Crystal-Chemical Origins of the Ultrahigh Conductivity of Metallic Delafossites

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO₂, PtCoO₂) are the most conductive oxides known, for reasons that remain poorly understood. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we report a new approach to PdCoO₂ crystal growth, using chemical vapor transport to achieve order-of-magnitude gains in size, the highest structural qualities reported, and record residual resistivity ratios (>440) [1]. Nevertheless, the first detailed mass spectrometry measurements on these materials reveal that they are not ultrapure, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this dichotomy, showing that the vast majority of impurities are forced to reside in Co-O octahedral layers, leaving the conductive Pd sheets highly pure. We thus conclude that a “sublattice purification” mechanism is essential to the ultrahigh low-temperature conductivity of metallic delafossites [1].

[1] Zhang et al., arXiv:2308.14257 (2023).