Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.

X-ray measurements of stresses and defects in EFG and large grained polycrystalline silicon ribbons. First quarterly report

The first model of a modified Bond goniometer has been built and tested for the precision measurement of interplanar spacings in Si-single crystals. A change in interplanar spacing ..delta..d/d approximately = to +- 10/sup -5/ can be detected which corresponds to surface stresses of the order of +- 1000 psi. A second version of the goniometer is being assembled incorporating a removable microscope for precision alignment of the Si-strip into the primary X-ray beam

TOPOLOGICAL AND CHEMICAL SHORT RANGE ORDER IN METAL-METAL GLASSES

No abstract availabl

A NEW METHOD FOR THE DETERMINATION OF THE TOTAL STRUCTURE FACTOR OF AMORPHOUS MATERIALS BY ENERGY-DISPERSIVE X-RAY DIFFRACTION (VARIABLE λ-METHOD)

Une nouvelle méthode est proposée pour déterminer le facteur de structure total de matériaux amorphes à l'aide d'une technique d'analyse en énergie des rayons X diffractés (EDXD, méthode de la longueur d'onde variable). Cette méthode est fondée sur l'évaluation directe du spectre d'énergie du faisceau primaire à partir des données de diffraction sans appliquer aucun procédé itératif. Les facteurs de structure de plusieurs alliages amorphes (Be Ti Zr, Ni Zr et Ni Co Zr) obtenus de cette manière sont comparés à ceux fournis par la méthode classique de dispersion angulaire (ADXD, méthode du 20 variable).A new method was developed for the determination of the total structure factor of amorphous materials using the energy-dispersive X-ray diffraction technique (EDXD, variable λ-method). It is based on the direct evaluation of the energy spectrum of the primary beam from the diffraction data without applying iterative procedures. The total structure factor for several amorphous alloys (BeTiZr, NiZr and NiCoZr) derived in this way is compared to the one obtained from the angular dispersive X-ray diffraction technique (ADXD, variable 2θ-method)

Curved anode wire chambers for x-ray diffraction applications

Curved position sensitive proportional detectors are described. The first has a radius of curvature of 135 mm and a 60/sup 0/ angular range. The second has a radius of curvature of 360 mm and a 45/sup 0/ angular range. For high quantum efficiency for x-ray energies up to 60 keV, a relatively large x-ray path and high gas pressure are required. The anode wires are suspended in circular arcs by the interaction of a current flowing through them and a magnetic field provided by two permanent magnets placed above and below the wire running parallel to it over the full length of the curved chambers. Anode wire stability under the combined action of the magnetic and electrostatic forces is discussed. (LEW

Structural study of Be43HfxZr57-x metallic glasses by X-ray and neutron diffraction

The Faber-Ziman partial structure factors of the Be43Hf xZr57-x glasses are calculated from the combination of one time-of-flight neutron diffraction measurement in the alloy with x = 25 at % and three X-ray diffraction measurements in the alloys with x = 5, 25, and 54 at % using the isomorphous substitution between Hf and Zr atoms. The Bhatia-Thornton partial structure factors are then deduced. From the partial radial distribution functions the partial coordination numbers are calculated and allow the generalized Warren chemical order parameter α 1 in the first coordination shell to be determined. The upper radius of the first shell is well defined by the position of the first minimum of the GNN function located at 3.8 A. Thus, we find a value of - 0.033 for α1 which indicates a slight tendency of chemical ordering.Les facteurs de structure partiels de Faber-Ziman des amorphes Be 43HfxZr57-x sont calculés à partir de la combinaison d'une mesure de diffraction neutronique en temps de vol dans l'alliage avec x = 25 at % et trois mesures de diffraction X dans les alliages avec x = 5, 25 et 54 at % en utilisant la substitution isomorphe entre les atomes d'hafnium et de zirconium. Les facteurs de structure partiels de Bhatia-Thornton sont ensuite déduits. A partir des fonctions de distribution radiale partielles, les nombres de coordination partiels sont calculés et permettent de déterminer le paramètre d'ordre chimique généralisé de Warren α1 dans la première couche de coordination. Le rayon extérieur de la première couche est bien défini par la position du premier minimum de la fonction GNN situé à 3,8 A. Nous trouvons ainsi une valeur de - 0,033 pour α1, qui montre une faible tendance de mise en ordre chimique

ATOMIC-SCALE STRUCTURE OF AMORPHOUS (Ni-Co)-(Zr-Hf) ALLOYS

Les structures des alliages amorphes Ni35Zr65 et (Ni-Co)-(Zr-Hf) ont été déterminées en employant la méthode de l'angle 2[MATH] variable pour les rayons X et la technique de la longueur d'onde variable pour les neutrons. Les fonctions partielles d'interférence (facteurs partiels de structure) Iij(K) ont été évaluées en utilisant la méthode de substitution isomorphe.The structures of amorphous Ni35Zr65 and (Ni-Co)-(Zr-Hf) alloys have been determined employing the variable 2[MATH] angle X-ray method and the variable wavelength neutron technique. The partial interference functions (structure factors) Iij(K) have been evaluated using the isomorphous substitution method