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

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Abstract
By combining 151Eu Mossbauer spectroscopy (on the Eu2+ S-state ion) and EXAFS (extended X-ray absorption fine structure) measurements in a series of amorphous RE1-xXx (RE=rare earth, X='sp' metal) the authors have investigated the angular correlations and the radial distribution functions (RDF) of the local atomic arrangement restricted to the first neighbour shell. The electric field gradient (EFG) parameters, including the modulus of qz, its balance in sign and the marginal distribution of the asymmetry parameter eta , display strong dependences on the nature of the element X. These various distributions rule out a single description of short-range order and they 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, with the extent of asymmetry depending upon the element X. No significant structural change is detected as a function of the rare-earth element, including rare earths with different valence (Eu2+, Gd3+). 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. On the other hand, no clear relationship can be established between short-range order in amorphous and crystalline phases. In the new amorphous metals Eu70Mg30, Eu70Zn30 and Eu75Cd25 which have been studied here, one observes similarly (from the EFG distribution) that the order is correlated with the enthalpy of formation.