Analysis Of Neutron Diffraction Data Research Articles

Structural chemistry and magnetic properties of Pr 3−xSr 1+xCrNiO 8

Polycrystalline samples of the n=1 Ruddlesden–Popper system Pr 3− x Sr 1+ x CrNiO 8 have been synthesized over the composition range 0.0< x⩽1.0 either by the ceramic method or from solution. They have been characterized by an appropriate combination of diffraction methods (X-ray, neutron and electron) and magnetometry (d.c. and a.c.). All compositions having x>0.1 adopt the tetragonal space group I4/ mmm; Pr 2.9Sr 1.1CrNiO 8 adopts the orthorhombic space group Fmmm. There is no evidence of Cr/Ni cation ordering in any composition. A maximum in the zero-field cooled magnetic susceptibility is observed at a temperature T f that decreases with increasing Sr content; 52⩽ T f (K)⩽13. The frequency dependence of T f observed in a.c. susceptibility measurements, together with the analysis of neutron diffraction data, suggests that the atomic magnetic moments in these compositions adopt a spin-glass-like state below T f.

Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data

The short-range order (SRO) correlations in liquid- and rotator-phase states of carbon tetrachloride are revisited here. The correlation of some angular magnitudes is used to evaluate the positional and orientational correlations in the liquid as well as in the rotator phase. The results show significant similitudes in the relative position of the molecules surrounding a central one but striking differences in their relative orientations, which could explain the changes in SRO between the two phases and the puzzling behavior of the local density in the liquid and rotator phases.

Neutron diffraction studies of the magnetic ordering in the spinel oxide system Mg xCo 1− xCr xFe 2− xO 4

Neutron diffraction studies were performed on the spinel oxide system Mg x Co 1− x Cr x Fe 2− x O 4 ( x = 0.0, 0.25, 0.50, 0.75 and 1.0) as a function of temperature. Presence of magnetic long-range order at low temperatures was confirmed from the evolution of Bragg reflections with the decrease of temperature. Magnetic structural properties were determined from the Rietveld analysis of neutron diffraction data recorded in the temperature range 10–810 K. Sublattice and net magnetic moments for various compositions at various temperatures were found out from the analysis. The transition temperatures were also determined for all the compositions. A decreasing ferrimagnetic order with increasing substitution was revealed from the decreasing ordered sublattice moments. Huge diffuse signals around the 111 Bragg diffraction line were observed in the neutron patterns of the composition x = 1.0 below the Curie temperature indicating the existence of magnetic spin clusters. Hysteresis and low field dc magnetization measurements were done on the compositions x = 0.75 and 1.0. Thermo-magnetic irreversibility in the zero-field cooled and field cooled magnetization together with a shallow transition below Curie temperature was also observed. The various observations suggest a canted ferrimagnetic ordering in the composition range x ≤ 0.75 and a semi-spin glass like behavior at x = 1.0. A super-paramagnetic behavior below the paramagnetic state is also suggestive from the observed features.

In situ observation of the tetragonal–cubic phase transition in the CeZrO4 solid solution – a high-temperature neutron diffraction study

The crystal structure of the compositionally homogeneous ceria-zirconia solid solution CeZrO(4) is refined by Rietveld analysis of neutron diffraction data measured in situ over the temperature range 296-1831 K. The CeZrO(4) exhibits a tetragonal structure with the space group P4(2)/nmc at temperatures from 296 to 1542 K (Z = 1), and a cubic fluorite-type form with the space group Fm\overline 3 m at 1831 K (Z = 2). The isotropic atomic displacement parameters of Ce and Zr atoms B(Ce,Zr) and O atoms B(O) are found to increase with temperature, with B(O) being larger than B(Ce,Zr), suggesting the higher diffusivity of oxygen ions. The ratio of the c axial length to the a length of the pseudo-fluorite lattice (c/a(F) axial ratio) for the tetragonal CeZrO(4) phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching unity between 1542 and 1831 K. The displacement of O atoms along the c axis in the tetragonal CeZrO(4) phase increased from 296 to 1034 K and decreased from 1291 to 1542 K, reaching 0.0 A between 1542 and 1831 K. These results indicate that the cubic-to-tetragonal phase transition between 1542 and 1831 K is accompanied by oxygen displacement along the c axis and the increase of the c/a(F) axial ratio from unity.

