Neutron Powder Diffraction Study Research Articles

Sr0.90Ba0.10Co0.95Ti0.05O3-δ cathode as an improved electrode for IT-SOFCs

The novel perovskite Sr0.90Ba0.10Co0.95Ti0.05O3-δ has been designed as cathode material to be used in intermediate-temperature solid oxide fuel cells (IT-SOFC). To enlarge the unit-cell size and enhance oxygen diffusion, Sr atoms have been partially substituted by a 10 % of Ba. This new compound was characterized through X Ray Diffraction (XRD) unveiling a tetragonal phase at RT. An in situ Neutron Powder Diffraction (NPD) study between RT and 800 °C showed a transition to a cubic perovskite at 400 °C, which was also noticeable in the thermogravimetric analysis (TGA) and dilatometric analysis. Adequate chemical and mechanical compatibility were achieved with the components of the cell. Electrical conductivity peaked at 400 °C, with values over 200 S cm−1, where it changed its behavior from semiconductor-like to insulator. Finally, polarization resistances as low as 0.062 Ω cm−2 were obtained for the working temperature of the cell in a symmetric cell, associated with the excellent power densities of 564 and 752 mW cm−2 obtained at 850 °C using pure H2 as fuel for the conventional test fuel cell and in a Pd-infiltrated one.

Neutron Vibrational Spectroscopic Study of the Acetylene: Ammonia (1:1) Cocrystal Relevant to Titan, Saturn's Moon.

The surface of Titan, Saturn's icy moon, is believed to be composed of various molecular minerals with a great diversity in structure and composition. Under the surface conditions, 93 K and 1.45 atm, most small molecules solidify and form minerals, including acetylene and ammonia. These two compounds can not only form single-component solids but also a 1:1 binary cocrystal that exhibits intriguing rotor phase behavior. This cocrystal is a putative mineral on Titan and other planetary bodies such as comets. In addition, the structure of the cocrystal is relevant to fundamental science as it can help better understand the emergence of rotor phases. Here, we present a detailed vibrational neutron spectroscopic study supported by a neutron powder diffraction study on the cocrystal and the single-phase solids. The experimentally observed spectral bands were assigned based on theoretical calculations. The established spectra-properties correlations for the cocrystal corroborate the observed properties. To the best of our knowledge, this study presents the first example of the application of neutron vibrational spectroscopy in studying Titan-relevant organic minerals.

High adsorption of ammonia in a titanium-based metal-organic framework.

We report the high adsorption of NH3 in a titanium-based metal-organic framework, MFM-300(Ti), comprising extended [TiO6]∞ chains linked by biphenyl-3,3',5,5'-tetracarboxylate ligands. At 273 K and 1 bar, MFM-300(Ti) shows an exceptional NH3 uptake of 23.4 mmol g-1 with a record-high packing density of 0.84 g cm-3. Dynamic breakthrough experiments confirm the excellent uptake and separation of NH3 at low concentration (1000 ppm). The combination of in situ neutron powder diffraction and spectroscopic studies reveal strong, yet reversible binding interactions of NH3 to the framework oxygen sites.

The Magnetism of Nonstoichiometric Sr2Cr1+xRe1-xO6 (0 < x < 0.5) Double Perovskites.

The effect of nonstoichiometry on the cation distribution, crystal structure, and magnetic properties of a series of Cr-rich Sr2Cr1+xRe1-xO6 samples has been investigated. The double perovskite structure is maintained over a wide solid solution range that extends from x = 0 to approximately x = 0.5. For most of the solid solution range, the Cr-rich octahedral site maintains a nearly constant occupancy, 87% Cr and 13% Re, that is comparable to prior studies of Sr2CrReO6, while Cr steadily replaces Re on the other octahedral site. As x approaches 0.5, long-range Cr/Re ordering drops precipitously. Analysis of X-ray powder diffraction peak shapes reveals antiphase boundaries, associated with Cr/Re ordering, the concentration of which increases steadily with increasing x. Neutron powder diffraction studies confirm antiferromagnetic coupling between antisite Cr3+ ions and Cr3+ ions that occupy the normal sites, leading to site-dependent ferrimagnetic ordering. Density functional theory calculations indicate that chromium maintains a +3 oxidation state across the series, while the oxidation state of rhenium increases with increasing x. Calculations are also used to explore the energies of competing magnetic ground states. Except for the most chromium-rich compositions (x ≈ 0.5), site-dependent ferrimagnetism is retained with only a modest reduction in TC. The saturation magnetization steadily decreases as the chromium content increases due to a combination of Cr/Re antisite disorder and antiphase boundaries.

