Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Powder diffraction”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate

Copper hexacyanoferrate, Cu II [Fe III (CN) 6 ] 2/3 nH 2 O, was synthesized, and varied amounts of K + ions were inserted via reduction by K 2 S 2 O 3 (aq). Ideally, the reaction can be written as Cu II [Fe III (CN) 6 ] 2/3 ∙ nH 2 O + 2x/3K + + 2x/3e⁻ ↔K 2x/3 Cu II [Fe II x Fe III 1- x (CN) 6 ] 2/3 nH 2 O. Infrared, Raman, and Mössbauer spectroscopy studies show that Fe III is continuously reduced to Fe II with increasing x, accompanied by a decrease of the a-axis of the cubic F$m\bar{3}$m unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction, ~20% of the Fe III , has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of ~26 wt % upon heating to 180 °C, above which the structure starts to decompose. The crystal structures of Cu II [Fe III (CN) 6 ] 2/3 ∙ nH 2 O and K 2/3 Cu[Fe(CN) 6 ] 2/3 ∙ nH 2 O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN) 6 groups are vacant, and the octahedron around Cu II is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the Cu N C Fe framework. In conclusion, the K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K + ions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Structural and Electronic Effects on the Properties of Fe 2 (dobdc) upon Oxidation with N 2 O

Here, we report electronic, vibrational, and magnetic properties, together with their structural dependences, for the metal–organic framework Fe 2 (dobdc) (dobdc 4 – = 2,5-dioxido-1,4-benzenedicarboxylate) and its derivatives, Fe 2 (O) 2 (dobdc) and Fe 2 (OH) 2 (dobdc)—species arising in the previously proposed mechanism for the oxidation of ethane to ethanol using N 2 O as an oxidant. Magnetic susceptibility measurements reported for Fe 2 (dobdc) in an earlier study and reported in the current study for Fe II 0.26 [Fe III (OH)] 1.74 (dobdc)(DMF) 0.15 (THF) 0.22 , which is more simply referred to as Fe 2 (OH) 2 (dobdc), were used to confirm the computational results. Theory was also compared to experiment for infrared spectra and X-ray powder diffraction structures. Structural and magnetic properties were computed by using Kohn-Sham density functional theory both with periodic boundary conditions and with cluster models. In addition, we studied the effects of different treatments of the exchange interactions on the magnetic coupling parameters by comparing several approaches to the exchange-correlation functional: generalized gradient approximation (GGA), GGA with empirical Coulomb and exchange integrals for 3d electrons (the GGA+U method), nonseparable gradient approximation with empirical Coulomb and exchange integrals for 3d electrons (GAM+U), hybrid GGA, meta-GGA, and hybrid meta- GGA. We found the coupling between the metal centers to be ferromagnetic in the case of Fe 2 (dobdc) and antiferromagnetic in the cases of Fe 2 (O) 2 (dobdc) and Fe 2 (OH) 2 (dobdc). The shift in magnetic coupling behavior correlates with the changing electronic structure of the framework, which derives from both structural and electronic changes that occur upon metal oxidation and addition of the charge-balancing oxo and hydroxo ligands.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17): Site-Selective Substitution, Electronic Structure, Chemical Bonding, and Structural Transformation

CoSn-type intermetallic compounds have emerged as a model platform for Kagome-derived flat-band physics, where subtle chemical perturbations can strongly influence electronic structure and phase stability. Here, we present a combined experimental and theoretical study of Sb-substitution in CoSn-type NiIn1–xSbx (0 ≤ x ≤ 0.17) to elucidate the interplay between site selectivity, solubility limit, chemical bonding, and electronic structure. Rietveld refinements on Neutron powder diffraction data confirmed the selective Sb-substitution at the electron-rich In2 (2d) site forming the honeycomb substructure, while the In1 (1a) site within the Kagome layer remains exclusively occupied by In. Density functional theory (DFT) calculations revealed that pristine CoSn-type NiIn hosts Ni 3d-dominated flat bands near the Fermi level (EF), originating from the Kagome-like Ni substructure. Partial replacement of In by Sb within the honeycomb layer alters these flat-band features below EF, reducing the density of states and suppressing the flat-band topology near the Fermi level. Orbital-resolved electronic structure and chemical-bonding analyses show that Sb-substitution enhances Ni-p-block (In/Sb) covalency and optimizes charge compensation, stabilizing the CoSn-type structure up to the solubility limit x ≈ 0.17. Beyond the limit, the higher-Sb compositions show satellite reflections consistent with an incommensurately modulated phase. These results establish a link between site-selective chemical substitution, bonding optimization, and flat-band electronic structure evolution, providing fundamental insights into how chemical substitution influences the electronic properties of Kagome-based intermetallic compounds.

