Cd₄Cu₇As, The First Representative of a Fully Ordered, Orthorhombically Distorted MgCu₂ Laves Phase

The ternary Laves phase Cd₄Cu₇As is the first intermetallic compound in the system Cu–Cd–As and a representative of a new substitution variant for <i>Laves</i> phases. It crystallizes orthorhombically in the space group <i>Pnnm</i> (No. 58) with lattice parameters <i>a</i> = 9.8833(7) Å; <i>b</i> = 7.1251(3) Å; <i>c</i> = 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. Cd₄Cu₇As 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 Cd₄Cu_6.9(1)As_1.1(1) was structurally characterized. It crystallizes cubic in the space group <i>Fd</i>3<i>m̅</i> with lattice parameter <i>a</i> = 7.0779(8) Å. Typically for quasi-binary Laves phases Cu and As are both occupying the 16<i>c</i> site. Chemical bonding, charge transfer and atomic properties of Cd₄Cu₇As were analyzed by band structure, ELF, and AIM calculations. On the basis of the general formula for Laves phases <i>AB</i>₂, Cd is slightly positively charged forming the <i>A</i> substructure, whereas Cu and As represent the negatively charged <i>B</i> substructure in both cases. The crystal structure distortion is thus related to local effects caused by Arsenic that exhibits a larger atomic volume (18 ų compared to 13 ų for Cu) and higher ionicity in bonding.