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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. 

Rocksalt structure nitrides emerge as a promising class of semiconductors for high-temperature thermoelectric and plasmonic applications. Controlling the bandgap and strain is essential for the development of a wide variety of electronic devices. Here we use (Ti 0.5 Mg 0.5 ) 1−x Al x N as a model system to explore and demonstrate the tunability of both the bandgap and the strain state in rocksalt structure nitrides, employing a combined experimental and computational approach. (Ti 0.5 Mg 0.5 ) 1−x Al x N layers with x ≤ 0.44 deposited on MgO(001) substrates by reactive co-sputtering at 700 °C are epitaxial single crystals with a solid-solution B1 rocksalt structure. The lattice mismatch with the substrate decreases with increasing x , leading to a transition in the strain-state from partially relaxed (74% and 38% for x = 0 and 0.09) to fully strained for x ≥ 0.22. First-principles calculations employing 64-atom Special Quasirandom Structures (SQS) indicate that the lattice constant decreases linearly with x according to a = (4.308 − 0.234 x ) Å for 0 ≤ x ≤ 1. In contrast, the measured relaxed lattice parameter a o = (4.269 − 0.131 x ) Å is linear only for x ≤ 0.33, its composition dependence is less pronounced, and x > 0.44 leads to the nucleation of secondary phases. The fundamental (indirect) bandgap predicted using the same SQS supercells and the HSE06 functional increases from 1.0 to 2.6 eV for x = 0–0.75. In contrast, the onset of the measured optical absorption due to interband transitions increases only from 2.3 to 2.6 eV for x = 0–0.44, suggesting that the addition of Al in the solid solution relaxes the electron momentum conservation and causes a shift from direct to indirect gap transitions. The resistivity increases from 9.0 to 708 μΩ m at 77 K and from 6.8 to 89 μΩ m at 295 K with increasing x = 0–0.44, indicating an increasing carrier localization associated with a randomization of cation site occupation and the increasing bandgap which also causes a 33% reduction in the optical carrier concentration. The overall results demonstrate bandgap and strain engineering in rocksalt nitride semiconductors and show that, in contrast to conventional covalent semiconductors, the random cation site occupation strongly affects optical transitions.