REE

Author: JIMÉNEZ MELERO ENRIQUE.
Year: 2004.
University: COMPLUTENSE DE MADRID [www.ucm.es].
Place of defense: FACULTAD DE CIENCIAS QUIMICAS.
Place of preparation: FACULTAD DE CIENCIAS QUIMICAS.

Summary: During this work has been done to study both Crystallographic as magnetic oxides RCrO4 (R = rare earth). To do so, the samples were prepared by the method of synthesis in the initial heat the mixture of nitrates in the presence of a flow of oxygen, in order to get the chromium in the state of oxidation 5 +. The optimization of temperature and reaction time has enabled the acquisition of various compounds as unique stages. In addition, the use of a process of freezing or "quenching" has led to the first isolation of polymorphic zirconia of PrCrO4. However, obtaining polymorphic of PrCrO4 has been possible only with the high pressure job during their synthesis. The structural characterization of the samples thus obtained has been carried out by X-ray diffraction neutron diffraction at room temperature. Data analysis has confirmed the structure type monazita taken by the LaCrO4, while the remaining compounds RCrO4 present the structural zirconia. Using neutron diffraction experiments were obtained values very precise coordinates of the oxygen atoms, which has in turn formed the basis for the correct description of the two structures, as well as for the evaluation of structural parameters depending rare earth ion. Moreover, there is in all these compounds a spatial distribution of micro - which turns out to be more pronounced along the diagonal measurement of the cell unit tetragonal high temperature. In the case of oxides RCrO4 (R = Nd, Tb, Tm, Yb), it has been observed experimentally using neutron diffraction data from high-resolution, the existence of a structural distortion at low temperatures, leading to a symmetry ortorrómbica. The group that characterizes the spatial symmetry is the Fddd (Nd, Tb, Tm) or Imma (Yb), respectively. The precise data obtained from distances and angles liaison coordination polyhedra [CrO4t And [ROS] 13-han allowed to study the change experienced by these polyhedra between the tetragonal phase high temperature and ortorrómbica low temperature. The study of the magnetic properties of these oxides has been initiated through measures magnetic susceptibility and magnetization. Experimental data clearly indicate that all compounds studied show an order magnetic at low temperatures. However, both the type of order (ferro or antiferromagnético) and the value of the critical temperature dependent on the rare earth ion present in the crystal structure. This information has been very useful in planning the neutron diffraction experiments at low temperatures, which have allowed the ultimate determination of the magnetic structure in each case. We must emphasize the use successfully for hot neutron diffraction experiments in the oxide GdCrO4, which led to the determination of their magnetic structure. In addition, it has employed a number of techniques in some of magnetic oxides RCrO4, with the aim of obtaining additional valuable information on their magnetic properties at the local level. These techniques spectroscopy Mossbauer of rare earth isotopes, Rotación-relajación of muones and Correlations angular disturbed. In this regard, it should be noted, for example, the first order magnetic transition observed in the sub-Tm3 + e induced by the order in Cr5 +. Finally, the detailed knowledge gained during this work on the magnetic properties of the compounds studied, it has proved very useful in the analysis and interpretation of experimental data Depolarización neutron. Thus, it has been assessed fOrIna pr 8 ecisa the 388 influence of the intrinsic magnetic anisotropy of the rare earth ions in the evolution of the magnetic domain structure with temperature and magnetic field applied.

Author: URONES GARROTE ESTEBAN.
Year: 2005.
University: COMPLUTENSE DE MADRID [www.ucm.es].
Place of defense: FACULTAD DE CIENCIAS QUÍMICAS.
Place of preparation: FACULTAD DE CIENCIAS QUÍMICAS.

Summary: This research work is the preparation and characterization estructural-microestructural by transmission electron microscopy techniques and associated X-ray diffraction and dust, new sulphides based Jan rare earth elements. Moreover, it has made the study of its potential applications as inorganic pigments greener plastics and paints. Systems that will be considered include different colored solid solutions, structural whose behavior is different. Firstly, the solid solution (Ce1-yNdy) 10S14 + xO1-x presents the structure type beta-Pr10S14O of sesquisulfuros of rare earths. The systems developed as M1-xR (2 / 3) x (1 / 3) xS (M = Ca, Mg, R = Yb) (cationic vacancies) present the structure type NaCl. When M: C has been observed the existence of intensity difractada diffuse in the electron diffraction diagrams, which may be associated with the presence of custers a fledgling cubic superstructure is observed at high concentrations of Yb3 +. In the case where: Mb, has been characterized using transmission electron microscopy and spectroscopy EELS the structural transition from the NaCl type to type spinel, through a short-range order, led by three-dimensional clusters and segretación defect length. The system described as M (3 / 2) yR2-y0.25 - (1 / 2) yS3 (M = C, R = Nd) presents the structure type gamma-Th3P4 to introduce Ca2 +, which is the stabilization of this type structural 1173K, whereas in the absence of CA2 + is observed phase alfa-Nd2S3. In addition, it has developed new phases in the system Mg-Yb-S, whose structures have been determined by TEM and associated techniques such as microdifracción electron. Its structure can be derived from the type NaCl by the repeated and regular implementation of the operation of crystallographic maclado chemical level cell unit. The esteuiometrías observed are MgYb2S4 and Mg2Yb6S11 (symmetry ortorróbmica, space group Cmcm); M3Yb8S15 and MgYb4S7 (monoclínica and symmetry space group C2 / m). The color of the samples were quantified using coordinates colorimetric L * a * b, obtained from the spectra corresponding to diffuse reflectance in the visible spectrum. The system Ce-Nd-SO colored red, whose tone varies with the composition of the solid solution. The system Ca-Nd-Sy Ca-Yb-S presented green high clarity, while the system Mg-Yb-S presents yellow, whose best quality was found in the nominal composition x = 0.30.