The distribution of the electric-field-gradient tensor parameters is evaluated for a randomly disordered structure displaying an intrinsic local symmetry. The second-order symmetries of the nonperturbed system are defined by the two central parameters and g0, then the corresponding distribution in the disordered modification is governed by one order parameter k hp/cr where cr is the variance in the components of the perturbing tensor. An approximate analytical expression is proposed with the purpose of interpreting experimental data in terms of relevant structural parameters for compounds where the local symmetry is randomly disturbed, e.g. , quasicrystals, amorphous alloys, or disordered rare-earth compounds with a symmetry lower than cubic. It should be emphasized that the intrinsic parameters characterizing the underlying symmetry (hp, rio) are significantly different from the corresponding mean values of the distribution, as soon as the disorder becomes finite.The modeling of disordered structures without longrange periodicity like amorphous and quasicrystalline materials, implies the knowledge of not only pair correlation functions (as deduced from scattering experiments) but also of the correlation functions that involve more than two atomic positions. Therefore, the local symmetry properties provide an important insight into the puzzle of these high-order correlations. These symmetries are reflected by multipolar fields (crystalline electric fields). In particular, the second-order crystal fields (B2o,B$) acting at rare-earth atoms2 or the electric-field-gradient tensor (EFGT) at the nucleus of S-state iona are direct probes of the second-order symmetries, i.e. , of the threeand fouratom correlation functions. 'There is an increasing amount of evidence that the amorphous order possesses the characteristics of some underlying crystal phase. s On the other hand, in order to test models of decoration for quasicrystalline lattices, it is crucial to determine whether these phases have an intrinsic point symmetry lower than cubic (and in turn lower than icosahedral) phases or whether the measured EFGT only arises from fluctuations due to the lack of periodicity.Hence, it is of primary importance to predict the EFGT distribution for a randomly perturbed structure showing an intrinsic second-order symmetry, i.e. , hp~o. With the help of numerical simulations, an approximate analytical expression is derived which should be very useful to analyze quadrupolar interaction data.Let V;k be the components of the 3x 3 EFG tensor [V].[V] being symmetric and traceless is fully determined by five independent quantities, e. g., three Euler angles (a, P, y) and two components of the trace in the eigenfrarne, i.e. , the principal component V"and the asymmetry parameter ri ( V» -V~~(/( V"[. In the presence of a distribution of the eigenvalues of [V], a polar coordinate representation (h, p) is more convenient since it avoids the unphysical discontinuities that are introduced by the definition of V"and q. The following results ca...

Fe,/Ru, hexagonal close-packed superlattices have been grown by MBE with various layers thickness 4 < x 12 b and 4 s v s 52 A. The superlattices show fully h.c.p.structure with sharp interfaces and with a periodic modulation along the [OOOl] growth axis. The h.c.p.-Fe layers correspond to a specific volume of 12.5A3 per F e atom, much larger than in regular close packed phases (11.3A3), which is likely related to a high-spin state for Fe.The ability to grow ultrathin films and to control interfaces at an atomic scale has open new possibilities t o synthetize metastable metallic structures [l]. Indeed, in some instances, epitaxial growth induces large and controlled variations of the interatomic distances, which are out of reach in bulk materials, in particular if a lattice expansion is considered. Regarding 3d elements and in particular Fe, magnetic properties are predicted to change dramatically upon changing the crystal structure and interatomic distances [21. Precise

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2014 La configuration électronique du fer, l'ordre structural à courte distance et les propriétés magnétiques de verres de composition (Fe2O3)x(BaO)y(B2O3)z sont établis par spectroscopie Mössbauer de 57Fe en combinaison avec des mesures macroscopiques. La coordinance moyenne du fer est déterminée par le rapport atomique Fe/O dans le milieu tandis que la configuration de valence du fer dépend principalement des conditions de préparation. Un ordre structural à courte distance très strict est mis en évidence autour des atomes de Fe ; il correspond à des fluctuations aléatoires des angles et longueurs de liaison Fe2014O. A faible concentration en Fe2O3, le verre reste paramagnétique jusqu'à 1,5 K; la mesure locale de moment révèle cependant la coexistence d'ions isolés et d'agrégats à l'échelle atomique. A concentration moyenne en Fe2O3 (~ 25 %), le verre présente une transition mictomagnétique. A concentration élevée en Fe2O3 (~ 50 %), le verre présente un comportement quasi-superparamagnétique, révélant une inhomogénéité de la répartition du fer dans le milieu. Les phases formées lors de la cristallisation du verre à concentration moyenne en Fe2O3 sont identifiées par spectroscopie Mössbauer.