Impedance spectroscopy measurements evidence superionic Li ⁺ mobility (>10 ⁻³ S cm ⁻¹ ) at room temperature and fast ionic mobility for Na ⁺ (5 × 10 ⁻⁶ S cm ⁻¹ ) in high entropy oxides, a new family of oxide-based materials with the general formula (MgCoNiCuZn) _1−x−y Ga _y A _x O (with A = Li, Na, K).

BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu₂Se₂)²⁻ and insulating (Bi₂O₂)²⁺ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi³⁺ with Ca²⁺. The resulting materials exhibit a large positive Seebeck coefficient of ~+330 μV K⁻¹ at 300 K, which may be due to the ‘natural superlattice’ layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of ~4.74 μW cm⁻¹ K⁻² at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (~0.9 W m⁻¹ K⁻¹) to 923 K (~0.45 W m⁻¹ K⁻¹). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young’s modulus, E~76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, γ~1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of ~0.9 at 923 K for Bi_0.925Ca_0.075CuSeO.

$p$ -type BiCuSeO, a layered oxyselenide composed of conductive ${\left({\text{Cu}}_2{\text{Se}}_2\right)}^{2-}$ layers alternately stacked with insulating ${\left({\text{Bi}}_2{\text{O}}_2\right)}^{2+}$ layers, shows an enhancement of the electrical conductivity after substituting ${\text{Bi}}^{3+}$ by ${\text{Sr}}^{2+}$ , from $470\text{S}{\text{m}}^{-1}$ (BiCuSeO) to $4.8\times {10}^4\text{S}{\text{m}}^{-1}$ $\left({\text{Bi}}_{0.85}{\text{Sr}}_{0.15}\text{CuSeO}\right)$ at 293 K. Coupled to high Seebeck coefficients, this leads to promising values of the thermoelectric power factor that exceeds $500\mu \text{W}{\text{m}}^{-1}{\text{K}}^{-2}$ at 873 K. Moreover, the thermal conductivity of these layered compounds is lower than $1\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$ at 873 K. Maximum $ZT$ values reach 0.76 at 873 K, making this family promising for thermoelectric applications in the medium temperature range.

We present a high ZT ∼ 1.4 in textured Bi_0.875Ba_0.125CuSeO obtained by a hot-forging process. The carrier mobility along the direction perpendicular to the pressing direction was significantly increased, resulting in increase in the electrical conductivity and maximization of the power factor at 923 K from 6.3 μW cm⁻¹ K⁻² for the sample before hot-forging to 8.1 μW cm⁻¹ K⁻² after the hot-forging process. Therefore, the maximum ZT was significantly increased from ∼1.1 to 1.4 through texturing for Bi_0.875Ba_0.125CuSeO, which is the highest ZT ever reported among oxygen containing materials.