Low Thermal Conductivity and High Thermoelectric Performance in Toxic-Element-Free Inverse-Perovskite Oxide

Given the recently increasing energy crisis, there has been a growing focus on thermoelectric technology for converting waste heat energy into electrical power. The energy conversion efficiency (<i>ZT</i>) of thermoelectric materials, defined as <i>ZT</i> = <i>S</i>²<i>σTκ</i>^–1, is determined by the key factors: the Seebeck coefficient (<i>S</i>), the electronic conductivity (<i>σ</i>), and thermal conductivity (<i>κ</i>), which includes both electronic (<i>κ</i>_ele) and lattice (<i>κ</i>_Lat) contributions. Therefore, high <i>ZT</i> thermoelectric materials require large <i>S</i> and high <i>σ</i> to achieve a high power factor (PF = <i>S</i>²<i>σ</i>), along with low <i>κ</i> to create a sizable temperature gradient. To date, high thermoelectric <i>ZT</i> values has primarily been demonstrated in heavy metal chalcogenides, but the use of toxic elements, such as Pb and Te, is not preferred for widespread practical applications of thermoelectricity. In this study, we propose inverse perovskite-type oxide Ba₃SiO as a novel class of toxic-element-free thermoelectric materials with high <i>ZT</i> reported so far. First, we experimentally demonstrate that undoped Ba₃SiO bulks exhibit rather high <i>ZT</i> = 0.16–0.84 at <i>T</i> = 300–623 K. In addition to these promising experimental results, we demonstrate that the maximum <i>ZT</i> is estimated to be 2.14 for Ba₃SiO at <i>T</i> = 600 K when optimally doped, based on the first-principles carrier and phonon transport calculations. The maximum <i>ZT</i> value of Ba₃SiO is notably higher than those of environmentally benign thermoelectric materials, and it is comparable to those of the high <i>ZT</i> thermoelectric materials composed of heavy and toxic elements in the same temperature range.<br/>We elucidate the origin of high <i>ZT</i> for Ba₃SiO and the underlying physical mechanisms through a combination of the experimental analysis and the first-principles calculations. We observed that Ba₃SiO bulks exhibit remarkably low lattice thermal conductivity (<i>κ</i>_Lat) of 1.00 W/(mK) at <i>T</i> = 300 K, which is significantly lower than 8.2 W/(mK) of the normal perovskite SrTiO₃ bulk, and even lower than 1.7–2.0 W/(mK) of Bi₂Te₃ and PbTe bulks. The crystal structure of Ba₃SiO is constructed from the highly distorted O–Ba₆ octahedra framework with weak O–Ba ionic bonds, which provides low phonon group velocity and strong phonon scattering. Additionally, Ba₃SiO possesses favorable band structures for achieving a high PF, where the valence band around the Fermi level arises from the <i>p</i> state of the negatively charged Si anion with large ion size, and their highly dispersive bands with multiple valley degeneracy enable both high <i>σ</i> and high <i>S</i>, simultaneously, leading to the high <i>ZT</i> for Ba₃SiO originating from the low <i>κ</i>_Lat and the high PF. These results suggest that inverse-perovskite oxides are promising and environmentally benign thermoelectric materials, offering a compelling alternative to current practical materials composed of heavy and toxic elements.