Papers by Author: S.-G. Eriksson

Abstract: Structural, magnetic, dielectric properties and Mossbauer effect were investigated on complex perovskite with composition AFe_2/3B_1/3O₃ (A=Ca,Sr,Pb,Ba; B=W,Te). The most striking feature of this type of complex perovskites is the coexistence of magnetic and antiferroelectric types of ordering in a certain temperature interval. It was found that ferrimagnetic Ca and Sr compounds belong to a partially ordered perovskite structure, and antiferromagnetic Pb phase to a disordered one. The possible models for nuclear and magnetic structures were proposed in accordance with the observed dielectric and magnetic properties.

Abstract: The double perovskites Ca_2-xSr_xMnWO₆ (x = 0.0, 0.5, 1.0, 1.5, 2.0) have been prepared as pure phases by a conventional solid-state reaction process. Stoichiometric amounts of CaCO₃,SrCO₃, MnO and WO₃ were mixed, ground and calcined in a nitrogen atmosphere. X-ray and neutron powder diffraction (NPD), together with magnetisation measurements were carried out in order to characterise the materials. The NPD and magnetisation measurements, performed at different temperatures, were made in order to correlate the materials structural and magnetic properties. Susceptibility- Temperature (ϰ-T) curves were measured between 2K and 200K in field cooled (FC) and zero-field-cooled (ZFC) modes with an applied field (H) up to 3T. Rietveld analysis of NPD data shows that the samples were B-site ordered perovskites with different types of lattice symmetry. Magnetisation measurements at low temperatures indicate an antiferromagnetic ordering of the magnetic moments.

Abstract: We have performed neutron powder diffraction (NPD) experiments on polycrystalline powders with nominal compositions (Ca_0.5La_0.5)(Ba_1.25La_0.75)Cu₃O₇- δ , (Ca_0.6La_0.4)(Ba_1.15La_0.85)Cu₃O₇-δ and (Ca_0.8La_0.2)(Ba_0.95La_1.05)Cu₃O₇-δ . The diffraction patterns, analysed by the Rietveld method, show that all samples consist mainly of a tetragonal Y-123 type phase. Unit cell parameters a and c shorten as the calcium content increases: a = 3.8660(2), 3.8634(3), and 3.8624(5) Å; c = 11.6325(11), 11.6143(14), and 11.5822(20) Å for x-values 0.5, 0.6, and 0.8, respectively. For the x = 0.6 and 0.8 samples the Rietveld refinement of calcium occupancies and EDX analysis suggest that the actual composition is closer to x ≈ 0.5. However, since the lattice parameters do change, it is also suggested that at these higher doping levels calcium does enter the Y- site to a larger extent than for the x = 0.5 composition. This is also in accordance with previously reported values for the T_c, which decreases slighty as x changes from 0.5 to 0.6 and has a pronounced change from 80 K to 73 K for x = 0.6 and 0.8, respectively.

Abstract: Solid solutions of Ga_xCoFe_1-xCrO₄ ( x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were obtained by solid state reactions in air at 1623K. X-ray and neutron powder diffraction measurements have been performed on the materials in order to characterize them. Rietveld refinement of neutron powder diffraction data showed cubic symmetry corresponding to the space group Fd-3m for all of the samples in the series. The results reveal that Ga has been substituted for Fe selectively. Ga exclusively occupies the octahedral site up to x = 0.8, while at x =1.0 a small amount of Ga (5%)enters into the tetrahedral site in the compound. Cr, Fe and Co cations are distributed between the tetrahedral 8 a and octahedral 16 d sites. Their proportion at each site varies with composition. The fractional coordinates of oxygen are almost the same in all the samples while the lattice parameter shows a slight decrease with increasing Ga content. The magnetic structure at room temperature was ferrimagnetic for the composition x = 0.0 and x = 0.2.

Abstract: The evolution of the crystal structures as a function of temperature of the new solid- solution KTiOPO₄ (KTP) isomorphs, namely K_1-xNb_xTi_1-xOPO₄ and K_1-xSb_xTi_1-xOPO₄ have been studied by neutron powder diffraction below and above the ferroelectric phase transition temperatures. On the basis of the experimental data obtained the cation distribution, among the sublattices of this structure, and the distortion of the framework at the phase transition, have been discussed. The possible correlation between the non-linear optical properties and the dominant structural features tailored by compositional variation has also been studied.