Single-crystal Neutron Diffraction Data Research Articles

Crystal chemistry and temperature behavior of the natural hydrous borate colemanite, a mineral commodity of boron

Colemanite, CaB3O4(OH)3⋅H2O, is the most common hydrous Ca-borate, as well as a major mineral commodity of boron. In this study, we report a thorough chemical analysis and the low-temperature behavior of a natural sample of colemanite by means of a multi-methodological approach. From the chemical point of view, the investigated sample resulted to be relatively pure, its composition being very close to the ideal one, with only a minor substitution of Sr2+ for Ca2+. At about 270.5 K, a displacive phase transition from the centrosymmetric P21/a to the acentric P21 space group occurs. On the basis of in situ single-crystal synchrotron X-ray (down to 104 K) and neutron diffraction (at 20 K) data, the hydrogen-bonding configuration of both the polymorphs and the structural modifications at the atomic scale at varying temperatures are described. The asymmetric distribution of ionic charges along the [010] axis, allowed by the loss of the inversion center, is likely responsible for the reported ferroelectric behavior of colemanite below the phase transition temperature.

The origin of the polar symmetry in huebnerite-type multiferroics

The magnetic structure of the multiferroic phase AF2 of huebnerite (MnWO4) has been re-investigated based on the polar space group P2, instead of P2/c widely accepted in literature. The site character between two independent sites for Mn2+, Mna and Mnb, fairly differs from each other in terms of bond valence sums. This hidden intrinsic dipole moment is the true origin of the chiral magnetic order in AF2 in the polar superspace group P2.1′ (α, 1/2, γ)0s. From structure refinements using single crystal neutron diffraction data at 10 K the ellipticity of the spin helix could be determined to be 0.92 and 0.77 at Mna and Mnb, respectively. The different contributions of two chiral spin-textures to electric polarisation microscopically measurable in the b axis (Pb) could be estimated with Pb (Mna) = 22.8 μCm−2 and Pb (Mnb) = 12.2 μCm−2.

Neutron diffraction study of the inverse spinels Co2TiO4 and Co2SnO4

We report a detailed single-crystal and powder neutron diffraction study of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)${}_{\text{M}}$ reflection due to ferrimagnetic ordering, which sets in below ${T}_{\text{N}}=48.6$ and 41 K for ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$, respectively. An additional low intensity magnetic reflection (200)${}_{\text{M}}$ was noticed in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we noticed a significant broadening of the magnetic (111)${}_{\text{M}}$ reflection, which possibly results from the disordered character of the Ti and Co atoms on the $B$ site. Practically, the same peak broadening was found for the neutron powder data of ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$. On the other hand, from our single-crystal neutron diffraction data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Co}}^{3+}$, the $2{t}_{g}$ levels split into a lower ${d}_{xy}$ level yielding a higher twofold degenerate ${d}_{xz}/{d}_{yz}$ level. As a consequence, one can expect a tetragonal distortion in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ with $c/a<1$, which we could not significantly detect in the present work.

2Flux growth and characterization of Ce-substituted [formula omitted] single crystals

Single crystals of (Nd1−xCex)2Fe14B, some reaching ∼6×8×8mm3 in volume, are grown out of Fe-(Nd, Ce) flux. This crystal growth method allows for large (Nd1−xCex)2Fe14B single crystals to be synthesized using a simple flux growth procedure. Chemical and structural analyses of the crystals indicate that (Nd1−xCex)2Fe14B forms a solid solution until at least x=0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements at 300K show only small decreases with increasing Ce content in saturation magnetization (Ms) and anisotropy field (HA), and Curie temperature (TC). First principles calculations are carried out to understand the effect of Ce substitution on the electronic and magnetic properties. For a multitude of applications, it is expected that the advantage of incorporating lower-cost and more abundant Ce will outweigh the small adverse effects on magnetic properties. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.