Anisotropic displacement parameters for H atoms using an ONIOM approach

X-ray diffraction data cannot provide anisotropic displacement parameters (ADPs) for H atoms, a major outstanding problem in charge-density analysis of molecular crystals. Although neutron diffraction experiments are the preferred source of this information, for a variety of reasons they are possible only for a minority of materials of interest. To date, approximate procedures combine rigid-body analysis of the molecular heavy-atom skeleton, based on ADPs derived from the X-ray data, with estimates of internal motion provided by spectroscopic data, analyses of neutron diffraction data on related compounds, or ab initio calculations on isolated molecules. Building on these efforts, an improved methodology is presented, incorporating information on internal vibrational motion from ab initio cluster calculations using the ONIOM approach implemented in GAUSSIAN03. The method is tested by comparing model H-atom ADPs with reference values, largely from neutron diffraction experiments, for a variety of molecular crystals: benzene, 1-methyluracil, alpha-glycine, xylitol and 2-methyl-4-nitroaniline. The results are impressive and, as the method is based on widely available software, and is in principle widely applicable, it offers considerable promise in future charge-density studies of molecular crystals.

Local order and frustration in the geometrically frustrated spinelsCd1−xZnxV2O4

Orbitally degenerate frustrated spinels, ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$ were investigated using elastic and inelastic neutron scattering techniques. In the end members with $x=0$ and 1, a tetragonal distortion $(c&lt;a)$ has been observed upon cooling mediated by a Jahn-Teller distortion that gives rise to orbital ordering. This leads to the formation of spin chains in the $ab$-plane that upon further cooling, N\'eel ordering is established due to interchain coupling. In the doped compositions, however, the bulk susceptibility, $\ensuremath{\chi}$, shows that the macroscopic transitions to cooperative orbital ordering and long-range antiferromagnetic ordering are suppressed. However, the inelastic neutron scattering measurements suggest that the dynamic spin correlations at low temperatures have similar one-dimensional characteristics as those observed in the pure samples. The pair density function analysis of neutron diffraction data shows that the local atomic structure does not become random with doping but rather consists of two distinct environments corresponding to $\mathrm{Zn}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{Cd}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. This indicates that short-range orbital ordering is present which leads to the one-dimensional character of the spin correlations even in the low temperature cubic phase of the doped compositions.

GLOBAL PROPERTIES OF BIOMIMETIC MEMBRANES: PERSPECTIVES ON MOLECULAR FEATURES

Global properties of biological model membranes such as, e.g., structure or elasticity, are known to be closely related to their local features. If a membrane active compound interacts with the membrane assembly, the membrane will primarily be affected on the local, molecular level. The local perturbation may than, through some coupling, translate into a global adjustment of the membrane. In order to address this coupling x-ray and neutron diffraction data analysis techniques have been developed that allow accurate monitoring of changes in global properties. This offers new perspectives on molecular membrane features that in combination with complementary techniques, such as differential scanning calorimetry, spectroscopy or dynamic scattering lead to a better understanding of biomimetic membranes. The present article reviews these aspects giving application examples for single- and multicomponent membranes, respectively.

Magnetization and neutron diffraction studies on Dy5Si2Ge2

The compound Dy5Si2Ge2 crystallizes in an orthorhombic structure (Sm5Ge4 type, space group Pnma). Magnetization measurements performed in the temperature range of 2–300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (TN) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. The magnetization approaches saturation in a field of 6 T with a moment value of ∼8μB∕Dy3+. Neutron diffraction measurements, carried out at 9.2 K, suggest that Dy moments arrange spirally along the a axis giving rise to a canted antiferromagnetic structure. The analysis of neutron diffraction data yields an ordered state magnetic moment of 7.63μBperDy3+ ion.

