Papers by Keyword: Neutron Powder Diffraction

Abstract: The structures of as-cast LaNi_3.8AlMn_0.2 alloys and subsequent compounds by means of annealing at different temperature (850, 900, 950, 1000 ^oC) were examined by using neutron powder diffraction (NPD) and X-ray diffraction (XRD). Based on the Rietveld method, the diffraction data was refined using FullProf software. The refined results demonstrate the structure types of all compounds are CaCu₅ type and their space groups are P6/mmm. Increasing the annealing temperature, the lattice parameters of LaNi_3.8AlMn_0.2 compounds did not possess clearly linear variation. It is noted that Mn atoms do not occupy the 2c sites but occupy the 3g sites in all compounds.

Abstract: The intermetallic alloy LaNi_3.8AlMn_0.2 and its deuteride LaNi_3.8AlMn_0.2D_3.2 were studied by neutron powder diffraction. The experimental results show that the crystal structure of LaNi_3.8AlMn_0.2 is CaCu₅ type with the hexagonal P6/mmm space group, the substituted Al atoms occupy 2c and 3g sites, while Mn atoms are only located on the 3g sites. For the corresponding deuteride LaNi_3.8AlMn_0.2D_3.2, the P6/mmm space group gives the best refinement, but D atoms enter two interstitial sites 6m and 12n.

Abstract: The effect of manganese doping on the magnetic and structural properties of strontiumtitanate (SrTiO3) was studied. Neutron powder diffraction, x-ray diffraction, magneticmeasurements, scanning electron microscopy and energy dispersive spectroscopy of x-rays wereutilized. Air-sintered Sr(MnxTi1-x)O3 (SMT) with x = 0:02 and x = 0:05 were homogeneoussingle phase orthorhombic (space group Pbnm) perovskite samples. The symmetry was orthorhombicalready at room temperature, and no symmetry change was observed down to 11K. An anomaly in the magnetic susceptibility was observed in x = 0:02 sample at 75 K. Incontrast to earlier reports, no ferromagnetism was observed.

Abstract: In 1949 Shull et al. [1] used for the first time neutrons for the determination of a magnetic structure. Ever since, the need for neutrons for the study of magnetism has increased. Two main reasons can be brought forward to explain this ongoing success: First of all a strong rise in research on functional materials (founding obliges) and secondly the increasing availability of easy to use programmes for the treatment of magnetic neutron diffraction data. The giant magnetoresistance effect, multiferroic materials, magnetoelasticity, magnetic shape memory alloys, magnetocaloric materials, high temperature superconductivity or spin polarized half metals: The last 15 years have seen the event of all these “hot topics” where the knowledge of the magnetism is a prerequisite for understanding the underlying functional mechanisms. Refinement programs like FULLPROF or GSAS and programs for magnetic symmetry analysis like BASIREPS or SARAH make the determination of magnetic structures accessible for non specialists. Following a historical overview on the use of neutron powder diffraction for the determination of magnetic structures, I will try to convince you of the easiness of using magnetic symmetry analysis for the determination of magnetic structures using some recent examples of own research on the rare earth iron borate TbFe3(BO3)4 and the rare earth transition metal telluride Ho6FeTe2.

Abstract: Crystal structures of Bi4Ti3O12 ferroelectrics have been investigated by high-temperature neutron powder diffraction study. The ferroelectricity in Bi4Ti3O12 is shown to originate from the in-plane displacement of TiO6 octahedra with respect to heavy Bi ions, and that the off-center Ti displacement in the octahedra plays a minor role. Bond valence analysis shows that the underbonding of Bi with the adjacent oxide ions at the perovskite A site in high-temperature paraelectric state plays an essential role in the ferroelectric phase transition in the Bi4Ti3O12 system.

Abstract: Sorption of ten different quantities of D2 within the porous coordination metal framework Cu3(1,3,5-benzenetricarboxylate)2 were studied using neutron powder diffraction. D2 sorption in the porous material reached saturation (at 25 K) at an amount equivalent to ~6.5 wt. % H2. Rietveld analyses revealed that at saturation there are two saturated small pore and three saturated large pore sites, in addition to the primary site at the coordinatively unsaturated Cu. We reveal that the linked bimodal pore system provides up to nine distinct sites for D2, at moderate D2 concentrations up to saturation. Intermediate sites are attributed to local D2-D2 interaction within the confined pore spaces. Within the small and large pores one, and two, intermediate sites are identified, respectively. The population of D2 at each site was found to affect the host lattice.

Abstract: The crystal structure of R-phase in Ti50.75.Ni47.75.Fe1.50 shape memory alloy (SMA) has been studied at a temperature of (290 ± 7) K on cooling by combined synchrotron and neutron powder diffraction using Rietveld refinement with generalized spherical harmonic (GSH) description for preferred orientation (PO). The results showed that (i) no significant improvement in the crystallographic RWP-factor was found when the inversion center was removed from the 3 P model, suggesting that the space group was indeed 3 P and not lower symmetry 3 P neither m P31 and (ii) the refined atomic parameters were converging only when the 3 P space group was used in the refinement.

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 magnetic structure of CrNiAs was determined from magnetisation and powder neutron diffraction experiments. In the 110-182 K temperature range, the compound is ferromagnetic, resulting mainly from the Cr moment of 1.25(5) μ_B. Below 110 K, neutron diffraction data reveals typical satellite reflections which propagation vector is 0.228 c*. A model of oscillating magnetic moments was refined from the integrated intensity of the few additional lines.

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.