A neutron diffraction study of the crystal of benzoic acid from 6 to 293 K and a macroscopic-scale quantum theory of the lattice of hydrogen-bonded dimers

Measurements via different techniques of the crystal of benzoic acid have led to conflicting conceptions of tautomerism: statistical disorder for diffraction; semiclassical jumps for relaxometry; quantum states for vibrational spectroscopy. We argue that these conflicts follow from the prejudice that nuclear positions and eigenstates are pre-existing to measurements, what is at variance with the principle of complementarity. We propose a self-contained quantum theory. First of all, new single-crystal neutron-diffraction data accord with long-range correlation for proton-site occupancies. Then we introduce a macroscopic-scale quantum-state emerging from phonon condensation, for which nuclear positions and eigenstates are indefinite. As to quantum-measurements, an incoming wave (neutron or photon) entangled with the condensate realizes a transitory state, either in the space of static nuclear-coordinates (diffraction), or in that of the symmetry coordinates (spectroscopy and relaxometry). We derive temperature-laws for proton-site occupancies and for the relaxation rate, which compare favorably with measurements.

Evidence for monoclinic distortion in the ground state phase of underdoped La1.95Sr0.05CuO4: A single crystal neutron diffraction study

The existing controversy about the symmetry of the crystal structure of the ground state of the critical doped La1.95Sr0.05CuO4 has been resolved by analyzing the single crystal neutron diffraction data collected between 5 and 730 K. We observed small but significant intensities for “forbidden” reflections given by extinction rules of the orthorhombic Bmab space group at low temperatures. A careful investigation of neutron diffraction data reveals that the crystal structure of La1.95Sr0.05CuO4 at 5 K is monoclinic with B2/m (2/m 1 1) space group. The monoclinic structure emerges from the orthorhombic structure in a continuous way; however, the structure is stable below ∼120 K which agrees with other observed phenomena. Our results on symmetry changes are crucial for the interpretation of physical properties also in other high temperature superconductors with similar structures.

Type II Bi1 - xWxO1.5 + 1.5x: a (3 + 3)-dimensional commensurate modulation that stabilizes the fast-ion conducting delta phase of bismuth oxide.

The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modulated δ-Bi2O3-related structure, in which the modulation vector ℇ `locks in' to a commensurate value of 1/3. The structure was refined in a 3 × 3 × 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 - xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 `tetrahedra of octahedra' and chains of corner-sharing WO6 octahedra along 〈110〉F directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with previous reports and with our own synthetic and analytical results.

A new polar symmetry of huebnerite (MnWO4) with ferrodistortive domains

Large-size single-crystal samples of huebnerite natural multiferroic MnWO4 were analyzed by neutron and synchrotron X-ray single-crystal diffraction as well as by polarized Raman spectroscopy. Both neutron and X-ray diffraction analyzes reveal polar space-group symmetry P2 for the nuclear structure of huebnerite via the detection of weak reflections h0l (l=odd) forbidden for the gliding plane c. Renninger scans of the reflection 301 on the neutron single-crystal diffractometer RESI (FRM-II) could confirm the absence of the gliding plane c in both para- and antiferromagnetic states of huebnerite. The symmetry breaking could be explained by structure analyzes with neutron single crystal diffraction data at 293K revealing that two Mn atoms in P2 are displaced independently along the b axis from their equilibrium position at a polar point site, C2 in P2/c. Micro X-ray diffraction and Raman-scattering mapping reveal a ferrodistortive domain texture in the room-temperature paramagnetic state of huebnerite, which is attributed to P2 domain formation through a proper ∼180° rotation about the reciprocal-space axis c*.

Structure of magnesium selenate enneahydrate, MgSeO4·9H2O, from 5 to 250 K using neutron time-of-flight Laue diffraction.

The complete structure of MgSeO4·9H2O has been refined from neutron single-crystal diffraction data obtained at 5, 100, 175 and 250 K. It is monoclinic, space group P2₁/c, Z = 4, with unit-cell parameters a = 7.222 (2), b = 10.484 (3), c = 17.327 (4) Å, β = 109.57 (2)°, and V = 1236.1 (6) Å(3) [ρ(calc) = 1770 (1) kg m(-3)] at 5 K. The structure consists of isolated [Mg(H2O)6](2+) octahedra, [SeO4](2-) tetrahedra and three interstitial lattice water molecules, all on sites of symmetry 1. The positions of the H atoms agree well with those inferred on the basis of geometrical considerations in the prior X-ray powder diffraction structure determination: no evidence of orientational disorder of the water molecules is apparent in the temperature range studied. Six of the nine water molecules are hydrogen bonded to one another to form a unique centrosymmetric dodecamer, (H2O)12. Raman spectra have been acquired in the range 170-4000 cm(-1) at 259 and 78 K; ab initio calculations, using density functional theory, have been carried out in order to aid in the analysis of the Raman spectrum as well as providing additional insights into the geometry and thermodynamics of the hydrogen bonds. Complementary information concerning the thermal expansion, crystal morphology and the solubility are also presented.

