Journal Article

Crystal-chemical origins of the ultrahigh conductivity of metallic delafossites

Sharma, Prachi, Haugstad, Greg, Ramberger, Justin, Leighton, Chris, Zhang, Yi, Fernandes, Rafael M., Evans, Guy N., Kaiser, Ben, Tao, Yu, Manno, Mike, Tutt, Fred, Garcia-Barriocanal, Javier, Chaturvedi, Vipul, Bayliff, Samuel, Seyfried, William E., Birol, Turan

2024

Overview

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds ( e.g ., PdCoO 2 , PtCoO 2 ) 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 μm. 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 different approach to PdCoO 2 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, detailed mass spectrometry measurements on these materials reveal that they are not ultrapure in a general sense, 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 sublattice purification mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites. Certain delafossite materials are the most conductive oxides known, for poorly understood reasons. This work elucidates this finding by uncovering a sublattice purification mechanism that enables ultrapure conductive planes even in impure crystals.

• Publisher: Nature Publishing Group UK,Nature Publishing Group,Nature Portfolio
• Subject: 140/58 / 147/135 / 639/301/119 / 639/766/119/995 / Conductivity / Crystal growth / Crystals / Electrons / Humanities and Social Sciences / Impurities / INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY / Low temperature / Mass spectrometry / Mass spectroscopy / multidisciplinary / Point defects / Purification / Residual resistivity / Room temperature / Science / Science & Technology - Other Topics / Science (multidisciplinary) / Scientific imaging / Thermal conductivity / Ultrahigh temperature