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Related Concept Videos

π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
Hybridization of Atomic Orbitals I03:24

Hybridization of Atomic Orbitals I

The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...

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Giant anharmonic phonon scattering in PbTe.

O Delaire1, J Ma, K Marty

  • 1Neutron Scattering Science Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, USA. delaireoa@ornl.gov

Nature Materials
|June 7, 2011
PubMed
Summary

Researchers discovered a key microscopic interaction in lead telluride (PbTe) that explains its low thermal conductivity. This finding is crucial for developing advanced thermoelectric materials for energy applications.

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Solid-State Chemistry

Background:

  • Efficient thermoelectric materials are vital for waste heat recovery and solid-state cooling.
  • Lead telluride (PbTe) exhibits excellent thermoelectric properties due to its inherently low thermal conductivity.
  • The microscopic origins of PbTe's low thermal conductivity remain unclear.

Purpose of the Study:

  • To elucidate the fundamental mechanisms responsible for the low thermal conductivity in lead telluride (PbTe).
  • To establish a microscopic understanding of heat transport in PbTe for guiding the design of new thermoelectric materials.

Main Methods:

  • Utilized inelastic neutron scattering (INS) measurements to probe lattice dynamics.
  • Performed first-principles calculations to compute phonon properties and interactions.
  • Analyzed the coupling between different phonon modes within the crystal structure.

Main Results:

  • Identified a significant anharmonic coupling between ferroelectric transverse optic (TO) modes and longitudinal acoustic (LA) modes in PbTe.
  • This strong coupling was observed to extend across a substantial region of reciprocal space.
  • The anharmonic coupling directly influences the behavior of heat-carrying LA phonons.

Conclusions:

  • The longitudinal acoustic-transverse optic anharmonic coupling is a primary factor contributing to the low thermal conductivity of PbTe.
  • This microscopic insight explains why many high-performance thermoelectric materials are found near ferroelectric lattice instabilities.
  • The findings provide a pathway for designing novel thermoelectric materials by tuning phonon interactions.