Lone pair electrons minimize lattice thermal conductivity

As over 93% of the world's energy comes from thermal processes, new materials that maximize heat transfer or minimize heat waste are crucial to improving efficiency. Here we focus on fully dense electrical insulators at the low end of the spectrum of lattice thermal conductivity k(L). We present an experimentally validated predictive tool that shows how the high deformability of lone-pair electron charge density can limit k(L) in crystalline materials. Using first-principles density-functional theory (DFT) calculations, we predict that several ABX(2) (groups I-V-VI2) compounds based on the rocksalt structure develop soft phonon modes due to the strong hybridization and repulsion between the lone-pair electrons of the group V cations and the valence p orbitals of group VI anions. In many cases, this creates lattice instabilities and the compounds either do not exist or crystallize in a different structure. Marginally stable ABX(2) compounds have anharmonic bonds that result in strong phonon-phonon interactions. We show experimentally how these can reduce k(L) to the amorphous limit.