Three-phonon scattering processes and thermal conductivity in IV-chalocogenides

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Physics

Background:

• IV-chalcogenide thermoelectric materials like PbTe, PbSe, PbS, and SnTe exhibit low thermal conductivity.
• Understanding phonon scattering is crucial for optimizing thermoelectric performance.

Purpose of the Study:

• To systematically investigate three-phonon scattering processes in IV-chalcogenides.
• To determine the relative contributions of acoustic and optical phonons to thermal conductivity.
• To compare different theoretical models for predicting thermal conductivity.

Main Methods:

• Numerical computation of thermal conductivity (κ) using the isotropic continuum scheme.
• Application and comparison of Callaway, Allen, and single-mode relaxation time theories.
• Analysis of phonon scattering contributions from acoustic (TA, LA) and optical (TO, LO) branches.

Main Results:

• Acoustic and transverse optical phonons contribute 10-25% to κ above room temperature, with κ(TO) > κ(TA) > κ(LA).
• Longitudinal optical phonons contribute negligibly in lead chalcogenides but significantly (22%) in SnTe.
• Thermal conductivity at high temperatures varies less than T(-1) due to high defect concentrations.
• SnTe resistivity below the Debye temperature shows a square-root dependence on point defect concentration, matching experimental data.

Conclusions:

• Three-phonon scattering processes, particularly involving acoustic and transverse optical phonons, are key to the low thermal conductivity of these materials.
• The Callaway and Allen theories provide a more comprehensive explanation of thermal conductivity than single-mode theories.
• Defect concentration plays a critical role in the temperature dependence of thermal conductivity and resistivity.