Neutron Diffraction Refinement Research Articles

Neutron diffraction determination of the cell dimensions and thermal expansion of the fluoroperovskite KMgF3 from 293 to 3.6 K

The cell dimensions of the fluoroperovskite KMgF3 synthesized by solid state methods have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 293–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data demonstrate conclusively that cubic \( Pm\overline{3} m \) KMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. Cell dimensions range from 3.9924(2) A at 293 K to 3.9800(2) A at 3.6 K, and are essentially constant within experimental error from 50 to 3.6 K. The thermal expansion data are described using a fourth order polynomial function.

Methane hydrate formation and decomposition: Structural studies via neutron diffraction and empirical potential structure refinement

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.

Structural and magnetic properties of LaMn1−xFexO3 (<x<1.0)

Electronic, structural, and magnetic properties of Mn-doped lanthanum ferrites were studied by neutron diffraction, superconducting quantum interference device, and impedance spectroscopy. Neutron diffraction refinements were performed with the constraint of full La occupancy, which showed the presence of excess oxygen when x<0.4. Mixed valent Mn cations and cation vacancies, therefore, exist in all the samples. The samples with x>0.7 are magnetically ordered at room temperature with orthorhombic symmetry (Pbnm). When x<0.3 the structure is rhombohedral and magnetically disordered above 16K. The majority carriers, electron holes, correspond to high oxidation states of Mn. The carrier concentration is determined from the Seebeck coefficients, and is a function of temperature and Fe concentration. The measurements of conductivity and Seebeck coefficients show polaron hopping at elevated temperatures.

Nonstoichiometry, Structure, and Electrical Properties of “SrPrO3”

The single-phase composition with the perovskite structure type in the Sr−Pr−O system has been identified as (Sr₀.₉₃₅Pr₀.₀₄₃□₀.₀₂₂)Pr₁.₀O₃ from neutron diffraction refinements, chemical analysis, and thermogravimetric analysis. The cation deficient oxide crystallizes in a GdFeO3-type orthorhombic structure, space group Pbnm, with lattice parameters a = 5.988 00(17), b = 6.121 36(17), c = 8.548 58(24) A, and Z = 4 at 300 K. Thermogravimetric analysis confirms that the compound is not oxygen deficient. (Sr₀.₉₃₅Pr₀.₀₄₃)Pr₁.₀O₃ exhibits semiconducting behavior. The bulk (grain interior) conductivity at 36 °C is 5.2 × 10⁻⁵ S cm⁻¹ with an activation energy for charge transport of 0.26 eV.

Structural characterisation and physical properties of Li MMnO 4 ( M=Cr, Ti) spinels

New spinel-type phases of general formula Li MMnO 4 ( M=Cr, Ti), derived from LiMn 2O 4 by substitution of Mn 3+ by Cr 3+ or Mn 4+ by Ti 4+, have been obtained and characterised. Neutron diffraction refinements confirm that both phases crystallise in the F d 3 m space group, giving the cation distributions [Li] 8 a [CrMn] 16 d O 4 and [Li 0.66Ti 0.34] 8 a [Li 0.34MnTi 0.66] 16 d O 4. Electrical conductivity has been examined by various techniques showing that these materials behave as semiconductors. The electrochemical behaviour indicates different oxidation–reduction steps in both cases concomitant with the insertion/deinsertion of lithium in non-reversible processes. X-ray diffraction patterns show that the above process is topotactic in LiCrMnO 4. Magnetic data and neutron diffraction measurements show that no long-range magnetic ordering is present, suggesting a spin-glass transition for M=Cr at low temperature, while for M=Ti the presence of non-magnetic ions in the octahedral sublattice provokes an inherent magnetic frustration.

