Cation role in the thermal properties of layered materials M1+M3+P2(S,Se)6(M1+=Cu,Ag;M3+=In,Bi)

Abstract

Thermal properties (thermal diffusivity, heat capacity, and thermal conductivity) have been retrieved for the layered compounds $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{A}{\mathrm{g}}_{x}\mathrm{In}{\mathrm{P}}_{2}{(\mathrm{S},\mathrm{Se})}_{6}, \mathrm{AgBi}{\mathrm{P}}_{2}{(\mathrm{S},\mathrm{Se})}_{6}$, and $\mathrm{CuBi}{\mathrm{P}}_{2}\mathrm{S}{\mathrm{e}}_{6}$ in the temperature range 30--350 K, using single crystals. A complete explanation of the thermal properties as ions are substituted has been developed, showing the role played by disorder, electronic levels hybridization, ion coordination, or size on the second-order Jahn-Teller effect. The evolution of the thermal diffusivity and the thermal conductivity with temperature has shown, on the one hand, strong thermal anisotropies and, on the other hand, that heat is effectively transferred by phonons, with some of the compounds (especially those with Bi) presenting very low values due to the enhancement of phonon scattering events, expressing a strong anharmonic behavior, which is justified in detail, case by case, on the basis of interactions among optical and acoustic phonon branches as well as the presence of electron lone pairs. This broad study opens up the possibility of designing new materials for applications where a low thermal conductivity is essential.