Gold–Gold Bonding: The Key to Stabilizing the 19-Electron Ternary Phases LnAuSb (Ln = La–Nd and Sm)

We report a new family of ternary 111 hexagonal LnAuSb (Ln = La–Nd, Sm) compounds that, with a 19 valence electron count, has one extra electron compared to all other known LnAuZ compounds. LaAuSb, CeAuSb, PrAuSb, NdAuSb, and SmAuSb crystallize in the YPtAs-type structure, and have a doubled unit cell compared to other LnAuZ phases as a result of the buckling of the Au–Sb honeycomb layers to create interlayer Au–Au dimers. The dimers accommodate the one excess electron per Au and thus these new phases can be considered Ln23+(Au–Au)0Sb23–. Band structure, density of states, and crystal orbital calculations confirm this picture, which results in a nearly complete band gap between full and empty electronic states and stable compounds; we can thus present a structural stability phase diagram for the LnAuZ (Z = Ge, As, Sn, Sb, Pb, Bi) family of phases. Those calculations also show that LaAuSb has a bulk Dirac cone below the Fermi level. The YPtAs-type LnAuSb family reported here is an example of the uniqueness of gold chemistry applied to a rigidly closed shell system in an unconventional way.本研究报道了一类全新的三元111相六方晶系LnAuSb（Ln=La~Nd、Sm）化合物，该类化合物的价电子数为19，相较于所有已知的LnAuZ类化合物多出一个价电子。LaAuSb、CeAuSb、PrAuSb、NdAuSb及SmAuSb均以YPtAs型结构结晶，相较于其他LnAuZ相，其晶胞尺寸扩大一倍，这源于Au-Sb蜂窝状层发生褶皱，进而形成层间Au-Au二聚体。此类二聚体可容纳每个Au原子对应的一个多余电子，因此该类新型相可被视为Ln₂³⁺-(Au-Au)⁰-Sb₂³⁻。能带结构、态密度及晶体轨道计算验证了这一模型，该模型表明体系在满电子态与空电子态之间存在近乎完整的带隙，可形成稳定化合物；据此我们可构建LnAuZ（Z=Ge、As、Sn、Sb、Pb、Bi）相家族的结构稳定性相图。上述计算同时显示，LaAuSb的费米能级下方存在体相狄拉克锥（Dirac cone）。本研究报道的YPtAs型LnAuSb相家族，是将金化学的独特性以非常规方式应用于严格闭壳层体系的典型范例。