Impact of Host-Guest Interactions on the Dielectric Properties of MFM-300 Materials.

Metal-organic framework (MOF) materials are attracting increasing interest in the field of electronics due to their structural diversity, intrinsic porosity, and designable host-guest interactions. Here, we report the dielectric properties of a series of robust materials, MFM-300(M) (M = Al, Sc, Cr, Fe, Ga, In), when exposed to different guest molecules. MFM-300(Fe) exhibits the most notable increase in dielectric constant to 35.3 ± 0.3 at 10 kHz upon adsorption of NH3. Structural analysis suggests that the electron delocalization induced by host-guest interactions between NH3 and the MOF host, as confirmed by neutron powder diffraction studies, leads to structural polarization, resulting in a high dielectric constant for NH3@MFM-300(Fe). This is further supported by ligand-to-metal charge-transfer transitions observed by solid-state UV/vis spectroscopy. The high detection sensitivity and stability to NH3 suggest that MFM-300(Fe) may act as a powerful dielectric-based sensor for NH3.

(Invited) Operando Observation of Fast Charging in Li-Ion Cells

Fast charging (<15 min) of lithium-ion batteries (LIBs) for electrical vehicles (EVs) is widely seen as the key factor that will greatly stimulate the EV markets, and its realization is mainly hindered by the sluggish diffusion of Li+. To have a mechanistic understanding of Li+ diffusion within LIBs, in this study, structural evolutions of electrodes for a Ni-rich LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cylindrical cell with high areal loading (2.78 mAh cm−2) are developed for operando neutron powder diffraction study at different charging rates. Via sequential Rietveld refinements, changes in structures of NMC622 and Li x C6 are obtained during moderate and fast charging (from 0.27 C to 4.4 C). NMC622 exhibits the same structural evolution regardless of C-rates. For phase transitions of Li x C6, the stage I (LiC6) phase emerges earlier during the stepwise intercalation at a lower state of charge when charging rate is increased. It is also found that the stage II (LiC12) → stage I (LiC6) transition is the rate-limiting step during fast charging. The LiC12 → LiC6 transition mechanism is further analyzed using the Johnson–Mehl–Avrami–Kolmogorov model. It is concluded as a diffusion-controlled, 1D phase transition with decreasing nucleation kinetics under increasing charging rates.

Ni-doping assisted modification of the non-collinear antiferromagnetic ordering in Mn5Si3 alloy

Ni-doped Mn5Si3 alloys of nominal compositions Mn5−xNixSi3 (for x = 0.05, 0.1, and 0.2) have been investigated through detailed neutron powder diffraction (NPD) studies in zero magnetic field and ambient pressure. At room temperature, all three Ni-doped alloys crystallize with D88 type hexagonal structure having P63∕mcm space group. These alloys undergo paramagnetic → collinear antiferromagnetic → non-collinear antiferromagnetic transitions on cooling from room temperature. A significant decrease in collinear to non-collinear antiferromagnetic transition temperature has been observed with increasing Ni concentration. The magnetic structure of both antiferromagnetic phases can be described by the magnetic propagation vector k = (0,1,0). However, the moment size and the orientation in the non-collinear antiferromagnetic phase are found to be notably affected by the Ni-doping. Approaching near-parallel arrangement of Mn-moments with increasing Ni-doping is found to be responsible for the gradual disappearance of unusual magnetic properties (inverted hysteresis loop, thermomagnetic irreversibility, etc.) observed in Mn5Si3 alloy.

Bonding mechanism and magnetic ordering in Laves phase λ1−MgCo2 intermetallic compound from theoretical and experimental studies

In the present work we have studied the bonding mechanism and magnetism in a hexagonal λ1−MgCo2 Laves phase intermetallic formed at rA/rB= 1.280. Ab initio theoretical calculations for MgCo2 using the projector-augmented wave method implemented in the VASP code showed that Mg carries a formal charge +1.7 |e| while Co atoms are also positively charged with around +0.7 |e| being donated to establish the interatomic covalent Co-Co bonding in a three-dimensional network of Co tetrahedra. MgCo2 is a metallic conductor with ferromagnetic properties and TC=303 K. Neutron Powder Diffraction study showed a collinear ordering of the Co moments, their alignment along the c-axis of the hexagonal unit cell with an average moment of 1.51(2) μB/atom Co. Ab initio computational studies indicate that the Co moments reach the experimentally observed value approx.1.5 μB when the unit cell volume exceeds 160 Å3 (average Co-Co distances 2.43 Å).

