María Luisa Veiga

Crystal and magnetic structure of the perovskites La2MTiO6(M = Co, Ni)

The crystal structures of the perovskites La2MTiO6 n(M = Co and Ni) were determined on the basis of powder neutron diffraction data collected at room temperature. Refinements of the data showed that these compounds have monoclinic symmetry, space group P21/n, and the common structure is a perovskite-type with a partially ordered arrangement of M(II) and Ti(IV) cations on the 6-coordination sites. n Magnetic structures of type II have been determined from neutron diffraction data collected at 1.5 K and it was found that there was a long range antiferromagnetic ordering where only the M2+ ions are responsible for their magnetic properties. The antiferromagnetic ordering is explained by means of a propagation vector [k n= n½, 0, ½] and the respective magnetic moments at 1.5 K are 2.88 μB for nCo and 2.05 μB for Ni.

Synthesis and magnetic properties of crystalline CrVO4

Crystalline CrVO/sub 4/ was prepared by solid state reaction from Cr/sub 2/O/sub 3/ and V/sub 2/O/sub 5/ at 723 K. Their crystallographic parameters, and atomic positions were refined by the powder X-Ray method and their i.r. absorption bands were assigned. Magnetic properties were discussed on the basis of a 1D model.

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.

Synthesis, crystal structure, and some physico-chemical properties of Te3SeO8

Abstract Monocrystals of a tellurium(IV) and selenium(IV) mixed oxide of composition Te3SeO8 have been prepared by solid state reaction between TeO3 and SeO2. The assignment of its IR absorption spectra has been achieved and its thermal decomposition process has been followed using TG and DSC techniques. The crystal structure of this new material has been determined by single crystal X-ray diffraction techniques: the compound has triclinic symmetry with lattice parameters: a = 4.854(1) A, b = 6.902(2) A, c = 11.515(2) A, α = 102.02(2)°, β = 100.80(1)°, and γ = 69.84(2)° (R = 0.027 for 1662 observed reflexions).

Magnetic characterization of the LaNi1−xSbxO3(0≤x≤13) perovskite-like system

Abstract The magnetic characterization of the LaNi1−xSbxO3 (0≤x≤ 1 3 ) perovskite-like system is reported. These compounds can be regarded as being derived from LaNiO3 by partial substitution of Ni3+ by Sb5+ cations with the concomitant partial reduction of Ni3+ to Ni2+. Formally, for x=0 only Ni3+ is present, whereas for x= 1 3 Ni cations are stabilized as Ni2+. Magnetic susceptibility measurements show that a transition from a Pauli paramagnetic behaviour (x=0.05) to complex magnetic interactions takes place in this system. The nature of such interactions is interpreted as antiferromagnetic with an important canting of the spin arrangement, giving rise to a noticeable ferromagnetic component. These results have been qualitatively confirmed using the EPR technique.

Cation ordering in distorted perovskites (MLa)(MgTe)O6, M = Na, K

The synthesis and crystal structure of the pseudo-orthorhombic (a≈b≈√2a0, c≈2a0) ordered perovskite MLaMgTeO6(M = Na and K) are reported. X-ray, electron and neutron diffraction and microscopy established that both compounds have a perovskite structure with ordering in the A and B sublattices.

Synthesis and structural characterization of a new perovskite series derived from LaNiO3: La5Ni4MO15(M Mo, Te, W)

The synthesis and crystal structure of the orthorhombic La5Ni4MO15(M Mo, Te, W) compounds are reported. These compounds can be regarded as being derived from LaNiO3 by substitution of 20% of Ni3+ by M6+ with the consequent reduction of some Ni3+ to Ni2+ Neutron, X-ray and electron diffraction data have established that these compounds are isostructural and have a distorted perovskite structure in which the cations placed in B sites are randomly distributed.

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.

New antimonate–vanadate with the rutile structure

VCrSbO6 has been prepared and its magnetic and electronic properties investigated. The crystal structure was refined from X-ray powder diffraction by the Rietveld method. The unit cell is tetragonal (space group P42/mnm, Z= 2), a= 4.5708(4)A and c= 3.0281(3)A. The compound is isostructural with the rutile phase FeNbO4 and stoichiometric with respect to all three constituent metals atoms. The electron diffraction pattern is consistent with a tetragonal symmetry.

Dielectric response and ionic conductivity of Cs(NbTe)O6

Abstract A study of the dielectric response in the frequency range 10 Hz to 10 7 Hz has been conducted on Cs(NbTe)O 6 defect pyrochlore oxides. Two different behaviours are identified: one characteristic of a volume conduction process by Cs + ions and other due to a contact effect. Ionic conductivity is derived from complex impedance plots and an activation energy of 0.67 eV is deduced for ion motion.