Neutron Diffraction Refinement Research Articles

Solvation of Inorganic Nitrate Salts in Protic Ionic Liquids

The bulk nanostructure of several inorganic salt solutions in protic ionic liquids is elucidated using neutron diffraction and empirical potential structure refinement modeling. The protic ionic liquids studied are ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN), which are mixed with either LiNO3, Mg(NO3)2, Ca(NO3)2, or Al(NO3)3 at 1:10 or 1:30 solute:solvent mol:mol ratios. The models show inorganic metal ions are solvated within the polar domains of the nanostructure and can induce significant differences in bulk solvent nanostructure from that of the pure ionic liquids. For EtAN, the uncharged groups aggregate and form an apolar domain in spite of the cation’s reduced amphiphilicity because the trans rotamer is favored when an inorganic salt is added.

Neutron and X-ray diffraction and empirical potential structure refinement modelling of magnesium stabilised amorphous calcium carbonate

Amorphous calcium carbonate (ACC) plays a key role in biomineralisation processes in sea organisms. Neutron and X-ray diffraction have been performed for a sample of magnesium-stabilised ACC, which was prepared with a Mg:Ca ratio of 0.05:1 and 0.25 H2O molecules per molecule of CO3. The empirical potential structure refinement method has been used to make a model of magnesium-stabilised ACC and the results revealed a fair agreement with the experimental diffraction data. The model has well-defined CO3 and H2O molecules. The average coordination number of Ca is 7.4 and is composed of 6.8 oxygen atoms from CO3 molecules and 0.6 oxygen atoms from H2O molecules. The average CaO bond length is 2.40Å. The distribution of Ca in the model is homogeneous with a uniformly distributed Ca-rich network and no evidence of the Ca-poor channels as previously reported for a reverse Monte Carlo model of ACC.

Nanostructure of an ionic liquid-glycerol mixture.

The nanostructure of a 50 : 50 vol% mixture of glycerol and ethylammonium formate (EAF), a protic ionic liquid (IL), has been investigated using neutron diffraction and empirical potential structure refinement (EPSR) fits. EPSR fits reveal that the mixture is nanostructured. Electrostatic interactions between IL charge groups leads to the formation of ionic regions. These solvophobically repel cation alkyl groups which cluster together to form apolar domains. The polar glycerol molecules are preferentially incorporated into the charged domains, and form hydrogen bonds with EAF groups rather than with other glycerol molecules. However, radial distribution functions reveal that glycerol molecules pack around each other in a fashion similar to that found in pure glycerol. This suggests that a glycerol channel runs through the ionic domain of EAF. The absence of significant glycerol-glycerol hydrogen bonding indicates that glycerol molecules are able to span the polar domain, bridging EAF charge groups. Glycerol can adopt six distinct conformations. The distribution of conformers in the EAF mixture is very different to that found in the pure liquid because hydrogen bonds form with EAF rather than with other glycerol molecules, which imparts different packing constraints.

Identification of complex anions in La1 − x A x MnO3 manganites (A = Ca, Sr) from neutron diffraction data and refinement of their structures on the basis of raman spectroscopy data

The consideration of the Mn(-O…Mn)6 structure unit of the LaMnO3 orthorhombic phase constructed on the basis of neutron diffraction data revealed the presence of atomic groups in the form of (MnO2)− complex anions and separate O2− ions. The Raman spectra have demonstrated that complex anions have a nearly linear O=Mn-O− structure. Comparison of the Raman spectra of the rhombohedral CaMnO3 and BaTiO3 phases and pyramidal molecules of the ZXY2 type has shown that the CaMnO3 phase is composed of pyramidal MnO32− molecules with one Mn=O double bond and two Mn-O− ordinary bonds. The complex anions are linked by O=Mn-O− intermolecular bonds to form planar zigzag Mn=O…Mn chains similar to C=C-C planar chains in some organic compounds exhibiting high electrical conductivity. With decreasing temperature, the Mn…O intermolecular distances decrease, and the Mn=O…Mn chains become even more similar to the C=C-C chains and exhibit high electrical conductivity.