Antiferromagnetic structure with strongly reduced ordered moment of p-electron in CsO2

We have performed a powder neutron diffraction study on CsO2, where the unpaired electron with in the orbital of the O ion is responsible for the magnetism. The magnetic reflections 0 0 and 0 1 were observed below the Néel temperature of about 10 K. An antiferromagnetic structure with a propagation vector of (0, , 0) and magnetic moments parallel to the a-axis is the most plausible. The magnitude of the ordered moment is about 0.2 µ B , which is considered to be strongly reduced due to the one-dimensionality of the system. We propose a possible orbital order that can explain the obtained magnetic structure, and discuss its relation to the one-dimensionality.

Location of Brønsted sites in deuterated L-zeolite: A combined neutron powder diffraction and computer modeling study

In this work both concentration and location of Brønsted acid sites (BASs) in a deuterated L-zeolite were determined by combining neutron powder diffraction with ab-initio molecular dynamics modelling. From the structure refinement of the acidic L zeolite, two Brønsted acid sites were identified. The first one, D1, is on the framework oxygen O5 pointing toward the center of the 8-membered ring channel; the second one, D2, located in proximity of the framework oxygen O1 pointing toward the 12-membered ring channel running parallel to the c-axis. On the whole, ∼7.7 acid sites were located in the unit-cell, corresponding to 58% and 14% for D1 and D2, respectively. The average structure of the simulated D-LTL was in good agreement with the experimental structure. The modelling revealed that the D2 BAS shows a large degree of mobility, consistent with the high thermal factor associated with that atomic position. The D1 BAS showed a behaviour consistent with static disorder, while the D2 BAS was characterized by a significant degree of dynamical disorder. In addition to these findings, the study also revealed that the O1 BAS had a higher Brønsted base character compared to the average basicity of the framework oxygens of the 12-membered ring channels.

Neutron powder diffraction studies of metal–organic frameworks for gas storage and separation

Neutron powder diffraction studies of metal–organic frameworks for gas storage and separation

Unconventional multiferroicity induced by structural distortion and magnetostriction effect in the layered spin-1/2 ferrimagnet Bi2Cu5B4O14

Complex systems with strongly entangled microscopic degrees of freedom often host a plethora of exotic properties. Herein, we report the unconventional and interesting multiferroic behavior of a layered noncentrosymmetric ferrimagnetic spin-1/2 system, ${\mathrm{Bi}}_{2}{\mathrm{Cu}}_{5}{\mathrm{B}}_{4}{\mathrm{O}}_{14}$. Neutron powder-diffraction study deciphers a robust ferrimagnetic ($\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) ordering below a temperature ${T}_{C}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which remains unaltered even under a high magnetic field of $H=10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Interestingly, two successive ferroelectric transitions are observed, one of which is triggered by the magnetostriction effect and emerges concomitantly with the ferrimagnetic ordering at ${T}_{C}={T}_{E2}=25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This is accompanied by a pronounced dielectric peak and a prominent magnetodielectric coupling at ${T}_{C}$. By contrast, other ferroelectric transition commences in the paramagnetic state at ${T}_{E1}=32\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is solely caused by a prominent structural distortion. This strong structural distortion is revealed by the synchrotron x-ray-diffraction study, which is further supported by a concurrent, sharp, anomalous phonon softening observed in the Raman spectra.

Noncryogenic Air Separation Using Aluminum Formate Al(HCOO)3 (ALF)

