Vacancy-ordering effects in AlB2-type ErGe2 - x(0.4 < x < or = 0.5)
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Jeppe Christensen1, Sven Lidin, Bernard Malaman
1Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden. jeppe@inorg.su.se
Abstract:
In the Er-Ge system, the compostion range ErGe(2) to Er(2)Ge(3) 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(alpha0gamma)0s, X representing the centring vector ((1/2), (1/2), 0, (1/2)). In this description the modulation vector q = (alphaa* + gammac*) is shown to be a direct measure of the Ge content as ErGe(2 - alpha) (alpha falls in the range 1\over 3 to (1/2)). The large composition range is achieved by extended vacancy ordering in the planar 6(3) net of Ge with subsequent relaxation.
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