Neutron Powder Data Research Articles

Thermal behaviors during lithium diffusion in Li0.4WO3 bronze studied by elastic and quasi-elastic neutron scattering.

Polycrystalline Li0.4WO3 bronze has been synthesized by solid state reaction carried out in a silica tube at 10-7 MPa and 973 K. The sample is characterized by temperature-dependent neutron elastic and quasielastic scatterings. The room-temperature neutron powder data Rietveld refinement confirmed the space group Im3̄ along with lithium occupancy found predominantly at the 6b crystallographic site. Upon increasing temperature above 300 K lithium at 6b site decreases and at 2a site increases, suggesting Li+ cation diffusion between these two sites demonstrated by quasielastic neutron scattering as well. The lattice thermal expansion anomaly is observed between 380 K and 450 K, which is explained in terms of lithium dynamic disorder (non-equilibrium) as complemented by elastic and quasielastic neutron scatterings. DFT calculations with different lithium distributions at two different crystallographic sites guide to understand the lattice expansion anomaly. The lattice thermal expansion is modeled using Grüneisen first-order approximation, where the Debye-Einstein-Anharmonicity approach provides the temperature-dependent vibrational energy. The DFT-calculated phonon density of states and bulk modulus help extract the characteristic Debye and Einstein frequencies.

Acta Crystallographica Section C: Chemistry Matters.

Chemistry matters to Acta Crystallographica Section C. Authors are encouraged to present new chemistry where there is a clear chemical story based around molecular structure. In this context, `structure' is used loosely and is not in any way restricted to structures determined by single-crystal X-ray crystallography, but includes those determined from powder X-ray or neutron data and NMR spectroscopy, and through theoretical calculations.

Neutron diffraction study of nitride perovskite LaReN3

AbstractNitride perovskites ABN3 are extremely rare with only two known instances of bulk‐phase materials, ThTaN3 and LaReN3. Here, we present a powder neutron diffraction study of the recently‐reported nitride perovskite LaReN3. A full structural model of LaReN3 at 300 K ( , a=5.58523(5), b=5.59297(5), c=7.90419(6) Å, α=89.9170(8), β=90.1729(7), γ=90.1709(7)°) has been refined simultaneously against powder neutron and previously‐collected synchrotron X‐ray diffraction data. This indicates that the triclinic distortion is due to cooperative electronic distortions of ReN6 octahedra, and also demonstrates that all nitrogen positions are fully occupied so the sample is stoichiometric LaReN3. Lattice strains derived from fits to variable temperature powder neutron data from 5 to 300 K indicate that LaReN3 undergoes structural transitions to higher symmetry perovskite tilt structures between 300 and 350 K.

The crystal chemistry and electrical properties of Fe doped Ca12Al14O33 (Mayenite)


 
 
 X-ray and neutron powder diffraction have been used to study the crystal chemistry of Fe doped mayenite (Ca12Al14-xFexO33). Solid- state synthesis was used to prepare Ca12Al14-xFexO33 where x = 0, 0.1, 0.2, 0.5 and 0.6 and the citrate gel route was used to prepare Ca12Al14-xFexO33 where x = 0, 0.05, 0.1, 0.2, 0.3 and 0.4. X-ray powder diffraction data indicate that samples with the same composition but synthesized by the citrate gel route were more likely to be phase pure than samples obtained by traditional solid-state synthesis. The refined lattice parameters were observed to increase with increasing Fe concentration, irrespective of the synthesis method. Refined neutron powder data confirm that Fe is going into Al site rather than Ca site. A 2-point probe was used to measure the electrical properties of the Fe doped citrate gel synthesized samples and showed that the resistivity increases for the Fe doped samples compared to the undoped mayenite.
 
 

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.

Weak ferromagnetism and superparamagnetic clusters coexistence in YFe1−xCoxO3 (0 ≤ x ≤ 1) perovskites

