Origin of the variation in lattice thermal conductivities in pyrite-type dichalcogenides

Abstract

Lattice thermal conductivity is a crucial parameter for thermoelectric (TE) applications. In pyrite-type metal dichalcogenides, it is observed that the type of metal atom has a significant influence in the phonon properties and lattice thermal conductivities, but the physical origin is not clear. In this paper, we show that, for the pyrite-type posttransition metal (such as Zn or Cd) dichalcogenides, because the fully occupied ${d}^{10}$ orbitals inside the valence bands are relatively localized, the enhanced symmetry-controlled $s\text{\ensuremath{-}}d$ coupling effects in the posttransition metal atoms when they vibrate away from the high-symmetry equilibrium positions can lead to soft phonon properties and strong anharmonicity. Thus, they behave as rattlinglike atoms, and these systems have extremely low lattice thermal conductivities. However, for some pyrite-type transition metal dichalcogenides, when the outermost $d$ electrons in transition metal atoms are only occupied by the ${T}_{2g}$-derived states, such as Fe with ${d}^{6}$ configuration, the symmetry-controlled $s\text{\ensuremath{-}}d$ coupling effects in the transition metal atoms are strongly cancelled by the varying crystal field splitting when symmetry is reduced during vibration. Therefore, these compounds exhibit hard phonon properties, weak anharmonicities, and high lattice thermal conductivities. In this paper, we reveal the electronic origin of the puzzling behaviors of pyrite-type dichalcogenides which show a wide range of thermal conductivity properties. The discussed mechanism can also be used to guide researchers in seeking promising TE materials with low lattice thermal conductivity.