Pallas Anna, Larsson Karin
Department of Materials Chemistry, Uppsala University, Box 538, Uppsala 751 21, Sweden.
J Phys Chem B. 2006 Mar 23;110(11):5367-71. doi: 10.1021/jp056652x.
Metal diborides (MB(2)) often have interesting thermal, mechanical, and superconducting properties. MgB(2) was put into focus some years ago for its high transition temperature (39 K) in combination with its simple AlB(2) structure. The boron structure in MB(2) is assumed to be dependent on the electron transfer from the nearby positioned metal atoms. An electronic and structural comparison has been performed here for various initially planar and puckered transition-metal borides, using quantum mechanical density functional theory (DFT) calculations under periodic boundary conditions. In comparison to MgB(2), the experimentally planar transition-metal diborides (ZrB(2), NbB(2), and MoB(2)) and the experimentally puckered ones (TcB(2), RuB(2), RhB(2), and PdB(2)) have been examined. The results indicate that the energetic stability generally follows the experimentally obtained results. The metals that are less electronegative than boron donate electrons to boron, which in turn induce planar boron structures (graphitic-like). The metals that prefer to be planar donate more than one electron, while the trend for metals which favor puckered B structures is that they donate less than one electron per metal atom. Two donated electrons per metal atom (or very close to) will result in the most stable AlB(2) structure.
金属硼化物（MB₂）通常具有有趣的热学、力学和超导性能。几年前，MgB₂因其较高的转变温度（39 K）以及简单的AlB₂结构而受到关注。MB₂中的硼结构被认为取决于附近金属原子的电子转移。在此，利用周期性边界条件下的量子力学密度泛函理论（DFT）计算，对各种初始为平面状和褶皱状的过渡金属硼化物进行了电子和结构比较。与MgB₂相比，研究了实验上为平面状的过渡金属硼化物（ZrB₂、NbB₂和MoB₂）以及实验上为褶皱状的过渡金属硼化物（TcB₂、RuB₂、RhB₂和PdB₂）。结果表明，能量稳定性总体上与实验结果相符。电负性比硼小的金属向硼提供电子，这反过来会诱导出平面硼结构（类石墨结构）。倾向于形成平面结构的金属提供一个以上的电子，而倾向于形成褶皱状硼结构的金属的趋势是每个金属原子提供的电子少于一个。每个金属原子提供两个电子（或非常接近两个电子）将导致最稳定的AlB₂结构。
