Phonon transport anomaly in metavalent bonded materials: contradictory to the conventional theory

We report a phenomenon occurring in some chalcopyrite structures with space group $$R\overline{3 }m$$ (space group number: 166) where heavier mean atomic mass does not necessarily lead to lower lattice thermal conductivity, in contradiction to the conventional Keyes’ theory of an inverse relation between lattice thermal conductivity and heavier elements. The lattice thermal conductivity is calculated for three chalcopyrite structures, namely TlBiS2, TlBiSe2, and TlBiTe2, at room temperature using phonon Boltzmann transport equation with interatomic force constants evaluated by density functional theory (DFT). According to Keyes’ theory, the decedent order of lattice thermal conductivity should follow: TlBiS2 > TlBiSe2 > TlBiTe2. However, our first-principles calculations demonstrate that TlBiSe2 has higher lattice thermal conductivity than TlBiS2, although the Se atoms are heavier than S atoms. Resorting to full first-principles calculations, we reveal the relatively lower lattice thermal conductivity of TlBiS2 compared to TlBiSe2 originates from the largely suppressed phonon relaxation time, in particular for optical phonons, which negatively affects thermal transport, although both heat capacity and phonon group velocity of TlBiS2 are higher than that of TlBiSe2, which should have led to higher lattice thermal conductivity in TlBiS2. Detailed analysis of phonon partial density of states (PDOS) demonstrates that the short phonon relaxation time in TlBiS2 is due to the extensive strongly anharmonic phonon interactions solely associated with the vibrations of S atoms as shown in PDOS, which reduces the life of the phonon transport in TlBiS2, while in TlBiSe2 and TlBiTe2 the thermally conductive collective modes are present. The higher anharmonic phonon–phonon interactions are also attributed to the higher transferred charges present in TlBiS2. We also provide a microscopic picture connecting the strong phonon anharmonicity to the stereo-chemically lone-pair electrons present in the chalcopyrite structures. Moreover by analyzing the atomic charges, our calculations show that these materials possess a recently discovered new type of bonding named metavalent bonding, which is featured by giant phonon anharmonicity, high transverse optical Gruneisen parameters, high Born effective charges, and high dielectric constants. These results provide deep understanding of metavalent bonding induced unusual heat transfer mechanism and offer a new route to designing novel materials with ultralow lattice thermal conductivity, which is extremely attractive for waste-heat recovery (thermoelectrics) and thermal insulation applications.