Understanding the effect of synthesis temperature on the structural and electrochemical characteristics of layered-spinel composite cathodes for lithium-ion batteries

The effect of synthesis temperature on the structural and electrochemical characteristics of the layered-spinel composite cathode system xLi[Li0.2Mn0.6Ni0.2]O2–(1 − x)Li[Mn1.5Ni0.5]O4 (0 ≤ x ≤ 1) has been investigated. With a joint neutron diffraction (ND) and X-ray diffraction (XRD) Rietveld refinement method, the composition and weight percent variations of the layered and spinel phases in this composite cathode system have been obtained as a function of x and synthesis temperature. While no significant composition and weight percent variations are found with the synthesis temperature, the electrochemical characteristics of both the layered and spinel phases in the composites are significantly affected by the synthesis temperature. In contrast to the layered sample (x = 1), the capacity of the layered phase in the composites increases with decreasing synthesis temperature due to an increase in surface area. Conversely, the effect of synthesis temperature on the spinel phase is similar in both the spinel sample (x = 0) and the composite samples. However, the lower synthesis temperature increases the cation ordering in the 16d octahedral sites of the spinel phase, which changes the voltage profiles below 3 V due to the decrease in the lattice distortion during lithium ion insertion into the empty 16c octahedral sites.

The atomic structure of deuterated boyleite ZnSO4{middle dot}4D2O, ilesite MnSO4{middle dot}4D2O, and bianchite ZnSO4{middle dot}6D2O

Deuterated boyleite ZnSO 4 ⋅4D 2 O, was synthesized and the atomic structure, including D positions, was successfully refined in a combined histogram neutron diffraction refinement. The cell dimensions for boyleite are a = 5.9144(2), b = 13.5665(4), c = 7.8924(2) Å, and β = 90.668(2)° with space group P2 1 /n and Z = 4. The atomic structure including D positions of the isostructural mineral ilesite, MnSO 4 ⋅4D 2 O, was refined and the cell dimensions are a = 5.9753(1), b = 13.8186(3), c = 8.0461(1) Å, and β = 90.826(2)°. Deuterated bianchite ZnSO 4 ⋅6D 2 O was synthesized and the atomic structure, including D positions, was successfully refined with a unit cell of a = 9.969(1), b = 7. 2441(7), c = 24.249(3) Å, and β = 98.488(5)° in space group C2/c and Z = 8. A comparison of the hydrogen bonding in M 2+ SO 4 ⋅4D 2 O with that in M 2+ SO 4 ⋅6D 2 O shows that bifurcated hydrogen bonds are common in the tetrahydrate sulfates but nonexistent in the hexahydrate structures. This is a result of the packing constraints of the rings of the sulfate and metal-containing octahedra in the tetrahydrates. In the hexahydrate sulfates there is no direct linkage between the sulfate and metal-containing octahedra and hydrogen bonds are optimized without packing constraints, and no bifurcated hydrogen bonds are observed.

Neutron diffraction study and ab-initio calculations of nanostructured doped ZnO

Nanostructured doped-ZnO system with various elements (M=Cr, Mn, Fe, Co, Ni, and In) at 10at.% doping concentration, was investigated. Neutron diffraction refinements confirm the stability of the würztite crystal structure of the parent compound ZnO, the localization of the doping elements within Zn sites, and some residual impurities. Magnetic measurements show a ferromagnetic behavior at room temperature where the magnetic parameters, saturation magnetization (Ms), remanence (Mr) and coercivity (Hc) vary considerable according to the nature of the doping elements, its valence and its solubility level: Ms has the highest value of 0.223emu/g for Ni and lowest value of 0.011emu/g for Cu. The ab-initio calculations using DFT method confirm the ferromagnetism of doped-ZnO in agreement with magnetic measurements. It is found that oxygen plays an important role to explain the magnetic properties observed in diluted magnetic semiconductors (DMS) of the studied doped-ZnO system. The estimated Curie temperature was found to vary considerably according to the nature of the doping element, with the lowest value for In (105K) and highest value for Co (737K).

A Monte Carlo revisiting of N-methylformamide and acetone

The pure liquids N-methylformamide and acetone have been revisited via Monte Carlo simulations in the NTP ensemble at 1 atm and 25 °C. The molecules are all-atom rigid structures, and the intermolecular potential used is the classical 6-12 Lennard-Jones plus Coulomb. The theoretical g(r)s of both liquids were compared with those obtained from neutron diffraction and empirical potential structure refinement simulations. The results point to the existence of H-bonds driving the N-methylformamide structure, while in acetone the correlations are dipole moment oriented. The structure of the liquid N-methylformamide is mainly guided by a dimer whose molecules are arranged in such a way that the angle between their dipole moments is 73°, while liquid acetone is much less organized and the orientation of the molecules changes from an antiparallel dipolar correlation at short distances to more parallel alignments of the molecular dipole moments for larger distances.

