M. Gaitán

Structure and magnetic properties of LiMVO4 (M = Co, Ni, Cu) spinels

The two-dimensional antiferromagnetic correlations found for LiCuVO4 below 30 K have been explained by considering the ordered distribution of lithium and copper in the octahedral sites.

Kinetic studies by DSC on the thermal decomposition of calcium oxalate

Abstract In the present paper, we have studied the kinetics of the thermal decomposition of calcium oxalate monohydrate, using a new method of kinetic study employing data obtained from differential scanning calorimetry, as previously proposed by us. Initial products and those obtained following decomposition have been identified by infrared spectroscopy and X-ray powder diffraction. A comparison between the results obtained in this paper and those reported in the literature has also been made.

Relationship between crystal structure and electric properties for lithium-containing spinels

Abstract The crystal structure of the spinel LiZnNbO 4 has been refined by Rietvelds analysis of powder X-ray diffraction data. The structure has tetragonal symmetry and the cell parameters are: a = 6.0824(1), c = 8.4035(7) A , space group P 4122 (Z = 4). The atomic positions are: Zn (4 c ), Li (4 b ), Nb (4 a ), 01 (8 d ) and 02 (8 d ). A different ionic conductivity behaviour for the isostructural spinels LiZnNbO 4 and Li 2 TeO 4 is detected and it is discussed on the basis of the mobile ion sites in both compounds.

A method for the kinetic analysis of nonisothermal decomposition reactions of solids

Abstract A new method, which allows the determination of the mechanism of a decomposition reaction of solids and leads to reliable kinetic parameters, is proposed. The test of the validity of the method is performed with seven simulated TG and DTG curves.

Application of a new method to the kinetic study of the thermal decarbonylation of sodium oxalate

Abstract The application of a new method for studying the kinetic parameters of the thermal decarbonylation of sodium oxalate in dinitrogen using differential scanning calorimetry is described. The activation energy and the pre-exponential Arrhenius factor for the loss of carbon monoxide are calculated from the DSC curves and the reaction order and mechanism are also determined. A comparison is made between the results obtained using the proposed method and those reported in the literature.

Synthesis, structural characterization and properties of solid solutions TiCuSbO with rutile structure

Abstract Solid solutions of stoichiometry Ti 1−x Cu x/3 Sb 2x/3 O 2 in the range 0.1 ≤ × ≤ 0.4 are obtained by thermal treatment of the appropriate amounts of simple oxides. These phases show a rutile-like structure in which Ti atoms are randomly substituted by Cu and Sb atoms in octahedral coordination of oxygen; the Jahn-Teller effect associated to Cu 2+ ions provokes an additions deformation of these octahedra, as afonction of the composition. Electron spin resonance and electrical conductivity measurements are also discussed.

New Methods of Synthesis of Mixed Oxides of te and Pb: Characterization of the New Phases PbTeO3 (Cubic) and PbTeO4 (Orthorhombic)

Abstract New methods of synthesis for mixed oxides of Te(IV) or Te(VI) and Pb(II) are described in this paper. The reactants were orthotelluric acid or tellurium trioxide, lead(II) acetate or hydrated oxide. The solid state reactions were carried out in air or in sealed tubes under vacuum. The isolated solid phases PbTeO3 (tetragonal and cubic), PbTeO3, Pb2Te3O8 and PbTeO4 were characterized by their X-ray diffraction powder patterns and by other techniques. Although some of these materials have been described previously, the experimental conditions employed in this work permit the isolation of some new phases.

Nb2Te2O9 and Ta2Te2O9: two new mixed oxides of Te(IV)

Nb/sub 2/Te/sub 2/O/sub 9/ and Ta/sub 2/Te/sub 2/O/sub 9/ were prepared by solid state reactions between amorphous TeO/sub 3/(s) and metallic pentoxides of Nb and Ta. A crystallographic analysis carried out by X-ray diffraction showed that these compounds are isostructural (space group: P2/sub 1//C. a = 6.883 A, b = 7.853 A, c = 14.591 A, ..beta.. = 103.66 for Nb/sub 2/Te/sub 2/O/sub 9/ and a = 7.10 A, b = 7.48 A, c = 14.62 A, ..beta.. = 102.9 for Ta/sub 2/Te/sub 2/O/sub 9/). The IR spectra and thermal decomposition processes of both mixed oxides were studied.

Structural study and magnetic measurements of some perovskites MLnLiTeO6

Abstract The crystal structures of the perovskites MLnLiTeO6 (M=Ca, Sr and Ba) were determined from X-ray diffraction patterns. Rietveld analysis of the data gave at ambient temperature an ordered distribution of Li+ and Te6+ formal cations in B sites whereas the M2+ and Ln3+ cations occupy the A sites at random. Space groups and lattice parameters of these oxides are discussed on the basis of the distortions on the ideal perovskite structure. On the other hand, magnetic measurements are compatible with this distorted model in which paramagnetic cations are isolated.

Thermal behaviour of hydrated pyrochlores K(NbW)O6 · H2O and K(TaW)O6 · H2O

The pyrochlore-type compounds KMWO6 · H2O (M = Nb and Ta) have been synthesized and their dehydration kinetics have been studied by thermogravimetric analysis. The proposed diffusion mechanism follows the structural model previously described in the literature.