Electron-Deficient Eu_6.5Gd_0.5Ge₆ Intermetallic: A Layered Intergrowth Phase of the Gd₅Si₄- and FeB-Type Structures

Abstract

A novel electron-poor Eu_6.5Gd_0.5Ge₆ compound adopts the Ca₇Sn₆-type structure (space group <i>Pnma</i>, <i>Z</i> = 4, <i>a</i> = 7.5943(5) Å, <i>b</i> = 22.905(1) Å, <i>c</i> = 8.3610(4) Å, and <i>V</i> = 1454.4(1) ų). The compound can be seen as an intergrowth of the Gd₅Si₄-type (<i>Pnma</i>) R₅Ge₄ (R = rare earth) and FeB-type (<i>Pnma</i>) RGe compounds. The phase analysis suggests that the Eu_7–<i>x</i>Gd_<i>x</i>Ge₆ series displays a narrow homogneity range of stabilizing the Ca₇Sn₆ structure at <i>x</i> ≈ 0.5. The structural results illustrate the structural rigidity of the _∝²[R₅X₄] slabs (X = <i>p</i>-element) and a possibility for discovering new intermetallics by combining the _∝²[R₅X₄] slabs with other symmetry-approximate building blocks. Electronic structure analysis suggests that the stability and composition of Eu_6.5Gd_0.5Ge₆ represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)₅Ge₄ phases