Orbit-lattice coupling leads to the intrinsic low thermal conductivity in MTe(M=Ge,Sn,Pb) thermoelectric materials

The intrinsic low thermal conductivity of ${A}^{\mathrm{IV}}\phantom{\rule{0.16em}{0ex}}{B}^{\mathrm{VI}}$ thermoelectric materials has been widely accepted as being closely related to specific chemical bonding or electronic states, for example, resonant bonding, the lone-pair effect, and metavalent bonding. These concepts have different characteristics of localized or delocalized electronic state mechanisms; i.e., resonant bonding corresponds to localized electronic states, the lone-pair effect is correlated with delocalized $ns$ electronic states, and metavalent bonding is characterized by the competition between localized and delocalized electronic states. It seems that those concepts are contradictory in describing ${A}^{\mathrm{IV}}\phantom{\rule{0.16em}{0ex}}{B}^{\mathrm{VI}}$ materials such as GeTe, SnTe, and PbTe simultaneously. Meanwhile, the direct connection between electrons, lattice vibration, and low thermal conductivity is still unclear. Herein, differently from most of the existing works, we focus on how electronic states couple with lattice vibration in the concept of the pseudo-Jahn-Teller effect. Then we propose a general theoretical interpretation (orbital-lattice coupling) to describe the intimate relationship between electronic states and ultralow lattice thermal conductivity in thermoelectric materials or any other strong anharmonic systems. Taking the classical thermoelectric materials (GeTe, SnTe, and PbTe) and the typical ionic crystal NaCl, all with high-symmetry rocksalt structure, as examples, we reveal that the electronic states of ${A}^{\mathrm{IV}}\phantom{\rule{0.16em}{0ex}}{B}^{\mathrm{VI}}$ materials tend to spontaneously break their lattice symmetry to avoid degeneracy. Afterwards, the dynamic charge transfer and electronic orbital overlapping under atomic distortion lower the total energy, effectively. The coupled electronic orbitals are therefore linked to lattice instability. Our results build a direct bridge between electrons and lattice, thus providing an important insight into the combination of novel electronic properties and inherent low thermal conductivity, which is general in understanding thermoelectric properties.