Structural invariance upon antiferromagnetic ordering in geometrically frustrated swedenborgite, CaBaCo2Fe2O7

Centimetre-sized single crystals of high-quality CaBaCo2Fe2O7were synthesized by the optical floating zone technique. The metal-to-metal stoichiometry and oxygen content were confirmed by spectroscopy and thermal reduction experiments. The hexagonal symmetryP63mc(No. 186) well describes the powder X-ray and neutron diffraction as well as single-crystal neutron diffraction at all measured temperatures. This symmetry is also consistent with optical second harmonic generation data obtained between 10 and 295 K. However, a satisfactory structure description from single-crystal neutron diffraction data needs an oxygen split position. Specific heat, magnetic susceptibility and powder neutron diffraction data indicate a magnetic phase transition atTN= 159 K to an antiferromagnetic ground state, but with a persisting hexagonal symmetry and intrinsic geometric frustration.

Successive spin-flop transitions of a Néel-type antiferromagnetLi2MnO3single crystal with a honeycomb lattice

We have carried out high magnetic field studies of single-crystalline Li$_2$MnO$_3$, a honeycomb lattice antiferromagnet. Its magnetic phase diagram was mapped out using magnetization measurements at applied fields up to 35 T. Our results show that it undergoes two successive meta-magnetic transitions around 9 T fields applied perpendicular to the ab-plane (along the c*-axis). These phase transitions are completely absent in the magnetization measured with field applied along the ab-plane. In order to understand this magnetic phase diagram, we developed a mean-field model starting from the correct Neel-type magnetic structure, consistent with our single crystal neutron diffraction data at zero field. Our model calculations succeeded in explaining the two meta-magnetic transitions that arise when Li$_2$MnO$_3$ enters two different spin-flop phases from the zero field Neel phase.

Lower resolution Laue neutron data yield correct nanocluster formulations

"The renaissance in Laue studies - at neutron sources - provides us with access to single crystal neutron diffraction data for synthetic compounds without requiring synthesis of prohibitively large amounts of compound or improbably large crystals. Such neutron diffraction studies provide vital data where proof of the presence or absence of hydrogen in particular locations is required and which cannot validly be proved by X-ray studies. Since the commissioning of KOALA at OPAL in 2009[1] we have obtained numerous data sets which demonstrate the vital importance of measuring data even where the extent of the diffraction pattern is at relatively low resolution - especially when compared to that obtainable for the same compound with X-rays. In the Laue experiment performed with a fixed radius detector, data reduction is only feasible for crystals in the ""goldilocks"" zone – where the unit cell is relatively large for the detector, a correspondingly low resolution diffraction pattern in which adjacent spots are less affected by overlap will yield more data against which a structure can be refined than a pattern of higher resolution – one where neighbouring spots overlap rendering both unusable (in our current methodology). Analogous application of powder neutron diffraction in such determinations is also considered. Single crystal neutron diffraction studies of several important compounds (up to 5KDa see figure below)[2] in which precise determination of hydride content by neutron diffraction was pivotal to the final formulation will be presented. The neutron data sets typically possess 20% or fewer unique data at substantially "lower resolution" than the corresponding X-ray data sets. Careful refinement clearly reveals chemical detail which is typically unexplored in related X-ray diffraction studies reporting high profile chemistry despite the synthetic route being one which hydride ought to be considered/excluded in product formulation."

