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

Abstract

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 (½, ½, 0, ½). 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\over 3 to ½). The large composition range is achieved by extended vacancy ordering in the planar 63 net of Ge with subsequent relaxation.