Influence of nonstoichiometry point defects on electronic thermal conductivity

Electronic contribution to thermal conductivity ( κ e) is proportional to electrical conductivity ( σ) as given by the Wiedemann–Franz law ( κ e = L σ T). The Lorenz number ( L) scales the thermal current associated with the electrical current and implies the electrons' capability of carrying heat. By experimental transport measurements and first-principles calculations, we show that electron transport overwhelmingly dominates thermal conductivity in β-Ag2Se, which has intrinsically low lattice thermal conductivity. The Lorenz number linearly decreases from Ag1.95Se to Ag2.03Se, as the point defect changes from a cation vacancy to a self-interstitial. This striking behavior reveals the inelastic electron scattering process due to nonstoichiometry point defects and suggests that the cation vacancies increase while self-interstitials reduce the amount of heat carried by electrons. Remarkably, the Lorenz number varies by 40% for such a narrow nonstoichiometry window, with the deviation as large as 36% from the Sommerfeld value. Finally, we predict the maximum Lorenz number that can be achieved in β-Ag2Se for various electron scattering mechanisms. This work provides insights into the physics of electronic heat conduction in solids containing point defects.