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Structure and Bonding in Yb4MgGe4:  Yb2+/Yb3+ Mixed-Valency and Charge Separation

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journal contribution
posted on 22.03.2006, 00:00 by Paul H. Tobash, Svilen Bobev
Reported are the synthesis and the structural characterization of a new derivative of the RE5Tt4 family (RE = Rare-earth; Tt = Tetrel, = Si, Ge, i.e., group 14 element), Yb5-xMgxGe4 (x ≈ 1). Crystal data for Yb4.04(1)Mg0.96(1)Ge4 at 23 °C:  orthorhombic, space group Pnma (No. 62), Z = 4; a = 7.155(2) Å, b = 14.769(5) Å, c = 7.688(2) Å; V = 812.5(4) Å3. This phase is an example of a substitution of lanthanide metal (Yb) with a nonmagnetic element (Mg) within this structure type. Its structure can alternatively be described as an intergrowth of the hypothetical Yb2MgGe2, which features flat infinite [MgGe2]4- layers and the hypothetical YbGe with [Ge2]6- dimers. The flat [MgGe2]4- layers propagate in two dimensions (a and c), and they are offset by a distance of 1/4·a with respect to one another and are interspaced with layers of [Ge2]6- dimers and Yb cations filling the space between them. According to the structural and physical property data, Yb4MgGe4 is a heterogeneous mixed-valent compound, i.e. a system where one of the two symmetry-inequivalent Yb sites has atoms in closed-shell Yb2+ configuration, whereas the Yb3+ cations occupy a different crystallographic site.