Transition metal carbides (TMCs) are considered to be potential superhard materials and have attracted much attention. With respect to titanium and carbon atoms, we confirm the pressure-composition phase diagram of the Ti–C system using structure searches and first-principles calculations. We firstly discovered stable TiC₄ which was expected to be synthesized at high pressure, as well as metastable TiC₂ and TiC₃. These layered titanium carbides are diamond net analogues due to the unusual C-layers in the form of puckered graphene-like, diamond-like and double diamond-like C-layers. The existence of diamond-like C-layers might help to understand the formation of diamond. All the studied titanium carbides could be recoverable at ambient pressure and exhibited great mechanical properties (strong ability to resist volume and shear deformations, small anisotropy, and high hardness). Moreover, we crystallized the structure of TiC₄ in other transition metal carbides and obtained five superhard TMC₄s (TM = V, Zr, Nb, Hf and Ta). Interactions between layers were revealed to be the source of the great mechanical properties and high hardness through combining detailed analyses of electronic structure and chemical bonding, namely, weak ionic interactions of neighboring Ti- and C-layers and the strong covalent interactions of C- and C-layers.

中文翻译：

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揭示超硬碳化钛中不寻常的刚性金刚石网类似物†

过渡金属碳化物（TMCs）被认为是潜在的超硬材料，受到了广泛关注。关于钛和碳原子，我们使用结构搜索和第一性原理计算确认了 Ti-C 系统的压力组成相图。我们首先发现了有望在高压下合成的稳定的TiC ₄以及亚稳定的TiC ₂和TiC ₃. 这些层状碳化钛是金刚石网类似物，因为具有褶皱状石墨烯、类金刚石和双类金刚石 C 层形式的不寻常 C 层。类金刚石 C 层的存在可能有助于理解金刚石的形成。所有研究的碳化钛都可以在环境压力下恢复，并表现出很好的机械性能（抗体积和剪切变形的能力强，各向异性小，硬度高）。此外，我们在其他过渡金属碳化物中结晶了TiC ₄的结构，得到了五种超硬TMC ₄s（TM = V、Zr、Nb、Hf 和 Ta）。通过结合对电子结构和化学键的详细分析，即相邻 Ti 层和 C 层的弱离子相互作用以及 C 和C 层。