Arbelio Penton-Madrigal

Direct determination of microstructural parameters from the X-ray diffraction profile of a crystal with stacking faults

The practical aspects of a recently deduced formalism for the direct solution of the powder diffraction pattern of a layer crystal are addressed. It is shown that the obtained solution is particularly well suited to combination with a deconvolution procedure based on a series development of the peak profile. The influence of noise and step size on the obtained solution is discussed. Computer simulation is used to assess the robustness of the proposed procedure. The developed procedure is applied to a powder sample of Y2Co17 that is affected by planar disorder.

Planar faulting in rare earth–Co 2:17 alloys

Abstract Planar faulting in R2Co17 intermetallic alloys were studied by high-resolution X-ray diffraction. For the light rare-earth elements, the Th2Zn17 rhombohedral structure is found, while for the heavy rare-earth atoms, the Th2Ni17 structure prevails. For intermediate rare-earth atom, the R2Co17 structure shows planar disorder. The occurrence of planar faulting is associated with the interlayer distance in the structure. A correlation was found between interlayer distance and stacking order. Simple qualitative explanation is given for the energy preference for one particular type of stacking order. Planar disorder does not introduce new nearest-neighbors environments for the rare-earth atoms apart from those already found in the hexagonal and rhombohedral variants, but merely changes the probability of occurrence of each neighborhood. Simple stacking fault models should prove insufficient to explain the observed planar disorder.

Extrinsic faulting in 3C close‐packed crystal structures: computational mechanics analysis

Extrinsic faulting has been discussed previously within the so-called difference method and random walk calculation. In this contribution it is revisited under the framework of computational mechanics, which allows expressions to be derived for the statistical complexity, entropy density and excess entropy as a function of faulting probability. The approach allows one to compare the disordering process of an extrinsic fault with other faulting types. The ℇ-machine description of the faulting mechanics is presented. Several useful analytical expressions such as probability of consecutive symbols in the Hägg coding are presented, as well as hexagonality. The analytical expression for the pairwise correlation function of the layers is derived and compared with results previously reported. The effect of faulting on the interference function is discussed in relation to the diffraction pattern.

Study of A and B sites order in lanthanide-doped lead titanate ferroelectric system

Pb 0.88 Ln 0.08 TiO 3 ferroelectric system, where Ln = La, Sm, Eu, and Dy, has been characterized using Scanning Electron Microscopy, Raman spectroscopy, and X-ray diffraction experiments. Softening of the lowest transverse optical phonon mode E (1TO) was evaluated as a function of the rare earths’ ionic radius suggesting partial occupation of lanthanide ions at the A and B sites of the perovskite structure. Using Rietveld refinements, it has been established a higher incorporation of Ln 3+ ions into the A sites of the perovskite structure than that of the B sites for the studied ceramics. The occupation at B sites increases slightly with the decreases of the ionic radii of the lanthanides.

Reduction of Cu2+in exchanged Ag+natural clinoptilolite: structural study

Modified zeolites with cations, clusters and metallic nanoparticles are of interest due to the possibility to develop materials with new properties [1]. A study concerning the thermal reduction under hydrogen flow of mixed bimetallic system Cu2+ and Ag+ exchanged natural clinoptilolite Tasajeras deposit, Cuba is presented. The influence of silver in the reduction of Cu2+ was studied. X-Ray diffraction experiments on natural clinoptilolite, as well as on ion-exchanged monometallic and bimetallic samples were performed. The peak intensities of the exchanged samples, are changed as a result of the ion exchange, and they are fundamentally associated with differences in nature, amount and position of the extra-framework ions in the channels of the natural zeolites [2, 3]. The fig 1 shows the magnitude of the Fourier Transform (FT) for the EXAFS signals of the natural clinoptilolite exchanged with Cu2+ and of the natural clinoptilolite exchanged with both Cu2+ and Ag+ and reduced at 150 oC . The TF of the experimental EXAFS signal of metallic Cu, has been also added. The amplitude of the peak at 1.82 Å, associated with the coordination of Cu2+ with 4 oxygen atoms, decreases upon Cu was added. A detailed analysis of the graph reveals also that for the exchanged and reduced samples a peak at 2.58 Å begins to emerge. This maximum coincides with the first maximum of the metallic Cu and may be associated with the formation of small clusters of Cu. The addition of Ag+ favors the decrease of the reduction temperature of Cu2+. The reduction of Cu2+ and Ag+ cations shows existence of notable interinfluence between both cations during this process. The Cu2+ reduction is favored by the presence of Ag+. The aggregation of the reduced highly dispersed species both for copper and silver is limited in this bimetallic system. The introduction of Ag+ as the second cation in the copper exchanged zeolites appears to be an efficient tool for the control of the size of the resultant reduced nanoparticles.