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr)2YbCu2O6.95(2)

The effect of Sr II -for-Ba II isovalent substitution on the magnetic irreversibility field ( H irr ) of Cu(Ba 1− y Sr y ) 2 YbCu 2 O 6.95(2) (Cu-1212) sample series ( y =0–0.4) is studied to reveal guiding rules for tailoring the intrinsic H irr characteristics of high- T c superconductors. It has been assumed that substitution of the larger alkaline-earth cation, Ba II , by the smaller, Sr II , might improve the H irr characteristics as a consequence of the decrease in the thickness of nonsuperconductive blocking block (BB). However, results of the present work show that Sr substitution rather depresses the H irr characteristics of the Cu-1212-phase superconductors even though the thickness of BB decreases. Both the amount of excess oxygen and the overall positive charge are confirmed to remain constant upon the Sr substitution by wet-chemical and X-ray absorption near-edge structure analyses, respectively. However, from neutron diffraction data analysis, it is found that Sr substitution breaks the conductive CuO chains in BB by shifting part of the excess oxygen atoms from the characteristic b -axis lattice site to the a -axis site. This is believed to decrease the concentration of mobile holes in the BB, as supported by the results of TEP measurements. The lower H irr ( T ) lines of the Sr-substituted samples may thus be attributed to the lower concentration of mobile holes in BB.

Proton positions in spinel H 0.9Li 0.1[Li 0.33Ti 1.67]O 4, an ion-exchanged spinel Li 1[Li 0.33Ti 1.67]O 4

Spinel Li 1[Li 0.33Ti 1.67]O 4 is an attractive candidate for the use as anode material in Li-ion batteries. This work focusses on the structural changes of spinel Li 1[Li 0.33Ti 1.67]O 4 upon the exchange of Li-ions with protons leading to the composition H 0.9Li 0.1[Li 0.33Ti 1.67]O 4. The structure has been determined by means of Fourier density difference analysis of neutron diffraction data on the protonated material, collected over a wide range of d-spacings with high counting statistics. The protons replace the Li-ions on the crystallographic 8a sites, and also occupy a fraction of the 48f sites that are located in the same ab-plane as the 8a sites. These positions suggest that the protons prefer to be located near the oxygen atoms. Based on these results, a strong influence on the properties as Li-ion battery electrode material may be anticipated. The small change in unit-cell dimension and the considerable amount of hydrogen that can be hosted in spinel Li 1[Li 0.33Ti 1.67]O 4 are favorable for the use as fuel-cell electrolyte material.

Effect of Sr-for-Ba isovalent substitution on the local structure, hole distribution and magnetic irreversibility of Cu(Ba,Sr) 2YbCu 2O 6.95(2)

The effect of Sr II-for-Ba II isovalent substitution on the magnetic irreversibility field ( H irr) of Cu(Ba 1− y Sr y ) 2YbCu 2O 6.95(2) (Cu-1212) sample series ( y=0–0.4) is studied to reveal guiding rules for tailoring the intrinsic H irr characteristics of high- T c superconductors. It has been assumed that substitution of the larger alkaline-earth cation, Ba II, by the smaller, Sr II, might improve the H irr characteristics as a consequence of the decrease in the thickness of nonsuperconductive blocking block (BB). However, results of the present work show that Sr substitution rather depresses the H irr characteristics of the Cu-1212-phase superconductors even though the thickness of BB decreases. Both the amount of excess oxygen and the overall positive charge are confirmed to remain constant upon the Sr substitution by wet-chemical and X-ray absorption near-edge structure analyses, respectively. However, from neutron diffraction data analysis, it is found that Sr substitution breaks the conductive CuO chains in BB by shifting part of the excess oxygen atoms from the characteristic b-axis lattice site to the a-axis site. This is believed to decrease the concentration of mobile holes in the BB, as supported by the results of TEP measurements. The lower H irr( T) lines of the Sr-substituted samples may thus be attributed to the lower concentration of mobile holes in BB.

