Inverse-Perovskite Ba₃BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity

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 Ba3BO (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 (kappa(lat)) 1.0-0.4 W m(-1) K-1 at T = 300-600 K are observed in Ba3BO. Highly distorted O-Ba-6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3BO. As a consequence of high PF and low kappa(lat), Ba3SiO (Ba3GeO) 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 Ba3SiO and 1.21 for Ba3GeO 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.