Solid solutions La 1− xAg yMnO 3+ δ: evidence for silver doping, structure and properties

Our experiments on the preparation of the composites of CMR manganites and metal silver have shown that in the case of La 1− x MnO 3+ δ the vacancies in the A-sublattice of the perovskite structure can be filled by Ag + ions under the soft conditions of the synthesis. Neutron diffraction refinement of La 0.8Ag 0.2MnO 3.00 clearly proves the silver doping in the A-sublattice. Depending on P(O 2)– T synthesis conditions the solid solutions with the broad stoichiometry range La 1− x Ag y MnO 3+ δ (0.3 ≥ x ≥ y) were prepared. We report here the evolution of the oxygen stoichiometry, the crystal structure as well as electro-physical properties in the series of the solid solutions. Curie temperature comes over a maximum exceeding the room temperature with the increase of silver content. La 1− x Ag y MnO 3+ δ solid solutions can be useful for the construction of the room temperature magnetic field sensors based on the magnetoresistance effect.

Electronic and magnetic behaviour of Li (1+ x)/2 Ti xCr (5−3 x)/2 O 4 spinels

A series of spinel type derivatives has been obtained from doping the parent compound, Li 4/3Ti 5/3O 4, with Cr 3+ giving rise to the phases of general formula Li (1+ x)/2 Ti x Cr (5−3 x)/2 O 4 (1.07⩽ x⩽1.6). X-ray and neutron diffraction refinements confirm that all these phases crystallise in the Fd3 m space group, in which Cr 3+ ions occupy octahedral sites whereas Ti 4+ and Li + are distributed in both octahedral and tetrahedral sites. Conductivity measurements on polycrystalline samples show that higher substituted samples behave as ionic conductors whereas for the lower limits of substitution the materials are predominantly electronic conductors. Magnetic susceptibility results indicate that the upper doped phases do not show magnetic cooperative interactions but these are significant for the less substituted materials, in which the effective magnetic moment is higher.

A combined diffraction (XRD, electron and neutron) and electrical study of Na 3MoO 3F 3

Na 3MoO 3F 3, a member of the A 2 BM VIO 3F 3 family of elpasolite-related oxyfluorides, has been prepared by the reaction of NaF with MoO 3 at 650°C. It is shown by a combined X-ray, electron and neutron diffraction study, that the true symmetry of Na 3MoO 3F 3 is not monoclinic (pseudo-orthorhombic) as previously reported but instead triclinic (metrically rhombohedral) P1. The superstructure unit cell is given by a=− a R+ b R, b= c R, c=( a R+ b R+ c R), when expressed with respect to the underlying rhombohedral parent structure. Neutron diffraction refinement of the rhombohedral sub-structure shows O and F to be fully ordered with alternate (111) R planes being occupied by O and then F. The nature of the distortion away from the high temperature Fm 3 ̄ m parent structure is consistent with a combination of φφφ type rotations of the octahedral framework together with ppp type displacements of the octahedral cations, which is not observed in any of the closely related A 2 BM VIO 3F 3 family members. Bond valence arguments are used to give a plausible explanation for this difference in behavior.

Electronic Structure of Sr2MoO4

The band structure for Sr2MoO4, which is isostructural with the unconventional spin-triplet superconductor Sr2RuO4, has been calculated within the local-density approximation. The results of the neutron-diffraction structure refinement used in the calculation are also presented. The elecronic structure of Sr2MoO4 resembles that of Sr2RuO4, while in Sr2MoO4 the anisotropy of Fermi velocity is considerably larger than that in Sr2RuO4. The neutron diffraction results and the structure optimization result suggest that the MoO6 octahedron is less elongated than the previously reported value.

