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

Abstract

The Na–Au–Zn system contains the two intermetallic phases Na_0.97(4)Au₂Zn₄ (<b>I</b>) and Na_0.72(4)Au₂Zn₂ (<b>II</b>) that are commensurately and incommensurately modulated derivatives of K_0.37Cd₂, respectively. Compound <b>I</b> crystallizes in tetragonal space group <i>P</i>4/<i>mbm</i> (No. 127), <i>a</i> = 7.986(1) Å, <i>c</i> = 7.971(1) Å, <i>Z</i> = 4, as a 1 × 1 × 3 superstructure derivative of K_0.37Cd₂ (<i>I</i>4/<i>mcm</i>). Compound <b>II</b> is a weakly incommensurate derivative of K_0.37Cd₂ with a modulation vector <i>q</i> = 0.189(1) along <i>c</i>. Its structure was solved in superspace group <i>P</i>4/<i>mbm</i>(00<i>g</i>)­00ss, <i>a</i> = 7.8799(6) Å, <i>c</i> = 2.7326(4) Å, <i>Z</i> = 2, as well as its average structure in <i>P</i>4/<i>mbm</i> with the same lattice parameters.. The Au–Zn networks in both consist of layers of gold or zinc squares that are condensed antiprismatically along <i>c</i> ([Au_4/2Zn₄Zn₄Au_4/2] for <b>I</b> and [Au_4/2Zn₄Au_4/2] for <b>II</b>) 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 (<b>I</b>) (shown) or between (<b>II</b>) 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