Separating oxygen from air to create oxygen-enriched gas streams is a process that is significant in both industrial and medical fields. However, the prominent technologies for creating oxygen-enriched gas streams are both energy and infrastructure intensive as they use cryogenic temperatures or materials that adsorb N2 from air. The latter method is less efficient than the methods that adsorb O2 directly. Herein, we show, via a combination of gas adsorption isotherms, gas breakthrough experiments, neutron and synchrotron X-ray powder diffraction, Raman spectroscopy, and computational studies, that the metal-organic framework, Al(HCOO)3 (ALF), which is easily prepared at low cost from commodity chemicals, exhibits substantial O2 adsorption and excellent time-dependent O2/N2 selectivity in a range of 50-125 near dry ice/solvent (≈190 K) temperatures. The effective O2 adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O2 in air) is ≈1.7 mmol/g, and at ice/salt temperatures (≈250 K), it is ≈0.3 mmol/g. Though the kinetics for full adsorption of O2 near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O2 adsorption are fast, suggesting that O2 separation using ALF with rapid temperature swings at ambient pressures is a potentially viable choice for low-cost air separation applications. We also present synthetic strategies for improving the kinetics of this family of compounds, namely, via Al/Fe solid solutions. To the best of our knowledge, ALF has the highest O2/N2 sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..

A combined powder x-ray and neutron diffraction studies on (1-x) NaNbO3-x Na0.50Bi0.50TiO3 solid solution

Solid solution of sodium niobate and sodium bismuth titanate [(1-x)NaNbO3-x(Na0.5Bi0.5)TiO3: xNBT)] is an important lead-free eco-friendly piezo-ceramic material. In the present work, we report an investigation of the phase stabilities of this solid solution using powder x-ray and neutron diffraction techniques for the entire range of composition. We observed the disappearance of certain peaks and changes in the main perovskite peaks in the powder diffraction data suggesting structural phase transitions with composition. Rietveld analysis of combined x-ray and neutron powder diffraction data is performed to obtain single correct structural model wherever possible. Detailed analyses of powder diffraction data for composition range 0.0 ≤ x ​< ​0.15 revealed the coexistence of orthorhombic (Pbma) and (Pmc21) phases and an orthorhombic phase with space group Pbnm and cell dimensions √2ap ​× ​√2 bp ​× ​6 cp gets stabilized for 0.15 ≤ x ​< ​0.20. On further increasing NBT content in NaNbO3 matrix (0.20 ≤ x ​< ​0.60), a mixed orthorhombic (Pbnm) phase and tetragonal (P4bm) phase is confirmed by Rietveld refinement. Further, in the composition range 0.60 ≤ x ​≤ ​0.80, the tetragonal (P4bm) phase coexists with the ferroelectric rhombohedral (R3c) phase and finally for composition x ​> ​0.80, it completely transforms to rhombohedral (R3c) phase. The presence of phase coexistence in various composition ranges indicates the first order nature of these phase transitions. A detailed structural phase diagram of the system is presented at ambient conditions.

Low-temperature crystal structures of the solvent dimethyl carbonate

Dimethyl carbonate (DMC) is an important industrial solvent but is additionally a common component of liquid lithium-ion battery electrolytes. Pure DMC has a melting point of 277 K, so encountering solidification under outdoor climatic conditions is very likely in many locations around the globe. Even eutectic, ethylene carbonate:dimethyl carbonate commercial LiPF6 salt electrolyte formulations can start to solidify at temperatures around 260 K with obvious consequences for their performance. No structures for crystalline DMC are currently available which could be a hindrance for in situ battery studies at reduced temperatures. A time-of-flight neutron powder diffraction study of the phase behavior and crystal structures of deuterated DMC was undertaken to help fill this knowledge gap. Three different orthorhombic crystalline phases were found with a previously unreported low-temperature phase transition around 50–55 K. The progression of Pbca → Pbcm → Ibam space groups follow a sequence of group–subgroup relationships with the final Ibam structure being disordered around the central carbon atom.

Temperature Dependent Crystal Structure of Nd2CuTiO6: An In Situ Low Temperature Powder Neutron Diffraction Study

Herein we reported the crystal structure and crystal chemistry of orthorhombic perovskite type Nd2CuTiO6 in between 2 K and 290 K as observed from the in situ temperature-dependent powder neutron diffraction (PND) studies. It is observed that the cations in octahedral sites are statistically occupied, and the ambient temperature orthorhombic structure is retained throughout the temperature range of the study. Absence of any long-range magnetic ordering down to 2 K is confirmed by both low-temperature PND and magnetization studies. The lattice shows strong anisotropic thermal expansion with increasing temperature, viz. almost no or feeble negative expansion along the a-axis while appreciably larger expansion along the other two axes (αb = 10.6 × 10−6 K−1 and αc = 9.8 × 10−6 K−1). A systematic change in the rotation of octahedral units with temperature was observed in the studied temperature range, while the expansion of unit cells is predominantly associated with the polyhedral units around the Nd3Ions. The temperature-dependent relative change in unit cell parameters as well as coefficients of axial thermal expansion show anomalous behavior at lower temperatures, and that seems to be related to the electronic contributions to lattice expansion.

