Andrei Novitskii

The pivotal role of intrinsic defects in nominally stoichiometric X(Fe_0.5Ni_0.5)Sb quaternary half-Heuslers (X = Ti, Zr, or Hf) is revealed. The Sb vacancies naturally occur during synthesis and can alter the conduction behavior from n-to p-type.

BiCuSeO has intrinsically low electrical conductivity, a high Seebeck coefficient, and low thermal conductivity. Therefore, the improvement of the thermoelectric performance of BiCuSeO mainly focuses on enhancing its electrical conductivity. In this study, we report the high thermoelectric performance of BiCuSeO achieved by optimizing the carrier concentration and phonon scattering through the composite effect induced by Cr addition. The enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to an almost one order of magnitude improvement of the power factor from ∼0.07 mW m⁻¹ K⁻² for the pristine BiCuSeO to ∼0.57 mW m⁻¹ K⁻² for the BiCu_0.96Cr_0.04SeO sample at 473 K. The Cr addition reduces the lattice thermal conductivity by ∼23%, as confirmed by both experimental results and Debye–Callaway model calculations. The analysis indicates that the lattice thermal conductivity can be reduced to 0.5 W m⁻¹ K⁻¹ at 773 K due to enhanced point defect scattering. The combination of optimized power factor and intrinsically low thermal conductivity yield a relatively high zT. The maximum zT value of ∼0.7 is obtained at 773 K for the BiCu_0.96Cr_0.04SeO sample. This study demonstrates the potential of utilizing the compositing approach for the development of efficient thermoelectric materials based on BiCuSeO oxyselenides.

The importance of the heat capacity measurement is reported to estimate and analyse accurately the structural and thermal transport behaviour in kesterite-Cu ₂ ZnSnS ₄ .

Bi2SeO2 is a promising n-type semiconductor to pair with p-type BiCuSeO in a thermoelectric (TE) device. The TE figure of merit zT and, therefore, the device efficiency must be optimized by tuning the carrier concentration. However, electron concentrations in self-doped n-type Bi2SeO2 span several orders of magnitude, even in samples with same nominal compositions. Such unsystematic variations in the electron concentration has a thermodynamic origin related to the variations in native defect concentrations. In this study, we use first-principles calculations to show that the selenium vacancy, which is the source of n-type conductivity in Bi2SeO2, varies by 1-2 orders of magnitude depending on the thermodynamic conditions. We predict that the electron concentration can be enhanced by synthesizing under more Se-poor conditions and/or at higher solid-state reaction temperatures (T_SSR), which promote the formation of selenium vacancies without introducing extrinsic dopants. We validate our computational predictions through solid-state synthesis of Bi2SeO2. We observe more than two orders of magnitude increase in the electron concentration simply by adjusting the synthesis conditions. Additionally, we reveal the significant effect of grain boundary scattering on electron mobility in Bi2SeO2, which can also be controlled by adjusting T_SSR. By simultaneously optimizing the electron concentration and mobility, we achieve a zT of ~0.2 at 773 K for self-doped n-type Bi2SeO2. Our study highlights the need for careful control of thermodynamic growth conditions and demonstrates TE performance improvement by varying synthesis parameters according to thermodynamic guidelines.

The demonstration of strong coupling between the processing conditions, microstructure, distribution, shape, and size of InSb precipitates in In-filled Co₄Sb₁₂.

The thermoelectric (TE) performance of pristine Bi₂Se₃ is inferior to that of Bi₂Te₃. Therefore, the study on Bi₂Se₃ has faced a decline. The lower performance is due to the low power factor and high thermal conductivity. In recent years, single aliovalent doping has been adopted to improve the TE performance of Bi₂Se₃. Here, we adopt an isovalent co-doping approach using indium and antimony to create a manifold enhancement in the TE performance of Bi₂Se₃ via the creation of neutral impurities and deep defect states (DDSs). A high figure of merit (ZT = 0.47) is obtained at 473 K for a doping concentration of 0.1 at. %. The TE performance obtained for Bi_2–xIn_xSb_2x/3Se₃, x = 0.1 at. %, is comparable to that obtained in several reports for pristine Bi₂Te₃. Our density functional theory calculation reveals an underlying DDS located at ∼15 eV below the Fermi level. This leads to enhanced electronic properties via density of states optimization induced by the co-doping. The isovalent doping is expected to create neutral impurities, which causes less scattering to conduction electrons while absorbing phonon vibration, thus improving the TE performance.