Thermal Properties of the Metallic Delafossite PdCoO₂: A Combined Experimental and First-Principles Study

Metallic delafossites have attracted much interest due to record-high oxide conductivities, but relatively little attention has been paid to thermal properties. Here, we address this via wide-temperature-range experimental studies of the crystal structure, thermal expansion, and specific heat of single-crystal PdCoO₂, combined with density functional theory (DFT) calculations of the electronic and phononic densities-of-states, and thus thermal properties [1]. PdCoO₂ retains the R-3m space group from 12-1000 K, with thermal expansion in quantitative agreement with DFT-based calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the Co³⁺ ions, which contrasts with Co-based perovskites. 1.9-400 K measurements of specific heat provide accurate values for the Debye temperature and Sommerfeld coefficient and can be modeled through a Debye-Einstein approach, quantitatively understood based on unusual features of the phonon density-of-states. These behaviors are remarkably closely reproduced by DFT, establishing quantitative understanding of key thermal properties of the metallic delafossite PdCoO₂.

* This work was primarily supported by the US Department of Energy through the University of Minnesota Center for Quantum Materials, under DE-SC0016371.

[1] Y. Zhang, A. Saha, F. Tutt, V. Chaturvedi, B. Voigt, W. Moore, J. Garcia-Barriocanal, T. Birol, and C. Leighton, Phys. Rev. Mater. (submitted).