Structural, electronic, and magnetic properties ofLaNi5−xTx(T=Fe,Mn)compounds

Structures and magnetic properties of the ${\mathrm{LaNi}}_{5\ensuremath{-}x}{\mathrm{Fe}}_{x}$ and ${\mathrm{LaNi}}_{5\ensuremath{-}x}{\mathrm{Mn}}_{x}$ compounds have been investigated using neutron diffraction and first-principles tight-binding-linear-muffin-tin-orbital methods. Both neutron diffraction refinement data and total energy calculations show that the Fe and Mn atoms preferentially occupy the $3g$ sites in the hexagonal ${\mathrm{CaCu}}_{5}$-type structure. The calculated magnetic moments of Fe and Ni atoms are of $2.4--2.5{\ensuremath{\mu}}_{B}$ and $0.2--0.5{\ensuremath{\mu}}_{B}$ in ${\mathrm{LaNi}}_{5\ensuremath{-}x}{\mathrm{Fe}}_{x},$ respectively. The magnetic structure exhibits more localized moments at Fe atoms in ${\mathrm{LaNi}}_{5\ensuremath{-}x}{\mathrm{Fe}}_{x}$ when $x<~1.0.$ Electronic structure calculations indicate that s-conduction electron spin polarization from the Ni or La atoms strongly interacts with Fe(Mn) d-spin moments in ${\mathrm{LaNi}}_{5\ensuremath{-}x}\mathrm{Fe}(\mathrm{Mn}{)}_{x}$ $(x\ensuremath{\ne}0)$ compounds, which gives rise to a very large valence transferred hyperfine field on the Ni or La sites. This $s\ensuremath{-}d$ hybridization may lead to an interaction among magnetic clusters in these kinds of materials and may cause a spin freezing effect at low temperature when the Fe(Mn) content is very low in ${\mathrm{LaNi}}_{5\ensuremath{-}x}\mathrm{Fe}(\mathrm{Mn}{)}_{x}.$ Mn atoms show magnetic moments of $3.0{\ensuremath{\mu}}_{B}$ per atom due to a large exchange splitting in ${\mathrm{LaNi}}_{5\ensuremath{-}x}{\mathrm{Mn}}_{x}$ $(x\ensuremath{\ne}0).$ ${\mathrm{LaNi}}_{4}\mathrm{Mn}$ is found to be ferrimagnetic, whereas antiferromagnetic exchange coupling between the Mn atoms is preferred for ${\mathrm{LaNi}}_{3}{\mathrm{Mn}}_{2}.$ Both ferrimagnetic and ferromagnetic exchange interactions between Mn atoms are found in the ${\mathrm{LaNi}}_{2}{\mathrm{Mn}}_{3}$ compounds. The calculated results are in good agreement with the experimental neutron data.

Electrochemical and Structural Characterization of LiNi1 − yCoy O 2 ( 0 ≤ y ≤ 0.2 ) Positive Electrodes during Initial Cycling

The initial charge/discharge behavior of positive electrodes has been closely examined by neutron diffraction, X‐ray diffraction, X‐ray absorption, and electrochemical methods. Charge/discharge experiments and neutron diffraction refinements show that capacity loss during the initial cycling of increases linearly with the excess nickel content controlled by cobalt substitution for nickel. Charge/discharge cycling with gradually increasing depth of charge indicates that most of the initial capacity loss takes place in the early stages of the first charge. X‐ray absorption near‐edge spectra reveal that the nickel and cobalt ions in are simultaneously oxidized and that the local structure of octahedra is slightly distorted upon lithium deintercalation. © 2000 The Electrochemical Society. All rights reserved.

ChemInform Abstract: The Simultaneous Powder X‐Ray and Neutron Diffraction Refinement of Two η‐Carbide Type Nitrides, Fe3Mo3N and Co3Mo3N, Prepared by Ammonolysis and by Plasma Nitridation of Oxide Precursors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Structure and electrochemical characterization of Li1+xMn2−xO4 spinels for rechargeable lithium batteries

