Inverse-Perovskite Ba₃ BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with...

High energy-conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic-element free Ba₃ BO (B = Si and Ge) with inverse-perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p-type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κ_lat ) 1.0-0.4 W m^-1  K^-1 at T = 300-600 K are observed in Ba₃ BO. Highly distorted O-Ba₆ octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba₃ BO. As a consequence of high PF and low κ_lat , Ba₃ SiO (Ba₃ GeO) exhibits rather high ZT = 0.16-0.84 (0.35-0.65) at T = 300-623 K (300-523 K). Finally, based on first-principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba₃ SiO and 1.21 for Ba₃ GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse-perovskites would be a new platform of environmentally-benign high-ZT thermoelectric materials.