Correlation of Local Crystal Structural and Physical Properties of the Delafossite CuCr1-xFexO2 (0 ≤ x ≤ 1) Series.

Abstract

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-xFexO2 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.