Roy, Nilanjan [National Institute of Technology Si↗
DOE OSTI2017

Systematic Investigation of Controlled Nanostructuring of Mn 12 Single-Molecule Magnets Templated by Metal–Organic Frameworks

This is the first systematic study exploring metal–organic frameworks (MOFs) as platforms for the controlled nanostructuring of molecular magnets. We report the incorporation of seven single-molecule magnets (SMMs) of general composition [Mn12O12(O2CR)16(OH2)4], with R = CF3 (1), (CH3)CCH2 (2), CH2Cl (3), CH2Br (4), CHCl2 (5), CH2But (6), and C6H5 (7), into the hexagonal channel pores of a mesoporous MOF host. The resulting nanostructured composites combine the key SMM properties with the functional properties of the MOF. Synchrotron-based powder diffraction with difference envelope density analysis, physisorption analysis (surface area and pore size distribution), and thermal analyses reveal that the well-ordered hexagonal structure of the host framework is preserved, and magnetic measurements indicate that slow relaxation of the magnetization, characteristic of the corresponding Mn12 derivative guests, occurs inside the MOF pores. Structural host–guest correlations including the bulkiness and polarity of peripheral SMM ligands are discussed as fundamental parameters influencing the global SMM@MOF loading capacities. These results demonstrate that employing MOFs as platforms for the nanostructuration of SMMs is not limited to a particular host–guest system but potentially applicable to a multitude of other molecular magnets. Such fundamental findings will assist in paving the way for the development of novel advanced spintronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

The Crystal Structure and Magnetic Behavior of Quinary Osmate and Ruthenate Double Perovskites La ABB 'O 6 ( A = Ca, Sr; B = Co, Ni; B ' = Ru, Os)

In this work, six LaABB'O 6 (A = Ca, Sr; B = Co, Ni; B' = Ru, Os) double perovskites were synthesized, several for the first time, and their crystal structures and magnetic behavior were characterized with neutron powder diffraction and direct-current and alternating-current magnetometry. All six compounds crystallize with P2 1 /n space group symmetry, resulting from a – a – c + octahedral tilting and complete rock salt ordering of transition-metal ions. Despite the electronic configurations of the transition-metal ions, either d 8 –d 3 or d 7 –d 3 , not one of the six compounds shows ferromagnetism as predicted by the Goodenough–Kanamori rules. LaSrNiOsO 6 , LaSrNiRuO 6 , and LaCaNiRuO 6 display long-range antiferromagnetic order, while LaCaNiOsO 6 , LaCaCoOsO 6 , and LaSrCoOsO 6 exhibit spin-glass behavior. These compounds are compared to the previously studied LaCaCoRuO 6 and LaSrCoRuO 6 , both of which order antiferromagnetically. Lastly, the observed variations in magnetic properties can be attributed largely to the response of competing superexchange pathways due to changes in B–O–B' bond angles, differences in the radial extent of the 4d (B' = Ru) and 5d (B' = Os) orbitals, and filling of the t 2g orbitals of the 3d ion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Structure and Phase Transformation in the Giant Magnetostriction Laves-Phase SmFe 2

As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for their abundant magnetic properties. Samarium-iron alloy system, SmFe 2 , is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe 2 has been identified as a cubic Fd$ \overline{3}\ $m structure at room temperature, however, the cubic symmetry does not match the spontaneous magnetization along the [111] cubic direction. Here we studied the crystal structure of SmFe 2 by high-resolution synchrotron X-ray powder diffraction and X-ray total scattering methods. SmFe 2 is found to adopt a centrosymmetric trigonal R$ \overline{3}\ $m structure at room temperature, which transforms to an orthorhombic Imma structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111] cubic to [110] cubic direction, and is further evidenced by the inflexion of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the FC-ZFC curve, and the anomaly in the specific heat capacity measurement. Lastly, the revised structure and phase transformation of SmFe 2 could be useful to understand the magnetostriction and related physical properties of other RM 2 -type pseudo-cubic Laves-phase intermetallic compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Crystal Structure Evolution and Notable Thermal Expansion in Hybrid Perovskites Formamidinium Tin Iodide and Formamidinium Lead Bromide

