Two homologous intermetallic phases in the Na-Au-Zn system with sodium bound in unusual paired sites within 1D tunnels

摘要

The Na-Au-Zn system contains the two intermetallic phases Na _(0.97(4))Au _2Zn _4 (I) and Na _(0.72(4))Au _2Zn _2 (II) that are commensurately and incommensurately modulated derivatives of K _(0.37)Cd)2, respectively. Compound I crystallizes in tetragonal space group P4/mbm (No. 127), a = 7.986(1) ?, c = 7.971(1) ?, Z = 4, as a 1 × 1 × 3 superstructure derivative of K _(0.37)Cd _2 (I4/mcm). Compound II is a weakly incommensurate derivative of K _(0.37)Cd _2 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) ?, c = 2.7326(4) ?, 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 ([Au _(4/2)Zn _4Zn _4Au _(4/2)] for I and [Au _(4/2)Zn _4Au _(4/2)] 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.