Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA.
Inorg Chem. 2013 Jun 3;52(11):6402-6. doi: 10.1021/ic4002219. Epub 2013 May 14.
We studied the binary carbide systems Li2C2 and CaC2 at high pressure using an evolutionary and ab initio random structure search methodology for crystal structure prediction. At ambient pressure Li2C2 and CaC2 represent salt-like acetylides consisting of C2(2-) dumbbell anions. The systems develop into semimetals (P3m1-Li2C2) and metals (Cmcm-Li2C2, Cmcm-CaC2, and Immm-CaC2) with polymeric anions (chains, layers, strands) at moderate pressures (below 20 GPa). Cmcm-CaC2 is energetically closely competing with the ground state structure. Polyanionic forms of carbon stabilized by electrostatic interactions with surrounding cations add a new feature to carbon chemistry. Semimetallic P3m1-Li2C2 displays an electronic structure close to that of graphene. The π* band, however, is hybridized with Li-sp states and changed into a bonding valence band. Metallic forms are predicted to be superconductors. Calculated critical temperatures may exceed 10 K for equilibrium volume structures.
我们使用演化和从头算随机结构搜索方法研究了高压下的二元碳化物系统 Li2C2 和 CaC2，以进行晶体结构预测。在常压下，Li2C2 和 CaC2 代表由 C2(2-)哑铃阴离子组成的类似于盐的乙炔化物。在中等压力（低于 20 GPa）下，这些系统发展成具有聚合阴离子（链、层、股）的半金属（P3m1-Li2C2）和金属（Cmcm-Li2C2、Cmcm-CaC2 和 Immm-CaC2）。Cmcm-CaC2 在能量上与基态结构竞争激烈。通过与周围阳离子的静电相互作用稳定的聚阴离子形式为碳化学增添了新的特征。半金属 P3m1-Li2C2 具有接近石墨烯的电子结构。然而，π*带与 Li-sp 态杂化，并变成成键价带。预测金属形式为超导体。平衡体积结构的计算临界温度可能超过 10 K。