The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI 3 , FA + = CH[NH 2 ] 2 + ) and formamidinium lead bromide (FAPbBr 3 ), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic Pm$\bar{3}$m (No. 221) to tetragonal P 4 /mbm (No. 127) for both materials upon cooling; this occurs for FAPbBr 3 between 275 and 250 K, and for FASnI 3 between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic Pnma (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI 3 and FAPbBr 3 are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K –1 for FASnI 3 at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the inorganic network and dynamic lone pair stereochemical activity on the B-site. Unusual pseudocubic behavior is displayed in the tetragonal phase of the FAPbBr 3 , similar to that previously observed in FAPbI 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Spark Plasma Sintering (SPS)-Assisted Synthesis and Thermoelectric Characterization of Magnéli Phase V 6 O 11

The Magnéli phase V6O11 was synthesized in gram amounts from a powder mixture of V 6 O 11 /V 7 O 13 and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V 6 O 11 has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K) –1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m) –1 , a Seebeck coefficient of α = –(35 ± 2) μV K –1 , which leads to a power factor of PF = 4.9 ± 0.8 × 10 –5 W (m K 2 ) –1 at ~600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V6O11 are involved. Here, this study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Structure Transformation and Cerium-Substituted Optical Response across the Carbonitridosilicate Solid Solution (La δ Y 1–δ ) 2 Si 4 N 6 C(δ = 0–0.5)

Following an investigation proving La 2 Si 4 N 6 C crystallizes in a monoclinic space group, isostructural to Y 2 Si 4 N 6 C, the reportedly hexagonal (La 0.5 Y 0.5 ) 2 Si 4 N 6 C was reinvestigated to examine the apparent crystal structure change across the solid solution. Initially, calculating the electronic structure and phonon density of states of (La 0.5 Y 0.5 ) 2 Si 4 N 6 C in the P6 3 mc space group revealed an imaginary phonon mode, which is indicative of a structural instability. Displacing the atoms along the pathway of the imaginary vibration led to a previously unreported space group for carbonitridosilicates, trigonal P31c. The assignment of the trigonal space group was subsequently confirmed by synthesizing (La 0.5 Y 0.5 ) 2 Si 4 N 6 C using high-temperature, solid state synthesis and analyzing the crystal structure with high-resolution synchrotron X-ray powder diffraction. Preparing the solid solution, (La δ Y 1–δ ) 1.98 Ce 0.02 Si 4 N 6 C (δ = 0–0.5), showed that the crystal structure changes from the monoclinic to the trigonal space group at δ ≈ 0.25. Finally, substituting Ce 3+ in the crystal structure to investigate the optical response via steady-state luminescent and photoluminescent quantum yield measurements reveals severe luminescent quenching with increasing La 3+ content, due to a combination of absorption of luminescence by the host structure and thermal quenching. Here, these results display the virtue of combining computational and experimental techniques to solve inorganic crystal structures and assess potential phosphor hosts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Radiation-Induced Changes in Quartz, A Mineral Analog of Nuclear Power Plant Concrete Aggregates

Quartz single-crystal samples consisting of α-quartz crystal structure were neutron irradiated to fluences of 5 × 10 18 , 4 × 10 19 , and 2 × 10 20 n/cm 2 (E > 0.1 MeV) at two temperatures (52 and 95 °C). The changes in the α-quartz phase as a function of these two conditions (temperature and fluence) were studied using X-ray powder diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM), and the results acquired using these complementary techniques are presented in a single place for the first time. XRD studies showed that the lattice parameters of α-quartz increased with increasing neutron flux. The lattice growth was larger for the samples that were neutron irradiated at 52 °C than at 95 °C. Moreover, an amorphous content was determined in the quartz samples neutron irradiated at 4 × 10 19 n/cm 2 , with the greater amount being in the 52 °C irradiated sample. Complete amorphization of quartz was observed at a fluence of 2 × 10 20 n/cm 2 (E > 0.1 MeV) using XRD and confirmed by TEM characterization and Raman spectroscopic studies. In conclusion, the cause for α-quartz lattice expansion and sample amorphization was also explored using XRD and Raman spectroscopic studies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Magnetic Ground State Crossover in a Series of Glaserite Systems with Triangular Magnetic Lattices