Apatite metaprism twist angle ( φ) as a tool for crystallochemical diagnosis

[ A I] 4[ A II] 6( BO 4) 6 X 2 apatites can flexibly accommodate numerous cationic, metalloid and anionic substitutions. Using a combination of new refinements and published structures, this paper reviews correlations between substituent type and framework adaptation through adjustment of the A IO 6 metaprism twist angle, φ. These systematics are illustrated through powder neutron diffraction refinement of the crystal chemistry of A 10(PO 4) 6F 2 ( A=Ca, Sr) fluorapatites. Variations in φ reflect changes in the relative size of the A I 4( BO 4) 6 framework and A II 6 X 2 tunnel content and can be used to quantitatively assess the reliability of A I/ A II cation partitioning coefficients determined by Rietveld analysis. In the simplest cases of bi-ionic substitution, the metaprism twist systematics conform to three principle trends (i) For A-type divalent substitution, the larger A 2 + species preferentially enters the channel before partitioning to the framework. This leads to parabolic modification in φ across the compositional series; (ii) For B-type pentavalent compounds, the φ variation will be linear in accord with the relative B 5+ ionic size; and (iii) For X-type substitution of halide anions, φ will be reduced as the average size increases. Departures from these trends may indicate polymorphism, compositional anomalies, A I /A II order disequilibrium, or poor structure refinement, and may be extended to chemically complex apatites with simultaneous substitutions over the A, B and X sites.

Low Temperature Neutron Diffraction Studies in [Mn3(suc)2(ina)2]n: An Homometallic Molecular 3D Ferrimagnet

Neutron diffraction techniques have been used to determine the low temperature crystal structure and to shed light on the magnetic behavior of the [Mn(3)(suc)(2)(ina)(2)](n) (suc = succinate and ina = isonicotinate) complex. The ferromagnetic signal observed below T(c) ≈ 5 K in this compound is due to a noncompensation of homometallic spins in the 3D framework. The Mn(II) magnetic moments obtained from neutron diffraction refinements are slightly lower than those observed for isolated Mn(II) ions; this can be due to covalent spin delocalization or geometrical magnetic fluctuations. A small discrepancy between the value of the magnetic moments of each Mn(II) site is also observed [Mn(1) 4.1(2) μ(B) and the Mn(2) 3.9(1) μ(B)]. These differences between the theoretical and observed manganese magnetic moments are not unexpected in this large spin metal complex, and qualitatively reasonable given the synergistic interaction between the metal ions through oxo-bridge. The competition among different interactions, principally those covalent through organic ligands and dipolar interaction, drive to a final 3D ferrimagnetic order.

Real time neutron diffraction and solid state NMR of high strength apatite–mullite glass ceramic

We report the real time crystallisation of an apatite–mullite glass ceramic, 4.5SiO 2–3.0Al 2O 3–1.5P 2O 5–3.0CaO–1.5CaF 2, analysed by time-of-flight (TOF) neutron diffraction and Rietveld refinement. The glass ceramic of this particular composition exhibits superior mechanical strength as was demonstrated earlier. The as-cast glass was heated up to 1200 °C with a 30 min isothermal hold, followed by cooling with the ramp stages at 2 °C min −1. On heating, fluorapatite (FAP—Ca 5(PO 4) 3 F ) crystallised at around 850 °C and stabilised at around 908 °C. Mullite (Al 6Si 2O 13) began to crystallise just above 1000 °C, which reinitiated the FAP crystallisation, with the FAP to mullite molar ratio decreasing linearly to around 1020 °C when the ratio stabilised to approximately 1:1. Dissolution of these two phases started at around 1130 °C but did not complete by 1200 °C. On cooling, cristabolite and quartz (berlinite) polymorphs of the aluminium phosphate, AlPO 4, crystallised at around 1025 °C. The neutron diffraction results are consistent with the effects on DSC trace for the ceramics. 19F MAS-NMR showed fluorine present in Al–F–Ca(n) sites and on heat treatment conversion to fluorapatite. The 29Si MAS-NMR showed a binary distribution of predominantly Q 3 and Q 4(3Al) species and mullite and residual glass on heat treatment. The 31P MAS-NMR showed pyrophosphate (Q 1) groups in the glass and on heat treatment formation of fluorapatite and aluminium phosphate. 27Al MAS-NMR showed the presence of predominantly Al(IV) in the glass and characteristic Al(IV) and Al(VI) sites of mullite and aluminium phosphate as well as residual amorphous material in the ceramics. The details of the structural transformation revealed from the real time measurements have significant implication for manufacturing glass ceramics and its mechanical performance and cannot be deduced based on room temperature measurements after cooling or quenching the material.