Local structure of Al-and Fe-based metallic glasses

The local atomic structures of Fe-and Al-based amorphous metallic glasseshave been investigated by the pair density function (PDF) analysis ofneutron diffraction data. In the Fe system, chemical substitution at thetransition metal ion sites induces significant changes of the local structuralenvironment. With the substitution of Nb for Zr and Mn for Fe, a pre-peak in thestructure function is observed that is indicative of short-range chemicalrearrangement. This is accompanied by a reduction of volume in realspace and a contraction of interatomic pair correlations that could inturn be associated with stronger glass forming ability. In the Al–Ni–Ndsystem, using the isotope difference neutron PDF technique, the localenvironments for Ni and Nd were separately determined. The Ni ions tendto form strong interactions with Al with significant shortening of thebond length and weaker interactions with Nd. The local Ni–Al cluster isalmost spherical with a coordination close to twelve nearest neighbours,while Ni–Ni chemical short-range ordering is quite strong as indicated bythe change of amplitude of the pre-peak with the isotopic substitution.On the other hand, the local Nd environment is more disordered withtwo separate Nd–Al coordination environments and weaker clustering.

Structural characterization and physical properties of the system La 1.33Li xCr xTi 2− xO 6

New phases which arise from partial substitution of Ti 4+ by Cr 3+ and Li + of the compound La 2/3TiO 3 have been obtained, giving rise to the series La 1.33Li x Cr x Ti 2− x O 6 ( x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic ( S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.

Multiple Li positions inside oxygen octahedra in lithiated TiO2 anatase.

Intercalation of Li in TiO2 anatase results in a phase separation in a Li-poor and a Li-rich phase. The local lithium configuration in the coexisting crystallographic phases is resolved by detailed analysis of neutron diffraction data. In each of the phases, two distinct positions within the octahedral interstices are found, with a temperature-dependent occupancy. A combination of quasi-elastic neutron scattering and force field molecular dynamics simulations shows that Li is hopping on a picosecond time scale between the two sites in the octahedral interstices. The results also suggest a specific Li arrangement along the crystallographic a direction, albeit without long range order. It is likely that multiple discrete Li sites within a distorted oxygen octahedron occur not only in intercalated TiO2 anatase but also in other (transition metal) oxides.

Local Order in Amorphous Fe-alloys

ABSTRACTThe pair density function analysis of neutron diffraction data of Fe-based metallic glasses of the Zr and Mo series shows how the local atomic structure changes by chemical substitution. The results provide evidence for short-range chemical reorganization accompanied by a volume contraction that could in turn be associated with stronger glass forming ability. While the existence of chemical short-range topological ordering is enhanced in both systems by alloying with a transition metal such as Mn, locally, the atomic structure changes in a way that corresponds to an increase in bonding interactions. The shortening of bonds is also related to volume contraction that can in turn be associated with a reduction of the ferromagnetic coupling of the Fe sublattice and to a lower Curie transition temperature.

Molecular Conformation and Structural Correlations of Liquid D-tert-Butanol at Room Temperature by Neutron Diffraction

Abstract An analysis of neutron diffraction data of liquid deuterated terr-butanol at room temperature to determine its molecular conformation is presented. Being a big molecule of 15 sites, the analysis is tricky and needs careful consideration. The resulting molecular parameters are compared with those obtained from other experimental data analysis and model calculations. The information about the intermolecular structural correlations, hydrogen-bonded molecular association in particular is also obtained from the diffraction data analysis. -PACS number: 61.25

Thermal Spray Yttria-Stabilized Zirconia Phase Changes during Annealing

Phase stability of thermal barrier deposits made from yttria partially stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000, 1200, and 1400 °C with times from 1 to 1000 h has been evaluated using the Rietveld analysis of neutron diffraction data. Results show that the yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, about 3.5 mol.% of YO1,5, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observed after annealing for 24 h at 1400 °C and was nearly 35% after 100 h at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 h. At 1000 °C, experimental times were not sufficient for monoclinic phase to form, although a decrease in the yttria concentration in the tetragonal phase was observed.