The magnetic properties are reported for three members of the glaserite series of compounds, Na 2 BaM(VO 4 ) 2 , M = Mn, Mn 0.6 Co 0.4 , and Co. Large single crystals are grown using a high-temperature hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M 2+ O 6 octahedra are interconnected with nonmagnetic (VO 4 ) 3– groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group $P\bar{3m}1$); however, at lower temperatures both Na 2 BaMn(VO 4 ) 2 and Na 2 BaMn 0.6 Co 0.4 (VO 4 ) 2 undergo a structural transition to lower symmetry (monoclinic, C2/c). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na 2 BaMn 0.6 Co 0.4 (VO 4 ) 2 does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na 2 BaMn(VO 4 ) 2 and Na 2 BaCo(VO 4 ) 2 were determined using neutron powder diffraction. At zero magnetic field, Na 2 BaMn(VO 4 ) 2 possesses an antiferromagnetic structure with the moments ordered in a Néel-type arrangement and aligned along the C 4 axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na 2 BaCo(VO 4 ) 2 represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the c-axis direction. Finally, the relationships between these structures and magnetic properties are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis, Crystal Structure, and Cooperative 3d–5d Magnetism in Rock Salt Type Li 4 NiOsO 6 and Li 3 Ni 2 OsO 6

Two new transition metal oxides with the nominal chemical compositions of Li 4 NiOsO 6 and Li 3 Ni 2 OsO 6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li 4 NiOsO 6 and Li 3 Ni 2 OsO 6 , respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. Furthermore, the magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni 2+ , depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and Sd electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os 5+ and Os 6+ -based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li 4 NiOsO 6 exhibits spin-glass-like behavior, while the transition temperature for Li 3 Ni 2 OsO 6 is nearly frequency independent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Structure Evolution of Chemically Degraded ZIF-8

Exposure of the metal–organic zeolitic imidazolate framework-8 (ZIF-8) to humid SO x gas cleaves Zn–N bonds, leading to framework disorder and a corresponding decrease in pore accessibility. This work shows that framework disorder exhibits a critical behavior with respect to water concentration in the gas stream. While the quantity of sulfate adsorbed at Zn nodes is at or below the detection threshold for the X-ray pair distribution function technique, precluding direct observation of the adsorbate, powder diffraction reveals clear structural alterations. Here, the onset of linker cleavage correlates to a large microstrain that diminishes as framework disorder increases. Energy-dispersive X-ray spectroscopy shows that the sulfur distribution throughout representative grains is uniform, implying the structure transformation is homogeneously nucleated and not dependent upon gas diffusion kinetics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Structural Characterization of Uranium Tetrafluoride Hydrate (UF 4 ·2.5H 2 O)

Uranium tetrafluoride is an important intermediate in the nuclear fuel cycle. Facile synthesis of its hydrate, uranium tetrafluoride hydrate (UF 4 ·2.5H 2 O), has recently been reported. The hydrate forms by contacting anhydrous UF 4 with neat H 2 O at room temperature for 24 h or by exposing anhydrous UF 4 to high relative humidity (>90%) conditions for several weeks. These pathways are of clear environmental relevance. Further understanding of the structure and optical spectra of UF 4 ·2.5H 2 O, especially of the water molecules, is therefore necessary. Herein, the structure of UF 4 ·2.5H 2 O was probed using time-of-flight neutron powder diffraction to improve understanding of the crystalline water environments in the structure. The complete structure was elucidated and compared to a previously reported partial structure for UF 4 ·2.5H 2 O and a predicted complete structure from density functional theory. The crystalline structure exhibits three distinct water environments: two of the three water sites are bound to uranium, and the third water is unbound or “free”. Furthermore, the completed structure reveals an extensive hydrogen bonding network involving water–fluorine and water–water interactions. One bound water site participates in hydrogen bonding with nearby fluoride ligands (O–H···F–U), and the second bound water site participates in hydrogen bonding with the unbound water (O–H···O) and a nearby fluoride ligand (O–H···F–U); the unbound water participates in hydrogen bonding with bound water (O–H···O–U). Low-temperature experiments and thermal analysis indicate UF 4 ·2.5H 2 O is thermally stable from 10 to 358 K, undergoes dehydration at higher temperatures, and is nearly dehydrated at 473 K. Structural measurements provide foundational understanding and will inform future investigations of the thermal and environmental stability of UF 4 ·2.5H 2 O.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Synthesis and Properties of the Helium Clathrate and Defect Perovskite [He 2– x $^\square$ x ][CaNb]F 6