Application of Charge & Band-flipping to Time-of-Flight Neutron Diffraction Data

Charge-flipping has become a popular approach to ab-initio structure solution from X-ray powder diffraction data in particular due to its speed and need for minimal input other than lattice parameters. Given the appetite of charge-flipping for low d-spacing reflections, time-of-flight (TOF) neutron data should be a good match from a resolution standpoint, with easy access to high Q and lack of form-factor drop-off. One obvious issue with neutron data is the presence of elements with negative scattering lengths, where the inherent assumption of atoms always having positive `density' in the algorithm breaks down. This means that portions of the structure can be effectively invisible. Given that some of these elements (e.g. H and Mn) are commonly found in samples of interest the issue is more than simple academic curiosity. Of course such atoms can be found by difference maps, but the issue has also been addressed within the charge-flipping algorithm with the `band-flipping' modification [1]. Although Oszlányi & Sütö demonstrated the approach was viable with simulated neutron single crystal data [1], to the authors' knowledge it hasn't been used previously with experimental single crystal or powder neutron diffraction data. Powder diffraction data from POWGEN and wavelength-resolved TOF Laue single crystal data from TOPAZ at the Spallation Neutron Source have been used to probe the relative ease of charge-flipping with different TOF data using the TOPAS software package [2]. In addition the effectiveness of different customized band-flipping approaches has been tested to extract positions for positive and negative scattering elements simultaneously.

Synthesis, crystal structure, and magnetic properties of novel intermetallic compounds R2Co2SiC (R = Pr, Nd).

The intermetallic compounds R2Co2SiC (R = Pr, Nd) were prepared from the reaction of silicon and carbon in either Pr/Co or Nd/Co eutectic flux. These phases crystallize with a new stuffed variant of the W2CoB2 structure type in orthorhombic space group Immm with unit cell parameters a = 3.978(4) Å, b = 6.094(5) Å, c = 8.903(8) Å (Z = 2; R1 = 0.0302) for Nd2Co2SiC. Silicon, cobalt, and carbon atoms form two-dimensional flat sheets, which are separated by puckered layers of rare-earth cations. Magnetic susceptibility measurements indicate that the rare earth cations in both analogues order ferromagnetically at low temperature (TC ≈ 12 K for Nd2Co2SiC and TC ≈ 20 K for Pr2Co2SiC). Single-crystal neutron diffraction data for Nd2Co2SiC indicate that Nd moments initially align ferromagnetically along the c axis around ∼12 K, but below 11 K, they tilt slightly away from the c axis, in the ac plane. Electronic structure calculations confirm the lack of spin polarization for Co 3d moments.

WinCSD: software package for crystallographic calculations (Version 4)

The fourth version of the program packageWinCSDis multi-purpose computer software for crystallographic calculations using single-crystal and powder X-ray and neutron diffraction data. The software environment and the graphical user interface are built using the platform of the Microsoft .NET Framework, which grants independence from changing Windows operating systems and allows for transferring to other operating systems. Graphic applications use the three-dimensional OpenGL graphics language.WinCSDcovers the complete spectrum of crystallographic calculations, including powder diffraction pattern deconvolution, crystal structure solution and refinement in 3 + dspace, refinement of the multipole model and electron density studies from diffraction data, and graphical representation of crystallographic information.

Hydrogen Bonding of O-Ethylxanthate Compounds and Neutron Structural Determination of C–H···S Interactions

A range of ethylxanthate (EtXn) salts, containing either protic or aprotic cations (guanidinium (1), methylammonium (2), dimethylammonium (3), trimethylammonium (4), tetramethylammonium (5), tetraethylammonium (6), and tetrapropylammonium (7)), have been synthesised and structurally characterised. The cations in these compounds differ in their degree of hydrogen-bonding ability, i.e. the number of donor groups, with significant structural consequences. Compounds 1–4 contain cations that are able to form N–H···S hydrogen bonds, with six, three, two, and one donor groups in 1–4 respectively. The number of donor atoms affects greatly the dimensionality of the hydrogen-bonding networks in the solid state. The structure of 1 has a 3-D hydrogen-bonding network, 2 and 3 form 2-D sheets and 1-D chains respectively, whereas the lone NH donor group in 4 has strong hydrogen bonding only within a discrete cation–anion pair. The tetraalkylammonium salts 5–7 have no strong hydrogen bonding, with only C–H···S and C–H···O interactions possible. To determine unambiguously the presence of such interactions, single-crystal Laue neutron diffraction data were obtained for compound 5, providing a fully anisotropic model, which can be used to rationalise potential close interactions in the other structures. The neutron structure of 5 confirms the existence of C–H···S hydrogen bonds, with the H···S distance falling well within the sum of the van der Waals radii of the atoms. The close-packing in 5–7 is mediated solely through these weak interactions, with the size of the cations influencing the structures.

