Manipulating lattice distortion to promote average thermoelectric power factor in metavalently bonded AgBiSe2

Local off-centering of atoms generally leads to high Grüneisen parameters and thus low lattice thermal conductivity, which is considered to be favorable for thermoelectric materials. Nevertheless, such severely distorted local structures may cause strong scattering for electrons and poor electrical performance. Here we demonstrate the relationship between thermoelectric properties and local structure in AgBiSe2 dual alloying with PbTe. It is disclosed that the introduction of Te anions in cubic (AgBiSe2)1−x(PbTe)x can release the distortion index of Bi/Ag/Pb cations in a octahedral shell, which substantially improves the local symmetry and meanwhile the weighted mobility in relevant alloys. More interestingly, it is elucidated that dually-alloyed AgBiSe2 displays a metavalent bonding character, first time reported in n-type cubic TE materials, as collectively verified by atomic coordination number, anomalously increased optical dielectric constant, moderate electrical conductivity, and large Grüneisen parameter. Consequently, a record-high average power factor of 0.52 mW m−1 K−2 between the measurement temperature interval, and a peak zT value of 0.92 at 800 K is obtained for metavalently bonded (AgBiSe2)0.75(PbTe)0.25.