Abstract Neutron and X-ray powder diffraction and Rietveld refinements have been carried out for the well-crystallized samples with a homogeneous composition Li 1+ x Mn 2− x O 4 (0≤ x ≤0.125). The lattice parameter of these spinels changes with both the composition and the synthesis temperature. SEM and TEM micrographs show that the crystallites appear as single crystals with a size of 0.1–2 μm. The phase transition from cubic Fd 3 m to orthorhombic Fddd around 280 K of samples Li 1+ x Mn 2− x O 4 ( x ≤0.04) was observed using low temperature XRD and DSC. The orthorhombic phase further converts to a single tetragonal phase I 4 1 / amd around 65 K. The charge/discharge capacity, and cyclability of these samples were examined in both the 3 V and the 4 V regions. We found that the discharge capacity as well as the cyclability of the cell Li/Li 1+ x Mn 2− x O 4 change largely with not only the value of x but also the crystal structure which depends on the synthesis conditions.

The simultaneous powder X-ray and neutron diffraction refinement of two η-carbide type nitrides, Fe3Mo3N and Co3Mo3N, prepared by ammonolysis and by plasma nitridation of oxide precursors

Abstract The intermetallic nitrides Fe3Mo3N and Co3Mo3N were synthesized by the ammonolysis of the ternary oxide precursors FeMoO4 and CoMoO4 as well as by plasma nitridation of the oxide precursors. The structures of Fe3Mo3N and Co3Mo3N were determined from the simultaneous Rietveld refinements of powder X-ray and neutron diffraction data. Both compounds were found to be isostructural with η-Fe3W3C which crystallizes with cubic symmetry in the space group Fd 3 m with Z=16. The lattice parameters were found to be a=11.0859(3) A and a=11.0270(6) A for Fe3Mo3N and Co3Mo3N, respectively. The general atomic positions that gave the best refinement for the intermetallic nitrides M3Mo3N are Mo on 48f, M on 32e and 16d and N on 16c. The structure consists of corner shared NMo6 octahedra with the metal atoms occupying the sites between the octahedra. Magnetic susceptibility data for Co3Mo3N was found not to follow the Curie law.

Study on the magnetic moments of PrFe 10.5V 1.5 and PrFe 10.5V 1.5N x compounds

The magnetic moments of PrFe 10.5V 1.5 and PrFe 10.5V 1.5N x have been studied by neutron diffraction measurements and 57Fe Mössbauer spectra measurements. Neutron diffraction refinements indicate that the directions of magnetic moments of Pr and Fe atoms change from ab-plane to c-axis and their magnitudes also increase upon nitrogenation. Mössbauer spectra studies show that hyperfine fields and isomer shifts of PrFe 10.5V 1.5 are strongly enhanced upon nitrogenation.

Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

The structure of Bi2.34U0.33La0.33O5 has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P, Z = 1, and lattice parameters aH = 4.01395(8), cH = 9.5423(2) A. X-Ray diffraction data are in favour of a model in which U and La atoms are situated in different crystallographic positions: i.e. 1a and 2d of the P space group respectively, forming mixed Bi/U and Bi/La layers which alternate along the c axis. The structure of this bismuth-based mixed oxide can be described as built up of interlocked edge-sharing (Bi/U)O8 and (Bi/La)O8 polyhedra. The co-ordination polyhedron around the Bi/U atoms is a hexagonal bipyramid, the equatorial plane being a puckered hexagon. The Bi/La atoms are co-ordinated to eight oxygen atoms forming a slightly distorted cube. Electron diffraction studies showed that the structure can be described as a fluorite-type superstructure, the relation to the fluorite sub-cell is a*H ≈ ⅓[4]a*C, b*H ≈ ⅓[ 22]a*C, c*H ≈ ⅓[111]a*C. This compound is a good oxide ion conductor (σ300 °C = 2.5 × 10–5 S cm–1). On the basis of the anion vacancies distribution determined from neutron diffraction refinement, a pathway for the oxide ion conduction is proposed.

