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Pregelj M.,Jozef Stefan Institute | Pregelj M.,Laboratory for Neutron Scattering | Jeschke H.O.,Goethe University Frankfurt | Feldner H.,Goethe University Frankfurt | And 12 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

A combination of density functional theory calculations, many-body model considerations, and magnetization and electron-spin-resonance measurements shows that the multiferroic FeTe 2O 5Br should be described as a system of alternating antiferromagnetic S=5/2 chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer model. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization. © 2012 American Physical Society.

Parekh K.,Indian Institute of Technology Gandhinagar | Almasy L.,Laboratory for Neutron Scattering | Lee H.S.,Korea Institute of Geoscience and Mineral Resources | Upadhyay R.V.,Charotar University of Science & Technology
Applied Physics A: Materials Science and Processing | Year: 2012

Zero field cooled dc-magnetization measurements of monodispersed Mn 0.5Zn 0.5Fe 2O 4 nanoparticles dispersed in kerosene exhibit two transitions at low temperatures. These transitions correspond to (i) the superparamagnetic to blocked superparamagnetic and (ii) the blocked superparamagnetic to surface spin-glass like/quantum superparamagnetic state upon lowering the temperature. The existence of a disorder surface is confirmed by recording small-angle neutron scattering data below and above the Curie temperature. Magnetic relaxation analysis shows a plateau at low temperature (below 5 K) with a slight minimum at 3 K, which is a characteristic of the surface spin-glass-like state. This is analyzed considering the energy distribution n(E)1/E. The existence of surface disorder dominates at low temperature and mimics the transition from superparamagnetic to quantum superparamagnetic states. © 2011 Springer-Verlag.

Pregelj M.,Jozef Stefan Institute | Pregelj M.,Laboratory for Neutron Scattering | Zorko A.,Jozef Stefan Institute | Zaharko O.,Laboratory for Neutron Scattering | And 5 more authors.
Physical Review Letters | Year: 2012

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3)mK (T/TN∼1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T→0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence. © 2012 American Physical Society.

Marczak W.,University of Silesia | Czech B.,University of Silesia | Almasy L.,Laboratory for Neutron Scattering | Almasy L.,Hungarian Academy of Sciences | Lairez D.,CEA Saclay Nuclear Research Center
Physical Chemistry Chemical Physics | Year: 2011

Small-angle neutron scattering proved that molecules in aqueous solutions of pyridine, 2-methylpyridine and 2,6-dimethylpyridine form clusters. The clusters are dynamic aggregates consisting of hydrogen-bonded water-amine complexes. Strengthening of the hydrogen bonds between water and amine molecules due to the methyl groups in the ortho position in the pyridine ring makes the structures more stable, as was evidenced by relatively long times of the structural relaxation. The strong intermolecular forces affect the thermal expansion of the systems. No aggregates similar to those in aqueous systems are present in the methanolic ones. That points to the crucial role of water in the molecular clustering. A molecule of methanol, although capable of hydrogen bonding with the amines, cannot participate in larger structures because of the lack of protons that could form the enhanced network. Thus, even if the amine-methanol complexes occur, they are incapable of further association. It was shown that the co-operative nature of hydrogen bonds and the propensity of water to association are the main factors that determine the properties of aqueous systems. © the Owner Societies. 2011.

Thakur N.,Bhabha Atomic Research Center | Yusuf S.M.,Bhabha Atomic Research Center | Paulose P.L.,Tata Institute of Fundamental Research | Keller L.,Laboratory for Neutron Scattering
Journal of Applied Physics | Year: 2012