Combined X-ray and Neutron Powder Diffraction Study on B-Site Cation Ordering in Complex Perovskite La2(Al1/2MgTa1/2)O6

Complex perovskite La2(Al1/2MgTa1/2)O6 (LAMT) crystallizes in a monoclinic unit cell with space group P21/n at room temperature. Its B-site cations are ordered in a rock-salt-type arrangement. Previously, the full occupancy of Mg on the 2c-Wyckoff position was deduced from powder X-ray diffraction (PXRD). However, conventional X-rays could not properly resolve the mixed occupation on the B-site, since there is little scattering contrast between the neighbouring elements Mg and Al of the periodic table. Hence, complementary neutron diffraction studies were carried out to verify the exact B-site cation ordering in the unit cell. In this specific configuration of the B-cations, with its occupancy ratio and the presence of a heavy element Ta as well as neighbouring elements Mg and Al, only the strategy of a combined Rietveld analysis using both the X-ray and neutron diffraction data simultaneously succeeded in elucidating an accurate B-site cation ordering in this complex perovskite system. A full occupancy of Mg on the 2c-Wyckoff position and each a half occupancy of Al and Ta on the 2d-Wyckoff position could be resolved for the rock-salt-type ordering of the B-site cations in the monoclinic unit cell of LAMT.

Canting Angle Behavior of Magnetic Moments in Y‐Substituted Tb2BaNiO5 and Its Relevance for Magnetoelectric Coupling

The Haldane‐spin chain compound, Tb2BaNiO5, has been known to be an exotic multiferroic system, exhibiting antiferromagnetic anomalies at T N1 = 63 K and T N2 = 25 K, with ferroelectricity appearing below T N2 only. Previous reports in addition have established that, interestingly, Tb ions play a direct and decisive role to lead to multiferroic properties with a critical canting angle of magnetic moments, unlike other well‐known multiferroics. Herein, the results of temperature‐dependent neutron powder diffraction studies on Tb2–x Y x BaNiO5 are reported, to get an insight into the critical canting angle for multiferroic behavior. While multiferroic transition temperature decreases linearly with Y concentration, there is an abrupt drop of relative canting angle (of Tb and Ni magnetic moments) with respect to that in the parent compound for an initial substitution of x = 0.5 in the multiferroic region, without any notable change thereafter. Therefore, it is inferred that this critical canting angle is made up of two components—cooperative (long‐range) and local (short‐range) contributions.

Magnetic structure of R[formula omitted]Sr[formula omitted]FeO[formula omitted] (R = Pr, Nd)

We present magnetization and neutron powder diffraction studies in the temperature range 2K to 300K for oxygen stoichiometric R1/3Sr2/3FeO3 (R = Pr and Nd). From full symmetry analysis, we proposed two magnetic models by a combined application of irreducible representations and magnetic space groups. Both models fit equally well the neutron powder diffraction data.

Display of converse and direct magnetoelectric effect in double perovskite LaYFe2O6

This work reports the simultaneous observation of converse magnetoelectric (CME) and direct magnetoelectric (DME) effects in LaYFe2O6. The structural, magnetic, and magnetoelectric properties of LaYFe2O6, prepared by the sol-gel auto-combustion method and sintered at various temperatures, have been studied. The x-ray powder diffraction study suggests the double perovskite structure with symmetries P21nm (∼90%) and Pbnm (∼10%). The alternate ordering of La and Y ions is confirmed by the neutron powder diffraction (ND) study, which also suggests the antiferromagnetic (AFM) ordering of spins. AFM behavior is also manifested by the magnetic field-dependent magnetization (M) measurement. A higher P21nm phase content is desirable in the context of magnetoelectricity. Magnetic transition (∼700 K) is asserted in the temperature-dependent M measurement. The isothermal magnetization study shows weak ferromagnetism probably due to gradually increasing spin canting with temperature until the transition temperature. The highest CME coefficient (∼2.26 mOe cm/V) as well as DME coefficient (∼0.45 mV/cm Oe) in this material are recorded. True magnetoelectricity for temperature as high as 400 K opens up a new avenue on the playground of magnetoelectric (ME)-based applications.