Heptane adsorption in silicalite-1: Molecular dynamics simulation

Molecular dynamics (MD) simulations have been used to study the adsorption process of n-heptane molecules in silicalite-1 at 300 K. MD simulated results were compared to experimental neutron diffraction (ND) and experimental self-diffusion coefficients. The analysis of MD data indicated a packing of the adsorbed molecules around 4 mol./u.c., which is not the consequence of an enthalpic effect but of an entropic effect. The role of the n-heptane chain flexibility ( cis– trans conformation) in relation with the silicalite-1 channel type (straight versus sinusoidal) was outlined and enabled to understand the mobility change arising at 4 mol./u.c., according to previous experimental results. The MD simulation also allowed to identify adsorption sites, three in the straight channels and three in the sinusoidal channels and to characterize their position, energy and occupation. Site position but only relative occupation data were in good agreement with neutron diffraction data. The assumption of a “commensurate freezing” to explain the step isotherm is discussed in the light of the MD simulation and ND refinement results.

Influence of the rare-earth element on the effects of the structural and magnetic phase transitions in CeFeAsO, PrFeAsO and NdFeAsO

We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron and antiferromagnetic ordering of rare-earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mössbauer spectroscopy and magnetization measurements. Transport and magnetic properties are affected strongly by the structural and magnetic transitions, suggesting significant changes in the band structure and/or carrier mobilities occur, and phonon–phonon scattering is reduced upon transformation to the low-temperature structure. Results are compared with recent reports for LaFeAsO, and systematic variations in properties as the identity of Ln is changed are observed and discussed. As Ln progresses across the rare-earth series from La to Nd, an increase in the hole contributions to the Seebeck coefficient and increases in magnetoresistance and the Hall coefficient are observed in the low-temperature phase. Analysis of hyperfine fields at the iron nuclei determined from Mössbauer spectra indicates that the moment on Fe in the orthorhombic phase is nearly independent of the identity of Ln, in apparent contrast to reports of powder neutron diffraction refinements.

Rapid determination of hydrogen positions and protonation states of diisopropyl fluorophosphatase by joint neutron and X-ray diffraction refinement

Hydrogen atoms constitute about half of all atoms in proteins and play a critical role in enzyme mechanisms and macromolecular and solvent structure. Hydrogen atom positions can readily be determined by neutron diffraction, and as such, neutron diffraction is an invaluable tool for elucidating molecular mechanisms. Joint refinement of neutron and X-ray diffraction data can lead to improved models compared with the use of neutron data alone and has now been incorporated into modern, maximum-likelihood based crystallographic refinement programs like CNS. Joint refinement has been applied to neutron and X-ray diffraction data collected on crystals of diisopropyl fluorophosphatase (DFPase), a calcium-dependent phosphotriesterase capable of detoxifying organophosphorus nerve agents. Neutron omit maps reveal a number of important features pertaining to the mechanism of DFPase. Solvent molecule W33, coordinating the catalytic calcium, is a water molecule in a strained coordination environment, and not a hydroxide. The smallest Ca-O-H angle is 53 degrees, well beyond the smallest angles previously observed. Residue Asp-229, is deprotonated, supporting a mechanism involving nucleophilic attack by Asp-229, and excluding water activation by the catalytic calcium. The extended network of hydrogen bonding interactions in the central water filled tunnel of DFPase is revealed, showing that internal solvent molecules form an important, integrated part of the overall structure.

Single-crystal polarized FTIR spectroscopy and neutron diffraction refinement of cancrinite

We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm−1 is polarized for E ⊥ c and is assigned to the ν3 mode. The second, at 3,531 cm−1, is also polarized for E ⊥ c and is assigned to ν1 mode. A weak component at 4,108 cm−1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm−1 range are strongly polarized for E ⊥ c, and are assigned to the vibrations of the CO3 group.

A powder neutron diffraction study of the crystal structure of the fluoroperovskite NaMgF3 (neighborite) from 300 to 3.6 K

The cell dimensions and crystal structures of the fluoroperovskite NaMgF3 (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data show that Pbnm NaMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF3 at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF3 from the aristotype cubic $$ Pm\ifmmode\expandafter\bar\else\expandafter\=\fi{3}m $$ structure is described in terms of the tilting of the MgF6 octahedra according to the tilt scheme a − a − c + . With decreasing temperature the antiphase tilt (a −) increases from 14.24° to 15.39°, whereas the in-phase tilt (c + ) remains effectively constant at ∼10.7°. Changes in the tilt angles are insufficient to cause changes in the coordination sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF3 is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those of Pbnm KCaF3.