The defect double perovskite [He 2– x $^\square$ x ][CaNb]F 6 , with helium on its A-site, can be prepared by the insertion of helium into ReO 3 -type CaNbF 6 at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ~60% of the A-sites become occupied. Helium uptake was quantified by both neutron powder diffraction and gas insertion and release measurements. After the conversion of gauge pressure to fugacity, the uptake of helium by CaNbF 6 can be described by a Langmuir isotherm. The enthalpy of absorption for helium in [He 2– x $^\square$ x ][CaNb]F 6 is estimated to be ~+3(1) kJ mol –1 , implying that its formation is entropically favored. Helium is able to diffuse through the material on a time scale of minutes at temperatures down to ~150 K but is trapped at 100 K and below. The insertion of helium into CaNbF 6 reduces the magnitude of its negative thermal expansion, increases the bulk modulus, and modifies its phase behavior. On compressing pristine CaNbF 6 , at 50 and 100 K, a cubic (Fm$\overline{3}$m) to rhombohedral (R$\overline{3}$) phase transition was observed at <0.20 GPa. However, a helium-containing sample remained cubic at 0.4 GPa and 50 K. CaNbF 6 , compressed in helium at room temperature, remained cubic to >3.7 GPa, the limit of our X-ray diffraction measurements, in contrast to prior reports that upon compression in a nonpenetrating medium, a phase transition is detected at ~0.4 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Structural and dynamical trends in alkali-metal silanides characterized by neutron-scattering methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH 3 , where M = K, Rb, Cs, K 0.5 Rb 0.5 , K 0.5 Cs 0.5 , and Rb 0.5 Cs 0.5 ) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH 3 – anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. Lastly, this dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Temperature-dependent Structural and Spectroscopic Studies of (Bi 1– x Fe x )FeO 3

We report on temperature-dependent structural and spectroscopic features of (Bi 1–x Fe x )FeO 3 perovskite for x = 0.15 and 0.25. Samples were synthesized by heating quantum crystalline precursors obtained by the polyol method. Crystal structures of each composition were obtained from in-house X-ray, synchrotron X-ray, and time-of-flight neutron powder diffraction data Rietveld refinements. Partial replacement of the Bi site by the Fe 3+ cation significantly changes the crystal physicochemical properties, such as thermal expansion, polyhedral distortion, Debye temperature, and vibrational and magnetic properties. Whereas BiFeO 3 is multiferroic, both Bi 0.85 Fe 0.15 FeO 3 and Bi 0.75 Fe 0.25 FeO 3 are found to be superparamagnetic, as observed by temperature-dependent Mössbauer and SQUID measurements. Lattice thermal expansion was modeled using the Debye–Einstein-anharmonicity approach. Debye temperatures obtained from the mean-squared atomic displacement parameter and lattice thermal expansion are compared. Furthermore, temperature dependence of selective Raman modes is also analyzed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH 3 , by Quasielastic Neutron Scattering

Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH 3 - anions in the monoalkali silanides (MSiH 3 , where M = K, Rb, and Cs) within the low-temperature ordered β-phases, and for CsSiH 3 , the high-temperature disordered α-phase and intervening hysteretic transition region. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 10 9 s -1 for β-KSiH 3 to 10 10 s -1 and higher for β-RbSiH 3 and β-CsSiH 3 . The β-phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi-C 3 symmetry axis. CsSiH 3 was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the β-phase region. As in α-KSiH 3 and α-RbSiH 3 , the highly mobile anions (with reorientational jump frequencies approaching and exceeding 10 12 s -1 ) in the disordered α-CsSiH 3 are all adequately modeled by H jumps between 24 different locations distributed radially around the anion center of gravity, although even higher anion reorientational disorder cannot be ruled out. QENS data for CsSiH 3 in the transition region between the α- and β-phases corroborated the presence of dynamically distinct intermediate ( i -) phase. The SiH 3 - anions within i -phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multidimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for α-phase anions but also an order-of-magnitude higher than those for β-phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this i -phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