Alemany Pere, Llunell Miquel, Canadell Enric
Departament de Química Física and Institut de Química Teòrica i Computacional, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.
Inorg Chem. 2009 Apr 6;48(7):2919-31. doi: 10.1021/ic801983a.
The electronic structure of the recently reported (Ca(7)N(4))[M((x))] (M = Ag, Ga, and In) phases has been studied by means of first principles density functional theory (DFT) calculations. It is shown that under the assumption of very weak host-guest interactions: (a) four calcium atoms per formula unit may be considered as Ca(1.5+), whereas the remaining three may be considered as Ca(2+) so that the guest atoms would be neutral, and (b) the Peierls distortions which could set in the guest linear chains are unlikely. These results are compatible with the experimental information. However, the first principles DFT calculations clearly show that very sizable host-guest interactions occur and drastically modify this situation. As a result, there is a substantial electron transfer from the framework to the guest atoms, and all calcium atoms of the framework are better described as Ca(2+). The stoichiometry and structure of these systems result from a competition between the natural tendency of the bare guest atoms to form uniform linear chains within the reduced space of the channels and the attempt to optimize their positions within the channels through interactions with the calcium atoms. Model calculations suggest that indium has a weaker tendency to form uniform linear chains and interacts in a stronger way with the host. It is shown that, for the (Ca(7)N(4))[M(1.33)] (M = Ag and Ga) phases, a structure built from three repeat units of the Ca(7)N(4) host framework containing uniform linear chains with a repeat unit of four guest metal atoms is compatible with the strong interaction scenario and the lack of correlation between the different linear guest chains. These phases should be metallic conductors, and the carriers have both host and guest character. In contrast, the guest atoms in (Ca(7)N(4))[In(1.0)] prefer to occur as a series of trimeric units. Although this phase is found to have a metallic band structure, the conductivity should be smaller than those of the (Ca(7)N(4))[M(1.33)] (M = Ag and Ga) phases.
通过第一性原理密度泛函理论（DFT）计算研究了最近报道的（Ca₇N₄）[Mₓ]（M = Ag、Ga和In）相的电子结构。结果表明，在主客体相互作用非常弱的假设下：（a）每个化学式单元中的四个钙原子可视为Ca¹·⁵⁺，而其余三个可视为Ca²⁺，这样客体原子将呈中性；（b）客体线性链中可能出现的佩尔斯畸变不太可能发生。这些结果与实验信息相符。然而，第一性原理DFT计算清楚地表明，主客体之间存在非常显著的相互作用，并极大地改变了这种情况。结果，有大量电子从骨架转移到客体原子，骨架中的所有钙原子都更宜描述为Ca²⁺。这些体系的化学计量和结构源于裸客体原子在通道缩小空间内形成均匀线性链的自然倾向与通过与钙原子相互作用来优化其在通道内位置的尝试之间的竞争。模型计算表明，铟形成均匀线性链的倾向较弱，且与主体的相互作用更强。结果表明，对于（Ca₇N₄）[M₁·³³]（M = Ag和Ga）相，由Ca₇N₄主体骨架的三个重复单元构建的结构，包含由四个客体金属原子组成重复单元的均匀线性链，与强相互作用情形以及不同线性客体链之间缺乏相关性是相符的。这些相应为金属导体，载流子兼具主体和客体特征。相比之下，（Ca₇N₄）[In₁·⁰]中的客体原子更倾向于以一系列三聚体单元的形式出现。尽管发现该相具有金属能带结构，但其电导率应小于（Ca₇N₄）[M₁·³³]（M = Ag和Ga）相的电导率。
