Cd₄Cu₇As, The First Representative of a Fully Ordered, Orthorhombically Distorted MgCu₂Laves Phase

Abstract

The ternary Laves phase Cd(4)Cu(7)As is the first intermetallic compound in the system Cu-Cd-As and a representative of a new substitution variant for Laves phases. It crystallizes orthorhombically in the space group Pnnm (No. 58) with lattice parameters a = 9.8833(7) Å; b = 7.1251(3) Å; c = 5.0895(4) Å. All sites are fully occupied within the standard deviations. The structure can be described as typical Laves phase, where Cu and As are forming vertex-linked tetrahedra and Cd adopts the structure motive of a distorted diamond network. Cd(4)Cu(7)As was prepared from stoichiometric mixtures of the elements in a solid state reaction at 1000 °C. Magnetic measurements are showing a Pauli paramagnetic behavior. During our systematical investigations within the ternary phase triangle Cd-Cu-As the cubic C15-type Laves phase Cd(4)Cu(6.9(1))As(1.1(1)) was structurally characterized. It crystallizes cubic in the space group Fd3m with lattice parameter a = 7.0779(8) Å. Typically for quasi-binary Laves phases Cu and As are both occupying the 16c site. Chemical bonding, charge transfer and atomic properties of Cd(4)Cu(7)As were analyzed by band structure, ELF, and AIM calculations. On the basis of the general formula for Laves phases AB(2), Cd is slightly positively charged forming the A substructure, whereas Cu and As represent the negatively charged B substructure in both cases. The crystal structure distortion is thus related to local effects caused by Arsenic that exhibits a larger atomic volume (18 Å(3) compared to 13 Å(3) for Cu) and higher ionicity in bonding.

Half Antiperovskites VI: On the Substitution Effects in Shandites InxSn2–xCo3S2

“…Intralayer distances d (Co–Co) = ½ a hex = 2.64 Å in In 2 Co 3 S 2 and Sn 2 Co 3 S 2 (2.68 Å, Table 3) are still longer than in the element Co (2.45 Å). This indicates weak interactions, contrary to Kagome layers in Laves phases 15. Stronger bonding is concluded from short Co–S distances (Figure 1b).…”

“…Due to preliminary analyses In1 forms two 4‐center bonds similar to some Laves‐phases,15,35 whereas In2 forms six 3‐center bonds within the layers. In case of Sn 2 Co 3 S 2 the bonding contributions of Sn1‐5p z are stronger.…”

“…This indicates weak interactions, contrary to Kagome layers in Laves phases. [15] Stronger bonding is concluded from short Co-S distances (Figure 1b). Layered networks [Co 3 S 2 ] for In 2 Co 3 S 2 [d(Co-S) = 2.17 Å] and Sn 2 Co 3 S 2 (2.18 Å) resemble sheets of grey arsenic as proposed for Rb[Cu 3 S 2 ].…”

“…Local effects are accounted for from electric field gradients (EFG), the electron localization function and the theory of atoms in molecules (ELF, AIM). Results are discussed with respect to the concept of half antiperovskites and recently investigated Laves phases 13–15…”

The Influence of the Valence Electron Concentration on the Structural Variation of the Laves Phases MgNi2–xGex

“…[16] A partial Cu/As substitution within the graphite-like 3 6 Cu-net of the cubic CdCu 2 Laves phase (C15) results in a new ordered, orthorhombically distorted Cd 4 Cu 7 As structure (superstructure of MgCu 2 ). [17] The system Mg/Ni/Ge shows a large variety of structure types, but only a few of them are structurally related to Laves phases. The Mg-poor compound MgNi 6 Ge 6 [18] crystallizes in the YCo 6 Ge 6 structure, [18] which can be derived from the CaCu 5 structure.…”