Giant Room-Temperature Power Factor in p-Type Thermoelectric SnSe under High Pressure

Materials that can efficiently convert heat into electricity are widely utilized in energy conversion technologies. The existing thermoelectrics demonstrate rather limited performance characteristics at room temperature, and hence, alternative materials and approaches are very much in demand. Here, it is experimentally shown that manipulating an applied stress can greatly improve a thermoelectric power factor of layered p‐type SnSe single crystals up to ≈180 µW K(−2) cm(−1) at room temperature. This giant enhancement is explained by a synergetic effect of three factors, such as: band‐gap narrowing, Lifshitz transition, and strong sample deformation. Under applied pressure above 1 GPa, the SnSe crystals become more ductile, which can be related to changes in the prevailing chemical bonding type inside the layers, from covalent toward metavalent. Thus, the SnSe single crystals transform into a highly unconventional crystalline state in which their layered crystal stacking is largely preserved, while the layers themselves are strongly deformed. This results in a dramatic narrowing in a band gap, from E (g) = 0.83 to 0.50 eV (at ambient conditions). Thus, the work demonstrates a novel strategy of improving the performance parameters of chalcogenide thermoelectrics via tuning their chemical bonding type, stimulating a sample deformation and a band‐structure reconstruction

Synthesis of Ilmenite-type εε-Mn2_2O3_3 and Its Properties

In contrast to the corundum-type A$_2$X$_3$ structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX$_3$ ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe$^{2+}$Ti$^{4+}$O$_3$ ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn2O3) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn$^{2+}$Mn$^{2´2+}$O$_3$ ($ε$-Mn$_2$O$_3$) is an n-type semiconductor with an indirect narrow band gap of E$_g$ = 0.55 eV. Comparative investigations of the electronic properties of $ε$-Mn$_2$O$_3$ and previously discovered quadruple perovskite $ζ$-Mn$_2$O$_3$ phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in $ε$-Mn$_2$O$_3$ below 210 K. The synthesis of $ε$-Mn$_2$O$_3$ indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A$_2$O$_3$, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO$_3$-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions