Thermal properties of the metallic delafossite PdCoO2: A combined experimental and first-principles study

Metallic delafossite materials (e.g., PdCoO2, PtCoO2), have attracted much recent attention due to record-high oxide conductivities, the origins of which remain unclear. Relatively little attention has been paid to their related thermal properties, however. We address this here 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. PdCoO2 is shown to retain the R (3) over barm space group from 12 to 1000 K, exhibiting (a) under bar- and (c) under bar -axis thermal expansion in good quantitative agreement with DFT-based lattice dynamics calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the nominally Co3+ ions, which is a notable difference between the edge-shared Co-O octahedra in PdCoO2 and the corner-shared octahedra in Co-based perovskites. Measurements of specific heat from 1.9 to 400 K provide accurate values for the Debye temperature and Sommerfeld coefficient, the phononic part being describable via a combined Debye-Einstein approach (accounting for high-frequency oxygen-related optical phonons), with excess intermediate-temperature specific heat due to a prominent low-energy peak in the phonon density of states. Most significantly, all electronic and phononic contributions to the specific heat are shown to be remarkably closely reproduced by DFT-based calculations, establishing quantitative understanding of key thermal properties of the metallic delafossite PdCoO2.