Cu-based delafossites offer multifunctional properties that include electrical, magnetic, optical, and thermal transport, and they also act as a photocathode for energy-harvesting applications. Such properties can be modified by bringing about subtle changes in the chemical environment, like introducing holes or excess electrons in the system. With the aim to understand the evolution of physical properties that take place upon systematically replacing 3d3 (Cr3+) with 3d5 (Fe3+), we performed a comprehensive study of structural, electrical and thermal transport, and magnetic properties of the CuCr1–x Fe x O2 series. In agreement with the different ionic radii of trivalent cations, high-intensity X-ray diffraction confirms a systematic increase of the unit cell parameters. Extended X-ray absorption fine structure spectroscopy confirms the uniform solution of mixed trivalent Cr3+/Fe3+ cations and demonstrates the changes in the hybridization between Cu 3d and O 2p orbitals. Cu K-edge near-edge spectra reflect a sharp 1s → 4p transition associated with the ligand–metal charge transfer “shakedown” process in the collinear O–Cu–O bond along c-axis, whereas the Cr and Fe K-edge absorption spectra show small reciprocal shifts in the edge energies. The charge carrier concentration increases with Fe substitution as confirmed from Hall effect measurement; both p-type conduction and a decrease in the electrical resistivity are observed. The electrical conduction follows a 3D variable range hopping at low-temperature and thermally activated transport at high temperature. The two end members of the series are well-studied, geometrically frustrated antiferromagnetic systems; the present study of magnetic properties of intermediate compositions show a systematic increase of FM interaction with rising Fe concentration and hence a competing magnetic state. A sharp transition in high-temperature (600 K) region identifies a truly paramagnetic state for Fe rich compounds. Thermal conductivity values are drastically affected by the spin fluctuation and spin-phonon scattering taking place in these compositions.

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We present the results of a comprehensive investigation of electric and thermal transport properties of polycrystalline Bi-substituted La0.95− xBi xSr0.05CoO3 for LBSCO-0, 1, and 2. The electrical resistivity reflects the semiconducting nature with n-type to p-type transition ∼52 K for LBSCO-1 and LBSCO-2 samples. In the low temperature region, the dominant transport mechanism is found to be variable range hopping with the hopping range decreasing with increasing temperature from 95 to 20 Å. The substitution of higher atomic weight element Bi at the La site drastically affects the overall thermal conductivity by reducing the lattice contribution (∼0.12 W/m-K at 50 K) and also enhancing the Seebeck coefficient ( S ∼ 354  μV/K). The increase in the resistivity and Seebeck coefficient for the Bi-substituted system is related to the decrease in the available charge carrier concentration (∼5.12 × 1020 cm−3). The overall variation in the Seebeck coefficient depicts a complex nature with a large decreasing trend below 50 K followed by an in-depth analysis of the Debye temperature (∼470 K) and e– ph coupling. These findings suggest that the Bi-substituted LBSCO system has phonon-mediated charge transport via the phonon drag effect below 50 K. Notably, we found a large increment in ZT ∼ 0.17 at room temperature for LBSCO-2 compositions that is one-order larger than pristine undoped LBSCO-0 and even higher than the other existing cobaltite-based thermoelectric choice.

A combination of high optical transparency and good electrical conductivity is realized in CuCrO 2 , a p-type transparent conducting oxide. With an aim to improve its physical properties, a CuCr 1-x Ti x O 2 (x ¼ 0, 0.05, 0.1) series was prepared and investigated for its structural, electrical transport, and optical properties. Along with the lattice parameter values, refinement of X-ray diffraction profiles confirm the solubility limit of Ti within the delafossite unit cell. Electrical resistivity and heat capacity measured by varying the temperature further characterize the Ti-substituted compositions. As determined from ultraviolet-visible spectroscopy, no significant changes take place in the optical gap of CuCrO 2 with Ti addition. Apart from confirming the 4þ valence state of Ti, the X-ray absorption near edge structure highlights the subtle changes taking place in the Cu-O hybridization upon Ti-substitution. The analysis of temperature dependent extended X-ray absorption fine structure spectroscopy, recorded at the Cr and Cu K-edge, emphasizes the impact of redistribution of charges on the local crystal structure. Cu-Cu hybridization along the a-axis appears to be influenced mainly by the temperature and only slightly by Ti substitution.

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