Reduced lattice thermal conductivity due to strong acoustic-optical scattering. (a) Temperature dependence of lattice thermal conductivity for Cu₂SnSe₃, Cu₂Sn_0.9Cd_0.1Se₃ and Cu_1.85Ag_0.15Sn_0.9Cd_0.1Se₃. (b) Room temperature sound velocity and (c) Debye temperature as a function of Ag content (y). (d) Low-temperature heat capacity for Cu₂SnSe₃, Cu₂Sn_0.9Cd_0.1Se₃, and Cu_1.85Ag_0.15Sn_0.9Cd_0.1Se₃. (e) Relationship of C_p/T³ versus T for Cu_1.85Ag_0.15Sn_0.9Cd_0.1Se₃ fitted with the Debye and Debye-Einstein models, respectively. (f) Relationship of C_p/T versus T². The individual contributions from electronic (γ), Debye (β) and two Einsteins terms (E₁, E₂) are shown in the figure. (g) Spectral lattice thermal conductivity (κ_s), (h) phonon relaxation time (τ_c), and (i) lattice thermal conductivity for the Cu_1.85Ag_0.15Sn_0.9Cd_0.1Se₃ fitted using the Debye-Callaway model. The contributions of Umklapp scattering (U), grain boundary scattering (B), point defect scattering (P), and acoustic-optical phonon scattering (AO) are calculated by the Debye-Callaway model.