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Misfit-layered cobaltate [Ca 2 CoO 3 ] p CoO 2 is a promising thermoelectric (TE) material that can operate at high temperature in air. Partial substitution of Ca with Sr has been suggested to enhance TE properties. To comprehend the unresolved effects of Sr-substitution on the TE properties, we have prepared single-phase polycrystalline samples of Sr-substituted [Ca 2 CoO 3 ] p CoO 2 , i.e., [(Ca 1- x Sr x) 2 CoO 3 ] p CoO 2 , using a spark plasma sintering (SPS) method followed by O 2 annealing. We analyzed the crystal structure by powder X-ray diffraction, evaluated the average valence state of Co ions by X-ray photoelectron spectroscopy, and measured temperature-dependent TE properties. Upon Sr-substitution, the average valence state of Co ions slightly increased. The electrical resistivity perpendicular to the SPS pressing direction increased, while that along the SPS pressing direction was almost unchanged. The Seebeck coefficient and the thermal conductivity were less affected by the Sr-substitution. The figure-of-merit zT perpendicular to the SPS pressing direction tended to decrease with Sr-substitution, reflecting the increase in electrical resistivity. [Display omitted] • Successfully prepared the single-phase of Sr-substitution for Ca in the Ca 2 CoO 3 BL of Co-121. • [(Ca 1- x Sr x) 2 CoO 3 ] p CoO 2 (0 ≦ x ≦ 0.20) samples. • The valence state of Co ions decreased upon Sr-substitution, but carrier concentration was unchanged. • The resistivity was changed due to the enhanced carrier scattering resulting from the structural modulation and/or substituted Sr ions. [ABSTRACT FROM AUTHOR]

To explore full-Heusler compounds with positive Seebeck coefficient, we calculated thermoelectric (TE) properties of several Co- and Mn-based full-Heusler compounds. Relatively high Seebeck coefficients were obtained in the calculation, indicating a potential of full-Heusler compounds as TE materials. As a candidate, we prepared a Mn 2 VAl sample and measured its TE properties. As predicted in the calculation, the Mn 2 VAl sample exhibited a positive Seebeck coefficient from 315 K to 1020 K. The maximum power factor was 2.7 × 10 −4 Wm −1 K −2 at 717 K. This study embodies great significance for future study on design and fabrication of full-Heusler compounds with high TE properties. [ABSTRACT FROM AUTHOR]

• An experimental and computational study on half-Heusler V(Fe 1 − x Ru x)Sb compounds are investigated. • An increase in room-temperature PF by ca. 25% was obtained for the x = 0.02 sample, which is consistent with the predicted doping level of Ru substitution in from the electronic structure analysis. • The minimum κ L value of 6.0 W/mK was achieved for the x = 0.08 sample at room temperature. • The room-temperature z T is improved from 0.1 to the value close to 0.2. [Display omitted] The effects of ruthenium (Ru) substitution on the thermoelectric properties of half-Heusler (HH) V(Fe 1 − x Ru x)Sb compounds have been theoretically and experimentally investigated. All V(Fe 1 − x Ru x)Sb samples with deficient HH structures reveal deficiencies in V 4 a and Fe 4 c sites, interstitial Fe occupied 4 d sites, and Ru substituted for Fe 4 c sites, by Rietveld analysis. Based on the refined crystal structures and electronic structure analysis, the Ru content x with a maximum power factor (PF) is predicted to be around x = 0.02. A small amount of Ru-substitution improves the PF by 26% at room temperature for the V(Fe 0.98 Ru 0.02)Sb sample, which is coincident with the predicted range of Ru content for maximum PF. The lattice thermal conductivity κ L of the V(Fe 1 − x Ru x)Sb samples decreases with an increase in x. The atomic mass and radius of Ru are substantially larger than those of Fe; therefore, κ L of the V(Fe 1 − x Ru x)Sb samples decreases mainly due to phonon scattering strengthened by the introduction of Ru at the Fe 4 c sites. A significantly reduced κ L of 5.8 W/Km at room temperature is obtained for the V(Fe 0.92 Ru 0.08)Sb sample, which is a ca. 40% decrease compared with the pristine HH-VFeSb sample. The dimensionless thermoelectric figure-of-merit reaches close to 0.2 at 300 K for the V(Fe 0.98 Ru 0.02)Sb sample. [ABSTRACT FROM AUTHOR]

• A series of the V(Fe 1 − x Co x)Sb compounds were successfully synthesised via arc-melting, spark plasma sintering, and annealing methods. • The V(Fe 1 − x Co x)Sb compounds were confirmed to have deficient half-Heusler structures by theoretical and experimental studies. • A maximum PF of 5.7 × 10 − 3 W/K 2 m with an optimised carrier concentration 2.1 × 10 20 cm − 3 was achieved at 300 K for the V(Fe 0.99 Co 0.01)Sb compound. [Display omitted] The effects of cobalt substitution at the Fe-site on the crystal structure, electronic structure and thermoelectric (TE) properties of half-Heusler (HH) V(Fe 1 − x Co x)Sb compounds are investigated. Crystal structural analysis using powder X-ray diffraction clarified that all compounds have d e f i c i e n t HH structures (Huang et al., C h e m. M a t e r., 32 , 5173-5181, 2020). With Co-substitution at the Fe 4 c sites, defects are observed in the V 4 a and Fe 4 c sites, and a small amount of Fe atoms enter into 4 d sites, which is normally vacant in an i d e a l HH structure. The deficient HH structures of the V(Fe 1 − x Co x)Sb compounds are supported by electronic structural analysis using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). All compounds exhibit n -type behaviour in both calculations and experiments with deficient HH structures. The absolute value of the Seebeck coefficient | S | , decreases with an increase in the cobalt content x. The tendency of the experimental | S | value to decrease corresponds well with the | S | determined by KKR-CPA calculations. Cobalt substitution acts as an electron dopant that effectively tunes the carrier concentration. The optimised carrier concentration of 2.1 × 10 20 cm − 3 leads to the highest power factor of 5.7 × 10 − 3 W/K 2 m at room temperature for the V(Fe 0.99 Co 0.01)Sb compound. The V(Fe 1 − x Co x)Sb compounds are thus potential candidates as n -type TE materials for power generation near room temperature. [ABSTRACT FROM AUTHOR]