YFe1-xCoxO3 (x=0, 0.3, 0.5, 0.7 and 1) perovskite solid solutions were synthesized by thermal decomposition of the cyano-metal complexes Y[Fe1-xCox(CN)6]·4H2O. All perovskites belong to the orthorhombic Pnma space group. Mössbauer spectra at room temperature show that samples with low Co3+ content are magnetically ordered, while samples with high Co3+ content are paramagnetic. A clear decrease of the magnetic order temperature was observed by magnetization vs. temperature experiments. At high temperatures, YFe0.3Co0.7O3 is paramagnetic and a non common increase of magnetization is observed. The later can be related to the thermal excitation of Co3+ in low spin (LS) configuration to intermediate (IS) or high spin (HS) configuration. Along the different compositions of YFe1-xCoxO3 solid solutions there is a competition between weak ferromagnetism (WFM) and superparamagnetic (SPM) clusters. WFM due to Fe3+ cations prevail in rich Fe3+ samples (x=0 and 0.3) and the Fe3+ cations that form SMP clusters prevail in the sample with x=0.7. Besides, in the sample with x=0.5 we postulate a slight prevalence of WFM due to Fe3+ cations. For YFe1-xCoxO3 with x=0.3, 0.5 and 0.7 neutron powder data at 2, 150 and 350K were refined with the irreducible representation Γ4 (Ax Fy Gz).

Structure and Magnetic Properties of KRuO4.

The crystal structure of KRuO4 is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, I41/a, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru7+ ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d1(e1). Curie-Weiss parameters are near free ion values for the effective moment and θ = -77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected R ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, J1, and the next-nearest-neighbor, J2, exchange constants are AF and of similar magnitude. A spin dimer analysis finds J1/J2 ≈ 25, indicating weak frustration, and a (dz2)1 ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the c axis. The Ru7+ moment is estimated to be 0.57(7) μB, reduced from an expected value near 1 μB. A recent computational study of isostructural, isoelectronic KOsO4 predicts a surprisingly large orbital moment due to spin-orbit coupling (SOC). However, the free ion SOC constant for Ru7+ is only ∼30% that of Os7+, so it is unclear that this effect can be implicated in the low ordered moment for KRuO4. The origin of the short-range spin correlations is also not understood.

Asymmetric band flipping for time-of-flight neutron diffraction data

Charge flipping with powder diffraction data is known to produce a result more reliably with high-resolution data, i.e. visible reflections at small d spacings. Such data are readily accessible with the neutron time-of-flight technique but the assumption that negative scattering density is nonphysical is no longer valid where elements with negative scattering lengths are present. The concept of band flipping was introduced in the literature, where a negative threshold is used in addition to a positive threshold during the flipping. However, it was not tested with experimental data at the time. Band flipping has been implemented in TOPAS together with the band modification of low-density elimination and tested with experimental powder and Laue single-crystal neutron data.

Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ5–TiNi2−xAl5, x=0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a=0.3984(2)nm, c=1.4073(3)nm, RF2=0.0133). Titanium atoms were unambiguously located from neutron powder data. τ5 is isotypic with the crystal structure of ZrNi2Al5. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ.=0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi2−xAl5−y, x=0.4, y=0.4 and HfNi2−xAl5−y, x=0.5, y=0.2. The crystallographic relations among the structure types of Cu, TiAl3, ZrNi2Al5 and Zr(Ni,Ga)7 have been defined in terms of a Bärnighausen scheme.

Complex room-temperature ferrimagnetism induced by zigzag stripes of oxygen vacancies inSr3YCo4O10+δ

The high temperature ferromagnetism in Sr3YCo4O10+delta perovskite, whose origin has been the subject of a considerable debate, has been studied by neutron powder diffraction and synchrotron X-ray diffraction measurements. Oxygen vacancy ordering creates a complex pattern of zigzag stripes in the oxygen-deficient CoO4+delta layers, where the Co ions are found in three distinct coordinations. The symmetry of this unprecedented structural modulation, in conjunction with the existence of different Co spin states, provide a straightforward explanation for the appearance of ferrimagnetism. A model for the magnetic structure compatible with these structural features is proposed, based on the refinement of powder neutron data. The macroscopic moment as a function of temperature that can be calculated from the values of the ordered spins extracted from refinements, is in excellent agreement with bulk magnetization. Unlike previous models, a collinear G-type magnetic structure with uncompensated moments due to distinct spin-states of Co imposed by different coordination is found.

Pyroxene from martian meteorite NWA856: Structural investigations by X-ray and neutron diffraction

Abstract Despite high mosaicity and complex micro structures in pyroxenes from Mars average occupancies and average atomic positions of pigeonite were determined from X-ray single crystal and neutron and X-ray powder diffraction data. The chemical composition of selected single crystals varied between En39Fs55Wo6 and En60Fs34Wo6. The average composition of the whole sample was determined from powder diffraction to be about En43Fs51Wo6. The ordering of the cations on the M1 and M2 positions, expressed by a partitioning or distribution coefficient K D, in the pigeonite structure was determined and a closure temperature of about 540 °C was estimated for crystals stemming from the core of our specimen. General trends of the crystal chemistry with K D are compared with literature data. From in-situ high temperature neutron powder data, however, it is deduced that a simple intra-phase-ordering process is not adequate, since diffusion processes between intimately intergrown augite and pigeonite phases at temperatures above 600 °C also take place. This makes the usual interpretation of structural data towards the prior history of the martian sample material doubtful.