Repulsive carbon–deuterium interactions in zirconium and titanium carbodeuterides

Abstract Zirconium and mixed zirconium/titanium carbodeuteride samples of nominal compositions ZrC 0.44 D 0.89 , Ti 0.66 Zr 0.33 C 0.57 D 0.92 and Ti 0.66 Zr 0.33 C 0.53 D 0.29 have been investigated by powder X-ray and neutron diffraction and joint Rietveld structure refinements. They contain two types of phases, one crystallizing with a face-centred cubic (fcc) metal atom arrangement and the other with a hexagonal close-packed (hcp) metal atom arrangement having trigonal symmetry. While the pure Zr sample contains both a cubic and a trigonal phase, the mixed Zr/Ti samples contain either a cubic phase only (C/D-poor sample), or two trigonal phases having different Ti/Zr ratios (C/D-rich sample). The carbon atoms occupy octahedral interstices in nearly ordered (hcp) or disordered (fcc) arrangements, and the deuterium atoms occupy either tetrahedral (hcp) or octahedral (fcc) interstices. In the hcp phase metal octahedra around carbon atoms contract while metal tetrahedra around deuterium atoms expand. The structure results are consistent with the assumption of repulsive interactions between non-metal atoms resulting in their longest possible separation (D–D > 2.1 A, C–D > 2.6 A).

Structural, magnetic, and transport properties of Zr-substituted La0.7Sr0.3MnO3

Zr-substituted perovskites La0.7Sr0.3Mn1−xZrxO3 with 0⩽x⩽0.20 were investigated by neutron diffraction (ND), magnetization, electric resistivity, and magnetoresistance measurements. ND refinements reveal that substituted Zr4+ goes only to the Mn site. Because of its large size, this leads to a Zr-solubility limit at x⩽0.10. The x⩽0.10 samples exhibit a rhombohedral structure (space group R3¯c) from 10K to room temperature. For the x⩽0.10 samples, the cell parameters a and c, and volume increase continuously with increasing Zr content. In addition, the structural distortion of the MnO6 octahedra increases with increasing Zr content. Zr-substituted La0.7Sr0.3MnO3 exhibits metallic behavior at low temperature. The field dependent resistivity suggests that electron-electron scattering is dominant and a two-magnon scattering process emerges with increasing temperature. The contribution from the two-magnon scattering in resistivity becomes larger with increasing Zr content. A maximum magnetoresistance (MRmax) of 35% is obtained for the x=0.03 sample at H=5T. The MRmax shifts to a higher temperature region upon application of an external magnetic field.

Structure and hydrogenation properties of the ternary alloys Ca 2− xMg xSi (0 ≤ x ≤ 1)

Partially Mg-substituted Ca 2Si alloys (Ca 2− x Mg x Si, 0 ≤ x ≤ 1) were synthesized via high-temperature evacuation of CaH 2/MgH 2/Si ball-milled mixtures. Neutron diffraction refinements showed that Ca 2− x Mg x Si (0 < x < 1) samples are not single-phase solid-solutions, but rather (1− x)Ca 2Si + xCaMgSi mixed phases. Hydrogen absorption measurements indicated that, although pure Ca 2Si ( x = 0) is readily hydridable and forms an unusual amorphous-hydride phase, pure CaMgSi ( x = 1) does not noticeably absorb hydrogen up to 7 MPa H 2 and 473–573 K. Thus, hydrogen absorption behaviors of the intermediate compositions (0 < x < 1) are dominated by the Ca 2Si component, as evidenced by the isotherm measurements and neutron vibrational spectra.

Structure of 2 molar NaOH in aqueous solution from neutron diffraction and empirical potential structure refinement

Neutron diffraction with isotopic substitution has been used to investigate aqueous solutions of $2M$ $\mathrm{Na}\mathrm{O}\mathrm{H}$ in the liquid state. The data were modeled using empirical potential structure refinement which allows for the extraction of the ion-water and water-water correlations. The data show that the ion-water radial distribution functions are in accordance with those found by previous studies on NaOH solutions and follow a trend which is dependent on the concentration of the solute. In particular, the shape of the hydroxide hydration shell is found to be concentration independent, but the number of water molecules occupying this shell increases with dilution. Additionally, the water-water correlations show that there is still a measurable effect on water structure with the addition of ions at this concentration, as the second shell in the water oxygen radial distribution function is compressed relative to the first shell. The data are also used to discuss the recent claims that the published radial distribution functions of water are unreliable, showing that data taken at different neutron sources, with different diffraction geometry and systematic errors lead to the same structural information when analyzed via a realistic modeling regime.