Polyclusters and Substitution Effects in the Na–Au–Ga System: Remarkable Sodium Bonding Characteristics in Polar Intermetallics

A systematic
exploration of Na- and Au-poor parts of the Na–Au–Ga
system (less than 15 at. % Na or Au) uncovered several compounds with
novel structural features that are unusual for the rest of the system.
Four ternary compounds Na1.00(3)Au0.18Ga1.82(1) (I), NaAu2Ga4 (II), Na5Au10Ga16 (III), and NaAu4Ga2 (IV) have been
synthesized and structurally characterized by single crystal X-ray
diffraction: Na1.00(3)Au0.18Ga1.82(1)(I, P6/mmm, a = 15.181(2), c =9.129­(2)­Å, Z = 30); NaAu2Ga4 (II, Pnma, a = 16.733(3), b = 4.3330(9), c =7.358­(3) Å, Z = 4); Na5Au10Ga16 (III, P63/m, a = 10.754(2), c =11.457­(2) Å, Z = 2); and NaAu4Ga2 (IV, P21/c, a = 8.292(2), b = 7.361(1), c =9.220­(2)­Å,
β = 116.15(3), Z = 4). Compound I lies between the large family of Bergman-related compounds and Na-poor
Zintl-type compounds and exhibits a clathrate-like structure containing
icosahedral clusters similar to those in cubic 1/1 approximants, as
well as tunnels with highly disordered cation positions and fused
Na-centered clusters. Structures II, III, and IV are built of polyanionic networks and clusters
that generate novel tunnels in each that contain isolated, ordered
Na atoms. Tight-binding electronic structure calculations using linear
muffin-tin-orbital (LMTO) methods on II, III, IV and an idealized model of I show that
all are metallic with evident pseudogaps at the Fermi levels. The
integrated crystal orbital Hamilton populations for II–IV are typically dominated by Au–Ga,
Ga–Ga, and Au–Au bonding, although Na–Au and
Na–Ga contributions are also significant. Sodium’s involvement
into such covalency is consistent with that recently reported in Na–Au–M
(M = Ga, Ge, Sn, Zn, and Cd) phases.