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ABSTRACT Transparent conductors with high conductivity, optical transparency, and thermal stability are critical for next‐generation optoelectronic devices, yet such materials are scarce. Here, we demonstrate that epitaxial Cr₂AlC(0001)/Al₂O₃(0001) layers exhibit a unique combination of these properties. The optical transmittance of ultrathin layers reaches 85% at 900 nm and remains above 21% across the near‐infrared (NIR)ultraviolet (UV) range, in good agreement with first‐principles optical properties predictions which reveal a small plasma frequency and a low cross‐section for interband transitions. Cr₂AlC has a room‐temperature bulk resistivityρ_o= 42.5 µΩcm and an exceptionally small resistivity scaling with a negligible (<4%) resistivity increase down to 10 nm, an effective bulk electron mean free pathλ< 2 nm at both 295 and 77 K, and a temperature‐independent effectiveρ_oλ< 3 × 10⁻¹⁶Ωm²that is smaller than that of all known elemental metals. An 11.1‐nm‐thick Cr₂AlC layer achieves a benchmarkΦ_TC≈ 0.004 Ω⁻¹in the near infrared (900 nm), comparable to state‐of‐art transparent conductors including ultra‐thin silver and carbon‐based materials. These results, in combination with the known oxidation stability to 1073–1373 K, establish Cr₂AlC as a uniquely robust, high‐temperature stable and scalable transparent conductor, paving the way for advanced extreme‐environment optoelectronics and photonics. 

The electron reflection probability r at symmetric twin boundaries Σ3, Σ5, Σ9, and Σ11 is predicted from first principles for the eight most conductive face-centered cubic (fcc) metals. r increases with decreasing interplanar distance of atomic planes parallel to the boundary. This provides the basis for an extrapolation scheme to estimate the reflection probability r r at random grain boundaries, which is relatively small, r r = 0.28–0.39, for Cu, Ag, and Au due to their nearly spherical Fermi surfaces, but approximately two times higher for Al, Ca, Ni, Rh, and Ir with a predicted r r = 0.61–0.72. The metal resistivity in the limit of small randomly oriented grains with fixed average size is expected to be proportional to the materials benchmark quantity ρ o λ × r r /(1 − r r ), where ρ o and λ are the bulk resistivity and bulk electron mean free path, respectively. Cu has the lowest value for this quantity, indicating that all other fcc metals have a higher resistivity in the limit of small randomly oriented grains. Thus, the conductivity benefit of replacement metals for narrow Cu interconnect lines can only be realized if the grains are larger than the linewidth or exhibit symmetric orientation relationships where r < r r .