Structural short-range order in amorphous rare-earth alloys from Moessbauer spectroscopy and EXAFS

by combining 151Eu Moessbauer spectroscopy and EXAFS (extended x-ray absorption fine structure) measurements in a series of amorphous REsub(1-x)Xsub(x) (RE=rare earth, X='sp' metal) the angular correlations and the radial distribution functions (RDF) of the local atomic arrangement restricted to the first neighbour shell, have been investigated. The electric field gradient (EFG) parameters, display strong dependences on the nature of the element X. The various distributions rule out a single description of short-range order and are discussed by reference to models of structure. The shape of the first peak of the partial RDF centred on the major (RE) and on the minor (X) atomic components is asymmetric, depending upon the element X. No significant structural change is detected as a function of the rare-earth element. The degree of structural short-range order is evaluated consistently from the EFG distribution and from the asymmetry of the RDF as determined from EXAFS. The decrease of the short-range order in the sequence Au, Ga, Al...is correlated with a decreasing enthalpy of formation of the corresponding crystalline compounds. No clear relationship can be established between short-range order in amorphous and crystalline phases. In the new amorphous metals Eu70Mg30, Eu70Zn30 and Eu75Cd25 it is observed that the order is correlated with the enthalpy of formation. (author)