Transition metal carbides find use in a wide range of advanced high-resilience applications including high-strength steels, heat shields, and deep-earth drills. However, carbides of the mid-to-late transition metals remain difficult to isolate and characterize on account of their metastability, which precludes the preparation of high-quality bulk single crystal samples using traditional solid-state methods. Herein, we report a combined computational and experimental survey of the cobalt–carbon binary system under high pressures and demonstrate that pressure offers a route toward the bulk synthesis of the metastable cementite-type cobalt carbide, Co₃C, which under ambient conditions can only be prepared in low-dimensional thin film or nanoparticle forms. First-principles calculations reveal two competitive low-energy stoichiometric phases under ambient pressures─Pnnm-Co₂C (Fe₂C-type) and Pnma-Co₃C (Fe₃C-type)─consistent with the known low-dimensional phases that have been studied for their promising magnetic properties. However, the calculated formation enthalpy of Pnma-Co₃C decreases steadily with the applied pressure, while that of Pnnm-Co₂C increases. We pursue these results using high-pressure laser-heated synthesis methods coupled with in situ X-ray diffraction and observe the formation of Pnma-Co₃C above 4.8 GPa. We determine the experimental bulk modulus of Co₃C to be K₀ = 237 GPa (K_p = 4.0). First-principles calculations of the phonon modes in Co₃C reveal dynamical instabilities at ambient pressure that are absent under compression. These results offer a promising new route for the synthesis of rare-earth-free magnets.

中文翻译：

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块状钴渗碳体、Co3C 的高压合成

过渡金属碳化物可用于各种先进的高回弹应用，包括高强度钢、隔热板和深地钻。然而，中晚期过渡金属的碳化物由于其亚稳定性仍然难以分离和表征，这排除了使用传统固态方法制备高质量块状单晶样品的可能性。在此，我们报告了在高压下对钴 - 碳二元系统进行的计算和实验联合调查，并证明压力为大量合成亚稳态渗碳体型碳化钴 Co ₃提供了途径C，在环境条件下只能以低维薄膜或纳米颗粒形式制备。第一性原理计算揭示了在环境压力下的两个竞争性低能化学计量相——Pnnm -Co ₂ C（Fe ₂ C 型）和Pnma -Co ₃ C（Fe ₃ C 型）——与已知的低维相一致已对其有希望的磁性进行了研究。然而，Pnma -Co ₃ C的计算生成焓随着施加的压力稳定下降，而Pnnm -Co ₂C 增加。我们使用高压激光加热合成方法与原位X 射线衍射相结合来追求这些结果，并在4.8 GPa 以上观察Pnma -Co ₃ C的形成。我们确定 Co ₃ C的实验体积模量为K ₀ = 237 GPa ( K _p = 4.0)。Co ₃ C 中声子模式的第一性原理计算揭示了在压缩下不存在的环境压力下的动力学不稳定性。这些结果为合成无稀土磁体提供了一条有前途的新途径。