extrinsic absorption a ex is assumed.…”

mentioning

confidence: 99%

Conduction-Electron Spin Density around Fe Impurities in Cu above and belowTK

1974

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ing small effect, moreover, might be masked by the finite lifetime of the phonons involved. The one-phonon density of states therefore is smeared out, and will not show any critical points, but rather broad maxima at the frequencies corresponding to symmetry points.The temperature dependence of the absorption in KBr, measured by Barker 21 and analyzed in Ref. 6 for 418 cm" 1 , is also in quantitative agreement with Eq. (3) for N = 3. The phonon curves of KBr, 18 in fact, indicate that the lowest-order processes at that wave number involve three phonons. In NaCl at 10.6 /im the lowest-order process involves four phonons. 19 The experimental data of HH can be explained in terms of Eq. (3) if an additional extrinsic absorption a ex is assumed. A good fit is obtained with a 0 = 0.0002 cm" 1 . Such a small extrinsic absorption, caused by a few parts per million of impurities, might well exist even in a good quality crystal. However, the experimental deviation from the simple four-phonon curve might equally well relate to the above-mentioned extension of the basic theory. It would be interesting to repeat the measurements of HH with even purer crystals and at lower temperatures in order to decide this intriguing alternative.To conclude, the complete temperature dependence of the absorption, in the particular case of NaF at 10.6 /xm, can be described as a pure three-phonon process. The different results from the two samples emphasize the importance of impurities for the small-absorption multiphonon region.The authors are indebted to V. Irniger and K. Gisler for technical assistance.Considerable effort, both theoretical and experimental, has gone into determining the nature of the ground state of a Kondo system, 1 " 3 but to

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“…21.2). In so doing, the dispersion curves are indeed slightly pushed towards the actual dispersion curves which were measured by RAUNIO et al (1969) . .…”

Section: Information Contained In Defect-induced Spectramentioning

confidence: 86%

Vibrational Infrared and Raman Spectra of Non-Metals

Bilz¹,

Strauch²,

Wehner³

1984

Light and Matter Id / Licht Und Materie Id

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The vibrational properties of crystals determine the photon infrared absorption, inelastic neutron scattering and, to a large extent, inelastic photon scattering by phonons, i.e., Raman scattering. The interpretation of infrared and Raman spectra requires, therefore, an understanding of the basic features of lattice dynamics. The quantum theory of solids describes the crystal properties in terms of elementary excitations and their mutual interactions. Dynamic properties are represented by phonons (lattice vibrations) and their interactions mainly with other phonons (anharmonicity), electrons (electron-phonon coupling), and photons (interaction with radiation). This characterizes the scope of the article. Its emphasis is on the interrelation between theory and experiment, i.e., on the microscopic or model interpretation of experimental spectra.Work on infrared absorption and Raman scattering of non-metallic solids was summarized twenty years ago in two review articles in this Encyclopedia by LECOMTE (1958) and by MIZUSHIMA (1958). Since then, many important new experimental data have become available, thanks to very refined techniques for measuring spectra of pure and imperfect single crystals. Furthermore, the theory of the interactions of phonons and photons has been much improved, stimulated by the remarkable development of the theory of lattice vibrations and the mathematical techniques of many-body physics.In this article we describe the dispersion and absorption of infrared radiation and Raman scattering in non-metallic crystals, both perfect and containing point defects. At present, quantitative calculations are usually restricted to diatomic crystals, especially those with simple structures such as the alkali halides or germanium and its homologues, since our knowledge of lattice vibrations is still rather poor for polyatomic and low-symmetry crystals. The theory of lattice vibrations, as reviewed by COCHRAN and COWLEY (1967) in Vol. XXV /2a of this Encyclopedia, constitutes the background and the natural starting point for our investigations. There are other recent reviews by LUDWIG (1967), COCHRAN (1971), MARADUDIN et al. (1971), SINHA (1973), and several articles in: HORTON and MARADUDIN (eds., 1974). A summary of the developments during the last few years is given in Chap. B.An important part of the theory of lattice vibrations is the construction of models. A good example is the so-called shell model for phonons (see Sect. 4) which describes the adiabatic linear electron-ion interaction in terms of localized charges and coupling constants. This provides a natural explanation of some long-range ion-ion forces in insulators in terms of induced dipole forces. Anharmonic extensions of this shell model and its modifications seem very desirable, and first attempts in this direction are discussed.Models can often give a qualitatively and sometimes quantitatively correct description of certain processes in terms of a few parameters. They provide, therefore, an orientation for microscopic approaches. Furthermo...

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Phonon Dispersion Relations in NaCl

Cited by 120 publications

References 25 publications

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Conduction-Electron Spin Density around Fe Impurities in Cu above and belowTK

Vibrational Infrared and Raman Spectra of Non-Metals

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