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))Au(2)Zn(4) (I) and Na(0.72(4))Au(2)Zn(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) Å, c = 7.971(1) Å, 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) 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 ([Au(4/2)Zn(4)Zn(4)Au(4/2)] for I and [Au(4/2)Zn(4)Au(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.

Lithiation-Induced Zinc Clustering of Zn3, Zn12, and Zn18 Units in Zintl-Like Ca∼30Li3+xZn60–x (x = 0.44–1.38)

“…Similar phenomena were also found in several tunnel structures such as Na 30.5 Ag 2.6 Ga 57.4 , 39 Na 30.1 Au 3.4 Ga 56.6 , 40 and Na 0.97 Au 2 Zn 4 and Na 0.72 Au 2 Zn 2 . 25 The structure solution of crystal 2 was quite similar to that of 1, except that the Ca5 site could be evidently represented by two parts, and an additional partially occupied Ca6 site was assigned to represent the electron density at the origin. As a result, the difference Fourier map was smooth (±2.4 e/Å 3 ).…”

“…Recently, interesting structural and bonding phenomena have been found among valence-electron-poor, polar intermetallics containing group 12 elements, especially those containing gold or platinum. For instance, the tunnel-like structures of Na 0.97 Au 2 Zn 4 and Na 0.72 Au 2 Zn 2 are respectively the 1 × 1 × 3 superstructure and an incommensurately modulated derivative (with a modulation vector q = 0.189 along c) of K 0.37 Cd 2 , although both structures feature layers of gold or zinc squares that are condensed antiprismatically along c. 25 The structure of the Mn 6 Th 23 -type Na 6 Cd 16 Au 7 consists of Cd 8 tetrahedral stars that are face-capped by six shared gold vertexes and further diagonally bridged via another gold to generate an o r t h o g o n a l , t h r e e -d i m e n s i o n a l f r a m e w o r k [Cd 8 (Au) 6/2 (Au) 4/8 ]. 26 Segregation of homoatomic clusters of group 12 elements is very impressive in the structure of Na 6 Cd 16 Au 7 .…”

Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization

“…27 This outcome demonstrates that a grand opportunity exists to obtain new polar intermetallics at low e/a values, at least for Au-based intermetallics. 28 Relatives discovered among neighboring Na/K-Au-Ga/In/Sn (A = alkali metal) [29][30][31][32] and Na/Ca/Sr/Ba-Au/Pt-Zn/Cd [33][34][35] systems are excellent suggestions of wonders to come in neighboring Au-rich polar intermetallic systems.…”

A Sodium‐Containing Quasicrystal: Using Gold To Enhance Sodium’s Covalency in Intermetallic Compounds

“…[15] Actually, we have confirmed [7] that the formation of a pseudogap for Na 13 The combination of Au with Na and Ga in the QC and its approximants contribute to the thermodynamic stability of the QC with respect to decomposition by establishing polarcovalent Au-Ga and even Au-Na metal-metal bonds. An experimental, albeit qualitative, indicator for strong Na-Au bonding also arises from unusually small Na-Au distances, that is, 3.06-3.15 , [17] observed in many Bergman-type crystalline phases and related structures. Accompanying theoretical studies point to the significance of Au-M polar-covalent bonding in these systems, such that these interactions often comprise 65-90 % of the total crystal orbital Hamilton population (COHP), which is a semi-quantitative measure of covalentbonding contributions in solids.…”

“…In contrast, the total Na-Au and Na-Ga contributions for the stuffed Bergman-type (A) is considerably smaller, approximately 12 % (see the Supporting Information, Table S2). An experimental, albeit qualitative, indicator for strong Na-Au bonding also arises from unusually small Na-Au distances, that is, 3.06-3.15 , [17] observed in many Bergman-type crystalline phases and related structures. Evidence of the typically stronger covalent bonding between alkaline-earth metals and gold have been noted before.…”