Crystal growth and neutron diffraction studies of LixCoO2 bulk single crystals

The first successful growth of neutron-size single-crystal LixCoO2 by the optical floating-zone technique is reported. Structural properties have been studied using the time-of-flight neutron Laue diffraction technique. Our experiment is the first report of the LixCoO2 single-crystal neutron study. The neutron diffraction profile yields sharp, strong Bragg reflections, indicating a single grain of high crystalline quality. The structural refinement from the single-crystal neutron diffraction data indicated a trigonal structure of space group R3¯m, and a Li concentration x=0.87. No superlattice reflections were detected. The surface morphology analysed by scanning electron microscopy revealed the absence of cracks. The magnetic susceptibility was measured in a field of 1T with Hc and H⊥c, and an antiferromagnetic transition was observed at ~10K, with no magnetic impurities.

Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

Contrary to previous studies that identified the ground state crystal structure of the entire ${R}_{3}$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_{3}$Co undergoes a structural phase transition at ${T}_{t}\ensuremath{\simeq}160$ K. Single crystal neutron diffraction data reveal that at ${T}_{t}$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_{3}$Co. In particular, a transversal acoustic phonon mode along the $(00\ensuremath{\xi})$ direction has imaginary frequencies at $\ensuremath{\xi}<1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\ensuremath{\xi})$ phonon mode below a critical temperature ${T}_{t}$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_{3}$Co as a system with the charge density wave instability.

Neutron and X-ray crystallographic analysis of the human α-thrombin–bivalirudin complex at pD 5.0: Protonation states and hydration structure of the enzyme–product complex

The protonation states and hydration structures of the α-thrombin–bivalirudin complex were studied by joint XN refinement of the single crystal X-ray and neutron diffraction data at resolutions of 1.6 and 2.8Å, respectively. The atomic distances were estimated by carrying out X-ray crystallographic analysis at 1.25Å resolution. The complex represents a model of the enzyme-product (EP) complex of α-thrombin. The neutron scattering length maps around the active site suggest that the side chain of H57/H was deuterated. The joint XN refinement showed that occupancies for Dδ1 and Dε2 of H57/H were 1.0 and 0.7, respectively. However, no significant neutron scattering length density was observed around the hydroxyl oxygen Oγ of S195/H, which was close to the carboxylic carbon atom of dFPR-COOH. These observations suggest that the Oγ atom of S195/H is deprotonated and maintains its nucleophilicity in the EP complex. In addition to the active site, the hydration structures of the S1 subsite and the Exosite I, which are involved in the recognition of bivalirudin, are presented.

Neutron and X-ray crystallographic analysis of Achromobacter protease I at pD 8.0: Protonation states and hydration structure in the free-form

The structure of the free-form of Achromobacter protease I (API) at pD 8.0 was refined by simultaneous use of single crystal X-ray and neutron diffraction data sets to investigate the protonation states of key catalytic residues of the serine protease. Occupancy refinement of the catalytic triad in the active site of API free-form showed that ca. 30% of the imidazole ring of H57 and ca. 70% of the hydroxyl group of S194 were deuterated. This observation indicates that a major fraction of S194 is protonated in the absence of a substrate. The protonation state of the catalytic triad in API was compared with the bovine β-trypsin–BPTI complex. The comparison led to the hypothesis that close contact of a substrate with S194 could lower the acidity of its hydroxyl group, thereby allowing H57 to extract the hydrogen from the hydroxyl group of S194. H210, which is a residue specific to API, does not form a hydrogen bond with the catalytic triad residue D113. Instead, H210 forms a hydrogen bond network with S176, H177 and a water molecule. The close proximity of the bulky, hydrophobic residue W169 may protect this hydrogen bond network, and this protection may stabilize the function of API over a wide pH range.