We have controlled the nature of magnetic ordering in Rb xBa yMn 3-(x 2y)/2Fe(CN) 6zH 2O prussian blue analogues by alkaline earth metal cation (Ba 2) substitution and application of an external magnetic field. Structural analysis, by employing Rietveld refinement of X-ray and neutron diffraction patterns, reveals a tetragonal crystal structure for all these compounds. The presence of Fe 3-C N-Mn 2 chains in the structure, and absence of both cyanide flipping (Mn 2-C N-Fe 3) and charge transfer from Mn 2 to Fe 3 (Fe 2-C N-Mn 3) have been confirmed by infrared and Mössbauer studies. The analysis of the low temperature neutron powder diffraction data reveals a long-range antiferromagnetic ordering in these compounds. The magnetic structure is layered, consisting of an antiparallel stacking of ferromagnetic sheets along the crystallographic c-direction. Within each sheet in the ab plane, the ordered moments of Mn 2 5.04(1) B for (x 0.84, y 0) and 4.99(7) B for (x 0.19, y 0.3) sample at 1.5 K and Fe 3 0.98(3) B for (x 0.84, y 0) and 0.99(3) B for (x 0.19, y 0.3) sample at 1.5 K are aligned ferromagnetically along the c-axis. Interestingly, dc magnetization study indicates that on application of an external magnetic field, an antiferromagnetic to ferrimagnetic-like phase transition occurs below ∼5 K. This phase transition becomes more and more prominent with increasing magnetic field as well as for higher Ba substitution. The role of a number of factors (such as ionic radius of Ba 2 ion and change in net ligand strength around Mn due to chemical insertion of Ba 2 ions) on the affecting nature of magnetic ordering has been discussed. The observed magnetism in these compounds is seen in light of the orbital model of superexchange interaction between magnetic ions (Mn 2 and Fe 3) mediated by cyanide ligands. © 2012 American Institute of Physics.

Cervellino A.,Paul Scherrer Institute | Cervellino A.,Laboratory for Neutron Scattering | Gvasaliya S.N.,Laboratory for Neutron Scattering | Gvasaliya S.N.,ETH Zurich | And 7 more authors.
Journal of Applied Crystallography | Year: 2011

The relaxor ferroelectric PbMg1/3Ta2/3O3 was studied by single-crystal neutron and synchrotron X-ray diffraction, and its detailed atomic structure modelled in terms of static Pb displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg1/3Nb2/3O3 or PbZn1/3Nb2/3O3 the diffuse scattering in the [H00]/[0K0] scattering plane has a butterfly shape around the h00 Bragg reflections and is orthogonal to the scattering vector for h h0 peaks. In the [H H0]/[00L] plane the diffuse scattering is elongated along the 112 directions and is orthogonal to the scattering vector for h h h reflections. It is found that a model consisting of correlated Pb displacements along the 111 directions reproduces adequately the main features of the diffuse scattering in PbMg 1/3Ta2/3O3 when the correlation lengths between the Pb-ion displacement vectors are longest along the 111 and shortest along the 11 and 1 0 directions. © 2011 International Union of Crystallography Printed in Singapore - all rights reserved.

Justino L.L.G.,University of Coimbra | Ramos M.L.,University of Coimbra | Knaapila M.,Institute for Energy Technology of Norway | Marques A.T.,University of Coimbra | And 9 more authors.
Macromolecules | Year: 2011

Interactions between polymer chains of poly(9,9-dioctylfluorene-2,7-diyl) (PF8) have been studied in toluene solution over a wide concentration range using multinuclear NMR spectral and relaxation measurements with both the fully protonated and alkyl chain deuterated polymers, small angle neutron scattering (SANS), together with theoretical calculations using DFT and semiempirical methodologies. Full assignment of the 1H and 13C chemical shifts in the NMR spectra of isolated chains of PF8 has been made using DFT, and are in good agreement with spectra in chloroform and in toluene solutions. Somewhat different behavior is seen in toluene solution, where, upon increasing polymer concentration, broadening of the alkyl chain resonances is seen, consistent with interactions between the side chains. Similar behavior is seen with the 2H resonance of PF8-d34. In both cases, line-narrowing and restoration of the structured alkyl chain resonances is seen upon studying the spectra of concentrated solutions using the magic angle spinning (MAS) technique, in agreement with the attribution of the broadening to interchain interactions between the octyl groups. Support for this interpretation comes from 1H and 13C spin - lattice relaxation time measurements, which also show differences in group dynamics along the alkyl chains. Semiempirical theoretical calculations, using PM3 and PM6 Hamiltonians, add further support to the importance of interactions between the alkyl groups of separate PF8 chains. SANS measurements on PF8 in toluene solutions from very dilute to concentrated solutions, extended to ultrasmall q ranges, provide further insight. Three concentration ranges can be identified. In dilute solutions, the results suggest that PF8 is present as fully dissolved polymer coils. Upon increasing polymer concentration, an intermediate region is observed, in which there are indications of transient contacts between the polymers, which the NMR results suggest involves side chain interactions. As a consequence of interactions between the chains, gel formation occurs. On the basis of these and previous results, some general considerations are presented upon the solubility and aggregation behavior of PF8, including indications of how interactions between alkyl chains may be important in the stabilization of the so-called β-phase of PF8. © 2010 American Chemical Society.

