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.