Vacancy-ordering effects in AlB2-type ErGe2-x (0.4<x≤0.5)

In the Er-Ge system, the compostion range ErGe2 to Er2Ge3 has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(α0γ)0s, X representing the centring vector ((1)/(2),(1)/(2),0,(1)/(2)). In this description the modulation vector q=(αa*+γc*) is shown to be a direct measure of the Ge content as ErGe2-α (α falls in the range (1)/(3) to (1)/(2)). The large composition range is achieved by extended vacancy ordering in the planar 63 net of Ge with subsequent relaxation. (orig.)