Strunz P.,Nuclear Physics Institute of Czech Republic | Petrenec M.,Academy of Sciences of the Czech Republic | Gasser U.,Laboratory for Neutron Scattering | Tobias J.,Academy of Sciences of the Czech Republic | And 2 more authors.
Journal of Alloys and Compounds | Year: 2014

Nickel base superalloy IN738LC has been studied after low-cycle fatigue by Small Angle Neutron Scattering (SANS). Samples subjected to high-temperature low-cycle fatigue were annealed at various temperatures to change the size and the distribution of precipitates. Ex and in situ SANS and TEM studies were performed. It was found that additional precipitates are formed either during slow cooling from high temperatures or after reheating above 570 C. Their size and distribution were evaluated. The precipitates arise regardless the application of the mechanical load. Nevertheless, these small precipitates influence low-cycle fatigue resistance. From the SANS data, it can be also deduced that the equilibrium volume fraction of γ′-precipitates at temperatures from room temperature to 825 C is significantly higher than 45%. The kinetics of formation of small and medium-size γ′ precipitates at 700 and 800 C was determined as well. © 2013 Elsevier Inc. All rights reserved.

Knaapila M.,Institute for Energy Technology of Norway | Evans R.C.,Trinity College Dublin | Garamus V.M.,Helmholtz Center Geesthacht | Almasy L.,Laboratory for Neutron Scattering | And 5 more authors.
Langmuir | Year: 2010

We report on the phase transitions, solution structure, and consequent effect on the photophysical properties of poly[3-(6-trimethylammoniumhexyl) thiophene] bromide (P3TMAHT) in aqueous sodium dodecylsulfate (SDS). Polythiophene was mixed with SDS or deuterated SDS to form P3TMAHT(SDS)x complex (x = the molar ratio of surfactant over monomer units) in D 2O and studied by small-angle neutron and X-ray scattering (SANS/SAXS) and optical spectroscopy. At room temperature, P3TMAHT forms charged aggregates with interparticle order. The addition of SDS eliminates the interparticle order and leads to rod-like (x=1/5) or sheet-like polymer-SDS aggregates (x=1/2 to 1) containing rod-like (x=1/5 to 1/2) or sheet-like (x=1/2 to 1) polymer associations. Partial precipitation occurs at the charge compensation point (x = 1). Ellipsoidal particles without interparticle order, reminiscent of SDS micelles modified by separated polymer chains, occur for x=2 to 5. Free SDS micelles dominate for x=20. Structural transitions lead to a concomitant variation in the solution color from red (P3TMAHT) to violet (x=1/5 to 1) to yellow (x > 2). The photoluminescence fingerprint changes progressively from a broad featureless band (x = 0) through the band narrowing and appearance of vibronic structure (x=1/5 to 1) to the return to a blue-shifted broad emission band (x=5). The polymer stiffness reaches a maximum for x=1, which leads to minimization of the Stokes shift (0.08 eV). This work gives fundamental information upon how surfactant complexation can influence both the solution structure and photophysical properties of a water-soluble polythiophene. © 2010 American Chemical Society.

Larregola S.A.,National University of San Luis | Alonso J.A.,CSIC - Institute of Materials Science | Sheptyakov D.,Laboratory for Neutron Scattering | Alguero M.,CSIC - Institute of Materials Science | And 3 more authors.
Journal of the American Chemical Society | Year: 2010

The synthesis, crystal structure, and dielectric properties of the novel double perovskite Pb2TmSbO6 are described. The room-temperature crystal structure was determined by ab initio procedures from neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (SXRPD) data in the monoclinic C2/c (No. 15) space group. This double perovskite contains a completely ordered array of alternating TmO6 and SbO 6 octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with an a-b-b- tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy a highly asymmetric void with 8-fold coordination due to the stereoactivity of the Pb2+ lone electron pair. This compound presents three successive phase transitions in a narrow temperature range (at T1 = 385 K, T2 = 444 K, and T3 = 460 K in the heating run) as shown by differential scanning calorimetry (DSC) data. The crystal structure and temperature-dependent NPD follow the space-group sequence C2/c → P21/n → R3̄ → Fm3̄m. This is a novel polymorph succession in the high-temperature evolution of perovskite-type oxides. The Tm/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/ antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature obtained from the Curie-Weiss fit of the reciprocal permittivity. © 2010 American Chemical Society.

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