ChemInform Abstract: A Novel Boroncarbide, Y2B3C2 - X-Ray Single Crystal and Neutron Powder Diffraction Analysis.

The crystal structure of Y2B3C2 was analysed by means of room temperature X-ray single crystal and neutron powder diffractometry. Y2B3C2 crystallises with the Gd2B3C2-type (space group: Cmmm - No. 65, Z=2). The unit cell dimensions are: a=0.3405(5), b=1.3765(6), c=0.3631(6) nm, b/a=4.044, V=0.1702 nm3 the X-ray density is: ρx=4.57 Mgm−3. The structure was solved by combined Patterson-Difference Fourier methods. Residual values are: RF=ΣΔF/ΣFo=0.027, Rw=0.020, for an asymmetric set of 232 independent reflections (Fo>3σ(Fo)). Boron atoms B1, each in triangular metal co-ordination, form infinite zigzag chains (dB1–B1=0.1923 nm) branched with carbon atoms. Bond angles in the chain are ϕB1–B1–B1=124.6°. Carbon atoms are at a distance of dB1–C=0.1552 in rectangular co-ordination of four metal atoms. The branched boron chains are connected by a bridging boron atom B2 linking the carbon atoms in form of a linear bridge at dC–B2=0.1442 nm. Rietveld refinement of the neutron powder data from Y2B3C2(11B–isotope) confirms the nuclear structure arrangement as well as the full occupancy of all the non-metal atom sites.

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc(0.5)Ta(0.5)O(3) (PST) and Pb(0.78)Ba(0.22)Sc(0.5)Ta(0.5)O(3) (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm3m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 111 cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R3c or R3 symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a(-)a(-)a(-) that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35 GPa.

Lithium Argyrodites with Phosphorus and Arsenic: Order and Disorder of Lithium Atoms, Crystal Chemistry, and Phase Transitions

Crystal chemical data of high- (HT) and low-temperature (LT) modifications of lithium argyrodites with the compositions Li(7)PCh(6) (Ch=S, Se), Li(6)PCh(5)X (X=Cl, Br, I), Li(6)AsS(5)Br, and Li(6)AsCh(5)I (Ch=S, Se) based on single-crystal, powder X-ray (113 K<T<503 K) and neutron measurements (5 K<T<293 K) are presented. In the HT modifications, the Li atoms are strongly disordered over a fraction of the available tetrahedral holes, whereas in the LT modifications they occupy ordered crystallographic positions with a pronounced site preference that is analysed on a crystal chemical basis. The Ch/X partial structures remain nearly unchanged upon the reversible phase transitions. Crystal chemical and crystallographic relations between HT and LT modifications based on the Frank-Kasper model of tetrahedral close packing are discussed. X-ray single-crystal data for HT-Li(6)PS(5)I show the electron density of the disordered Li to be smeared out over an extended region preferably inside face-sharing double tetrahedra. A series of temperature-dependent powder neutron data for Li(6)PS(5)I gives clear evidence for an HT/LT phase transition at approximately 175 K with an ordering of the Li atoms in different polyhedra with coordination numbers between three and four.

Texture and structural refinement using neutron diffraction data from molybdite (MoO3) and calcite (CaCO3) powders and a Ni-rich Ni50.7Ti49.30alloy

Preferred orientation or texture is a common feature of experimental powder patterns. The mathematics of two commonly used models for preferred orientation—the March-Dollase and the generalized spherical-harmonic models—is reviewed. Both models were applied individually to neutron powder data from uniaxially pressed molybdite (MoO3) and calcite (CaCO3) powders in Rietveld analyses, as well as the as-received powders. The structural refinement results are compared to single-crystal structures. The results indicate that reasonable refinement of crystal structures can be obtained using either the March model or generalized spherical-harmonic description. However, the generalized spherical-harmonic description provided better Rietveld fits than the March model for the molybdite and calcite. Therefore, the generalized spherical-harmonic description is recommended for correction of preferred orientation in neutron diffraction analysis for both crystal structure refinement and phase composition analysis. Subsequently, the generalized spherical-harmonic description is extended to crystal structure refinement of annealed and the aged polycrystalline Ni-rich Ni50.7Ti49.30shape memory alloys.