Neutron diffraction and extended X-ray absorption fine structure studies of Pr/sub 3/(Fe/sub 1-x/Co/sub x/)/sub 27.5/Ti/sub 1.5/ permanent magnet compounds

Neutron diffraction (ND) and extended X-ray absorption fine structure (EXAFS) measurements were carried out to determine the long-range and short-range structural properties of Co-substituted Pr/sub 3/Fe/sub 1-x/Co/sub x/Ti/sub 1.5/ compounds. ND refinement analysis indicates Ti atoms occupy the 4g and 4i Fe sites and Co atoms occupy, without preference, those Fe sites not shared with the Ti atoms. EXAFS measurements show the local environment of the Pr experiences increasing static disorder with the introduction of Co, and its coordination sphere contracts in the vicinity of those Co near neighbors. The Fe local environment remains unaffected with the introduction of Co suggesting that substitution in excess of 40% can be achieved in this structure.

Nonstoichiometric Li-Pseudobrookite(ss) in the Li2O-Fe2O3-TiO2System

Phase equilibria in the Li2O-Fe2O3-TiO2system were studied at 900 and 1000°C using the quenching method. Two new ternary solid solutions, based on the pseudobrookite and ramsdellite structure types, were identified and structurally characterised using Rietveld refinements of powder X-ray data. The pseudobrookite(ss) showed deviations from M3O5stoichiometry towards metal-rich compositions which increased with increasing substitution of Li+Ti for Fe, from Fe2TiO5to Li0.81Fe0.27Ti2.09O5(≡M3.17O5). A neutron diffraction refinement was made on Li0.81Fe0.27Ti2.09O5, space group Ccmm, witha=9.6576(2),b=3.7532(1),c=9.8820(2) Å. Titanium is ordered in the eight-fold M2 site while lithium, together with minor iron and titanium, is ordered in the four-fold M1 site. The excess lithium (0.17 Li per formula unit) was located in interstitial sites in the structure, having square-pyramidal coordination. A local-ordering model is presented which involves Li3clusters with Li-Li=2.3 Å.

Article

Ab initio density functional calculations have been carried out on the ordered phase of ammonium chloride and the results compared with experimental data. Powder neutron diffraction measurements have been carried out on 14ND4Cl and 15ND4Cl at a temperature of 5 K; the lattice parameter is 3.8224(4) Å and the N-D bond length is 1.0318(8) Å. Calculated structural and spectroscopic quantities agree well with the results of the neutron diffraction refinements, and with measured spectroscopic data. The calculated barrier to reorientation of the ammonium ion about the fourfold axis is 29 kJ mol-1, and the calculated compressibility is 3.78 × 10-11 Pa-1. The octopole moment of the ammonium ion in the crystal has been estimated from the calculated electron density, and is about 10-20% greater than for the free ion. The octopole moment does not vary appreciably with the orientation of the ammonium ion in the lattice. There is no evidence of any covalent interaction between the ammonium and chloride ions.Key words: ammonium chloride, density functional theory, neutron diffraction, ab initio calculations.

An ab initio and neutron diffraction study of ammonium chloride

Ab initio density functional calculations have been carried out on the ordered phase of ammonium chloride and the results compared with experimental data. Powder neutron diffraction measurements have been carried out on 14 ND 4Cl and 15 ND 4Cl at a temperature of 5 K; the lattice parameter is 3.8224(4) A and the N—D bond length is 1.0318(8) A. Calculated structural and spectroscopic quantities agree well with the results of the neutron diffraction refinements, and with measured spectroscopic data. The calculated barrier to reorientation of the ammonium ion about the fourfold axis is 29 kJ mol -1 , and the calculated compressibility is 3.78 ◊ 10 -11 Pa -1 . The octopole moment of the ammonium ion in the crystal has been estimated from the calculated electron density, and is about 10-20% greater than for the free ion. The octopole moment does not vary appreciably with the orientation of the ammonium ion in the lattice. There is no evidence of any covalent interaction between the ammonium and chloride ions.