Understanding the ultrahigh conductivity and thermal properties of metallic delafossites

Abstract

Metallic delafossites (e.g., PdCoO2, PtCoO2), have attracted much recent attention due to record-high oxide conductivities. Despite their highly anisotropic complex-oxidic nature, PdCoO2 and PtCoO2 are the most conductive oxides known. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 m. Significant advances have been made with bulk crystals of these compounds, and various potential applications have been proposed. However, the origin of their ultrahigh conductivities remains unclear. It is widely accepted that these materials must be ultrapure and have ultralow defect density to achieve these properties. However, these extraordinary levels of crystalline perfection have not been proven. This situation is further complicated by the rudimentary nature of the crystal growth methods applied thus far, which produce only small crystals and are not typically capable of such ultrahigh purity. In addition, relatively little attention has been paid to the important thermal properties related to the electronic and phononic structure of these compounds.Using PdCoO2 as the gateway, we address the above problems starting from crystal growth. We first develop a new chemical vapor transport (CVT) method to grow PdCoO2 single crystals, achieving order-of-magnitude gains in crystal size, the highest structural qualities, and a deeper understanding of the growth mechanism. Using these high-quality crystals, the less-investigated thermal properties are studied by experimental probes of the crystal structure, thermal expansion, and specific heat of PdCoO2, combined with density functional theory (DFT) calculations. PdCoO2 is shown to retain the R3 ̅m space group from 12-1000 K, where the measured thermal expansion coefficients are in good quantitative agreement with DFT-based lattice dynamics calculations. The Co-O bond lengths also explain the stability of the low-spin state of the nominally-Co3+ ions. 1.9-400 K measurements of the specific heat provide accurate values for the Debye temperature and Sommerfeld coefficient, the phononic part being describable via a combined Debye-Einstein approach. Notably, all electronic and phononic contributions to the specific heat are shown to be closely reproduced by DFT calculations, establishing quantitative understanding of key thermal properties of PdCoO2. We then turn to magnetic, electronic, and chemical characterizations to directly investigate the origin of the ultrahigh conductivity. Our CVT crystals have the highest residual resistivity ratios (>440) yet reported and the lowest magnetic impurity concentrations. Detailed mass spectrometry, however, reveals that these crystals are not ultrapure in general, typically harboring 100s-of-parts-per-million elemental impurities. Through crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure. The Pd site impurity concentrations are in quantitative agreement with measured residual resistivities. We thus conclude that a sublattice purification mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites. This is further demonstrated by substituting Co with Cr in PdCoO2, where significant increases in the Cr concentrations only lead to weak rises in the measured residual resistivities in these PdCo1-xCrxO2 solid solutions (x = 0.0046 – 0.047). The Cr3+ ions also introduce local moments to the weakly paramagnetic PdCoO2, where spin-glass formation and the Kondo effect are also observed, adding to the richness of the electronic transport and magnetic phenomena in these highly conductive, layered pseudo-two-dimensional oxides.