Synthesis and Characterization of the Ultramarine-Type Analog Na8−x[Si6Al6O24]·(S2,S3,CO3)1−2

The use of nitrite sodalite as a precursor to prepare new blue ultramarine analogs has been investigated. The encapsulation of the chromophores inside of beta cages was achieved by heating the nitrite sodalite precursor with a mixture of sodium carbonate, sulfur, and a reducing agent at about 1000 K under airtight conditions. The obtained new blue ultramarine-type material was characterized by means of X-ray and neutron powder diffraction, transmission electron microscopy, 29Si magic-angle-spinning NMR, IR, Raman, and diffuse reflectance spectroscopies. Thermogravimetric analysis and magnetic measurements were performed in order to determine water and chromophore contents, respectively. On the basis of the spectroscopic results and Rietveld analysis of neutron powder data, the beta cage filling was found to be yellow S2(-) and blue S3(-) chromophores along with carbonate anions. The determined chromophore concentration, about 0.31 mol per formula unit, is enough to have greenish-blue ultramarines, which proves that ultramarine analogs can be obtained from nitrite sodalite.

Water and Cation Distribution in Fully and Partially Hydrated Li-LSX Zeolite

Faujasite type zeolite Li-LSX with unit cell composition Li96[Si96Al96O384]·xH2O is a very promising sorption material for heat storage and energy transformation. To understand the hydration and dehydration processes and to optimize the sorption properties, the structures of partially and fully hydrated Li-LSX with water loadings of x = 8, 16, 32, 48, 96, and 270 H2O/D2O were analyzed based on neutron and synchrotron powder data. In addition, forcefield simulations were performed. At low loading of water the cations occupy three sites: SIC (in the center of a 6-ring window of the β-cage), SIIC (in the center of another 6-ring window of the β-cage), and SIII (in front of the 4-ring of the β-cage). Increasing hydration leads to the disordering of Li+ at site SIII, which interacts with water molecules. As a consequence, Li+ moves from SIII sites to other crystallographic sites. In total six different crystallographic water sites (W1−W6) were identified. Nearly all water molecules occupy the supercage; 2.3 wa...

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

For the first time, the 7M modulated structure, frequently observed in ferromagnetic shape memory Ni–Mn–Ga martensitic phases, is solved by powder diffraction analysis. Two polycrystalline samples with composition Ni 2Mn 1.2Ga 0.8 and Ni 2.15Mn 0.85Ga, respectively, showing a 7M martensitic state stable at room temperature, were studied. The determination of the modulated crystal structure of Ni 2Mn 1.2Ga 0.8 martensite was achieved by refining the X-ray powder diffraction pattern by the Rietveld method. The basic structure belongs to monoclinic symmetry. The crystal structure, solved within the superspace approach, is found to show an incommensurate 7M modulation with q = 0.308 c ∗. The Rietveld refinement for Ni 2.15Mn 0.85Ga martensite on the basis of neutron powder data surprisingly provides a very similar incommensurate 7M structure with the same periodicity and analogous modulation function. The incommensurate structure presents typical displacive modulation with several analogies with the Zhdanov (5, 2 ¯ ) 2 stacking sequence.

Cocrystallization with Acetylene: Molecular Complex with Methanol

Methanol forms a cocrystal with acetylene, of which single crystals were grown in situ. The methanol chains of the β-methanol are retained, and acetylene is intercalated between the layers. The weight ratio of host to guest is 1.23, the lowest and most efficient value for all known acetylene cocrystals. Further, the first single crystal structure for the low-temperature α-form of methanol is reported confirming previous neutron powder data. For the β-form of methanol, the space group Cmc21 is favored in contrast to former data in Cmcm.

Accurate molecular structures of chlorothiazide and hydrochlorothiazide by joint refinement against powder neutron and X-ray diffraction data

The compounds chlorothiazide and hydrochlorothiazide (crystalline form II) have been studied in their fully hydrogenous forms by powder neutron diffraction on the GEM diffractometer. The results of joint Rietveld refinement of the structures against multi-bank neutron and single-bank X-ray powder data are reported and show that accurate and precise structural information can be obtained from polycrystalline molecular organic materials by this route.