  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. Their room-temperature conductivity can
exceed that of Au, while their low-temperature electronic mean-free-paths reach
an astonishing 20 microns. 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 first report a
new approach to PdCoO2 crystal growth, using chemical vapor transport methods
to achieve order-of-magnitude gains in size, the highest structural qualities
yet reported, and record residual resistivity ratios (>440). 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
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 (~1 ppm impurity concentrations). These purities are shown to be in
quantitative agreement with measured residual resistivities. We thus conclude
that a previously unconsidered "sublattice purification" mechanism is essential
to the ultrahigh low-temperature conductivity and mean-free-path of metallic
delafossites.