Raman phonons and light scattering in RMnO 3 (R=La, Pr, Nd, Ho, Er Tb and Y) orthorhombic and hexagonal manganites

RMnO 3 compounds present the Pbnm orthorhombic structure for R=La, Pr, Nd while compounds with smaller ionic radii (R=Ho, Er Tb and Y) can be obtained either in the Pbnm or the P6 3 cm hexagonal structure. Their magnetic and transport properties are strongly dependent on the rare earth and, therefore, on their detailed structure. We have studied these compounds through the Raman phonons of both structures as a function of the rare earth ion and temperature. The phonon frequencies of the main peaks of Pbnm structure have been correlated to structural characteristics, such as bond-lengths or bond angles, obtained from the analysis of neutron diffraction data. According to the structural and Raman behavior, these compounds can be classified into two groups: one includes La, Pr, and Nd, and the other includes the rest. This classification also correlates to the different behavior of their magneto transport properties when doped. A structural transition to a cubic or quasi-cubic perovskite, above 800 K, can be deduced from the phonon spectra for YMnO 3 orthorhombic compound.

Neutron scattering studies of anantiferromagnetic Kondo compound: Ce8Pd24Ga

The antiferromagnet Ce8Pd24Ga with TN = 3.1 K has beeninvestigated using neutron diffraction, inelastic neutronscattering, electrical resistivity, magnetoresistance and magneticsusceptibility measurements. Rietveld analysis of neutrondiffraction data reveals that Ce8Pd24Ga crystallizes inthe cubic structure with space group Pm3̄m. Inelastic neutronscattering (INS) studies show two well-defined crystal-fieldexcitations at 3.2 meV and 12.8 meV. The crystal-field parametershave been estimated from the analysis of INS data. The heatcapacity calculated from the crystal-field level scheme shows aSchottky peak at 15 K which agrees well with the reportedexperimental results. The resistivity exhibits -ln T behaviour athigh temperature followed by a peak at 8 K and eventually drops atTN. The peak in the resistivity at 8 K arises due to thecombined effect of crystalline electric fields and Kondointeractions. The analysis of the resistivity data in themagnetically ordered state reveals a gap of 16.1 K in the spin-wavespectrum. At 1.8 K the magnetoresistance is positive and it changes tonegative at 3 K. The positive magnetoresistance at 1.8 K isconsistent with the antiferromagnetic ground state. The negativemagnetoresistance shows a scaling behaviour that yields a low-temperature Kondo temperature of 5.8 K. The magnetic susceptibilityexhibits Curie-Weiss behaviour between 20 K and 300 K with aneffective paramagnetic moment µeff = 2.33 µB andparamagnetic Curie temperature θp = -18.8 K. The presentstudies reveal that the physical properties of Ce8Pd24Gaare governed by the Kondo, crystal-field andRuderman-Kittel-Kasuya-Yosida (RKKY) interactions.

Low-temperature crystal and magnetic structures of the chain-ladder composite materialSr0.4Ca13.6Cu24+yO41+z:Hole redistribution and antiferromagnetic order

The low-temperature crystal structure of a one-dimensional chain-ladder composite material ${\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{13.6}{\mathrm{Cu}}_{24+y}{\mathrm{O}}_{41+z}$ was determined by the Rietveld analysis of neutron diffraction data using a superspace group approach. The hole distribution between the chain and ladder planes was estimated by the bond-valence sum (BVS) calculation based on Cu-O interatomic distances. The minimum of the distance between ladder-copper and chain-oxygen atoms [Cu(1)\char21{}O(3)] was revealed to expand with lowering temperature. The BVS calculation indicated that such a structural change corresponds to a redistribution of holes from the ladder to the chain and that almost all of the holes are localized in the chain below or near the N\'eel temperature. By assuming reasonable magnetic interactions between hole-unoccupied Cu sites on the chain plane, we propose a possible magnetic structure model taking into account the distribution of holes and observed magnetic neutron Bragg reflections. The results suggest the presence of spin dimers, spin trimers, and ``lone'' spins in the chain, of which the latter two have effective magnetic moments. These moments may be an origin of staggered antiferromagnetic spin modulation onto the spin-liquid state.