Two Homologous Intermetallic Phases in the Na–Au–Zn System with Sodium Bound in Unusual Paired Sites within 1D Tunnels

The Na–Au–Zn system contains the two intermetallic phases Na0.97(4)Au2Zn4 (I) and Na0.72(4)Au2Zn2 (II) that are commensurately and incommensurately modulated derivatives of K0.37Cd2, respectively. Compound I crystallizes in tetragonal space group P4/mbm (No. 127), a = 7.986(1) Å, c = 7.971(1) Å, Z = 4, as a 1 × 1 × 3 superstructure derivative of K0.37Cd2 (I4/mcm). Compound II is a weakly incommensurate derivative of K0.37Cd2 with a modulation vector q = 0.189(1) along c. Its structure was solved in superspace group P4/mbm(00g)­00ss, a = 7.8799(6) Å, c = 2.7326(4) Å, Z = 2, as well as its average structure in P4/mbm with the same lattice parameters.. The Au–Zn networks in both consist of layers of gold or zinc squares that are condensed antiprismatically along c ([Au4/2Zn4Zn4Au4/2] for I and [Au4/2Zn4Au4/2] for II) to define fairly uniform tunnels. The long-range cation dispositions in the tunnels are all clearly and rationally defined by electron density (Fourier) mapping. These show only close, somewhat diffuse, pairs of opposed, ≤50% occupied Na sites that are centered on (I) (shown) or between (II) the gold squares. Tight-binding electronic structure calculations via linear muffin-tin-orbital (LMTO) methods, assuming random occupancy of ≤ ∼100% of nonpaired Na sites, again show that the major Hamilton bonding populations in both compounds arise from the polar heteroatomic Au–Zn interactions. Clear Na–Au (and lesser Na–Zn) bonding is also evident in the COHP functions. These two compounds are the only stable ternary phases in the (Cs,Rb,K,Na)–Au–Zn systems, emphasizing the special bonding and packing requirements in these sodium structures.Na-Au-Zn体系包含两个金属间化合物相：Na₀.₉₇(₄)Au₂Zn₄（相I）与Na₀.₇₂(₄)Au₂Zn₂（相II），二者分别为K₀.₃₇Cd₂的共格调制与非共格调制衍生物。相I以四方晶系空间群P4/mbm（编号127）结晶，晶胞参数为a = 7.986(1) Å、c = 7.971(1) Å，Z=4，属于K₀.₃₇Cd₂（空间群I4/mcm）的1×1×3超结构衍生物。相II则是K₀.₃₇Cd₂的弱非共格调制衍生物，调制矢量q = 0.189(1) 沿c轴方向。其结构通过超空间群P4/mbm(00g)­00ss解析，晶胞参数为a = 7.8799(6) Å、c = 2.7326(4) Å，Z=2；其平均结构亦在空间群P4/mbm下解析，晶胞参数与之完全一致。两相中的Au-Zn网络均由金或锌的正方形层构成，沿c轴以反棱柱方式缩合（相I为[Au₄/₂Zn₄Zn₄Au₄/₂]，相II为[Au₄/₂Zn₄Au₄/₂]），由此形成分布均匀的隧道结构。隧道内的长程阳离子排布均可通过电子密度（傅里叶）作图清晰且合理地确定，结果显示仅存在紧密且略有弥散的成对对立钠位点，其占位率≤50%：在相I中该位点位于金正方形的中心（已通过图示展示），在相II中则位于金正方形之间。通过线性 muffin-tin轨道（Linear Muffin-Tin Orbital, LMTO）方法开展紧束缚电子结构计算，假设未配对钠位点的占位率随机分布于约100%以内，结果表明两相中的主要哈密顿键合布居均来自极性异原子Au-Zn相互作用。晶体轨道哈密顿布居（Crystal Orbital Hamilton Population, COHP）函数中也可清晰观测到Na-Au（以及强度较弱的Na-Zn）键合特征。上述两种化合物是(Cs,Rb,K,Na)-Au-Zn体系中仅有的稳定三元相，凸显了这类钠基结构的特殊成键与堆积需求。