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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

X-ray diffraction investigations of structural modifications in In-doped tin pyrophosphates

Laboratory and synchrotron x-ray powder diffraction were used to investigate the structural modifications that occur upon indium doping of tin pyrophosphate. The data collected under air, vacuum, and inert gas sample environments at temperatures (T) from 50 °C to 300 °C show that regardless of the In-doping level (0 ≤ x ≤ 0.18) all InxSn1-xP2O7 samples are isomorphic (have the same P a -3 cubic crystal structure) at all temperatures and under all the conditions investigated. The cubic lattice parameter (a) increases linearly with T at all doping levels, but the “a vs. x|T“ isotherms exhibit a robust peak at x = 0.1 when data are collected on samples measured in air. On the other hand, Rietveld refinements against data collected on InxSn1-xP2O7 samples yield values of OO bond lengths and POP bond angles that show no major changes at x = 0.1 at any temperature. This is significant, as the Sn0.9In0.1P2O7 (x = 0.1) compound is known to exhibit the highest proton conductivity within the series, but the microscopic details responsible for the increased proton conductivity are not understood. Finally, the peak observed in the “a vs. x|T“ curves vanishes if the measurements are taken on samples kept either under vacuum or in an inert gas environment. This is a remarkable behavior as it lends further support to our hypothesis that a key microscopic feature responsible for the large proton conductivity of the Sn0.9In0.1P2O7 compound is the enlargement of the lattice constant at x = 0.1.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Hydration mechanisms of two polymorphs of synthetic ye'elimite

Ye'elimite is the main phase in calcium sulfoaluminate cements and also a key phase in sulfobelite cements. However, its hydration mechanism is not well understood. Here we reported new data on the hydration behavior of ye'elimite using synchrotron and laboratory powder diffraction coupled to the Rietveld methodology. Both internal and external standard methodologies have been used to determine the overall amorphous contents. We have addressed the standard variables: water-to-ye'elimite ratio and additional sulfate sources of different solubilities. Moreover, we report a deep study of the role of the polymorphism of pure ye'elimites. The hydration behavior of orthorhombic stoichiometric and pseudo-cubic solid-solution ye'elimites is discussed. In the absence of additional sulfate sources, stoichiometric-ye'elimite reacts slower than solid-solution-ye'elimite, and AFm-type phases are the main hydrated crystalline phases, as expected. Moreover, solid-solution-ye'elimite produces higher amounts of ettringite than stoichiometric-ye'elimite. However, in the presence of additional sulfates, stoichiometric-ye'elimite reacts faster than solid-solution-ye'elimite.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural analysis of highly porous γ-Al{sub 2}O{sub 3}

Two highly porous γ-aluminas, a commercial catalyst obtained from the calcination of boehmite and a highly mesoporous product obtained from amorphous aluminum (oxy)hydroxide via a sol–gel-based process were investigated by {sup 27}Al nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and atomic pair distribution function (PDF) analysis of synchrotron powder diffraction data. NMR data showed for both materials a distribution of tetrahedrally and octahedrally coordinated Al at a 0.30:0.70 ratio, which is typical for γ-aluminas. TEM studies revealed that rod-shaped particles with about 5 nm in thickness are the building blocks of the porous structure in both materials. These particles often extend to a length of 50 nm in the commercial catalyst and are considerably shorter in the sol–gel-based material, which has a higher surface area. Refinement of PDFs revealed the presence of a ∼1 nm scale local structure and the validity of a tetragonal average structure for both materials. This tetragonal average structure contains a substantial fraction of non-spinel octahedral Al atoms. It is argued that the presence of local structure is a general feature of γ-alumina, independent of precursor and synthesis conditions. The concentration of “non-spinel” Al atoms seems to correlate with surface properties, and increases with increasing pore size/surface area. This should have implications to the catalytic properties of porous γ-alumina. - Graphical abstract: Boehmite-derived and sol–gel synthesized porous γ-Al{sub 2}O{sub 3} possess identical structural properties, featuring a nm scale local structure and a tetragonal average structure. - Highlights: • Porous γ-Al{sub 2}O{sub 3} generally possesses a nm-scale local structure. • The tetragonal average structure contains a substantial fraction of “non-spinel” Al atoms. • The concentration of “non-spinel” Al atoms correlates with surface properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Temperature-dependent structural property and power factor of n type thermoelectric Bi{sub 0.90}Sb{sub 0.10} and Bi{sub 0.86}Sb{sub 0.14} alloys

Thermal variation of structural property, linear thermal expansion coefficient (α), resistivity (ρ), thermopower (S), and power factor (PF) of polycrystalline Bi{sub 1-x}Sb{sub x} (x = 0.10 and 0.14) samples are reported. Temperature-dependent powder diffraction experiments indicate that samples do not undergo any structural phase transition. Rietveld refinement technique has been used to perform detailed structural analysis. Temperature dependence of α is found to be stronger for Bi{sub 0.90}Sb{sub 0.10}. Also, PF for direct band gap Bi{sub 0.90}Sb{sub 0.10} is higher as compared to that for indirect band gap Bi{sub 0.86}Sb{sub 0.14}. Role of electron-electron and electron-phonon scattering on ρ, S, and PF has been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

X-ray and Raman spectroscopic studies on Ru substituted LiMn{sub 2}O{sub 4}

Phase pure Ru substituted LiMn{sub 2}O{sub 4} charge frustrated spinel compounds have been synthesized. Ru substitution for Mn is possible only up to 7.5%. Powder diffraction measurements confirm sustenance of charge frustrated cubic phase with space group Fd3m upon substitution of Ru for Mn. An increase in lattice parameter has been observed indicating replacement of Mn as Ru{sup 3+} or Ru{sup 4+} rather than Ru{sup 5+}. Raman spectroscopic measurements show that the crystal structure is robust against an increase of laser power and the observed bands show an expected red shift. In the case of Ru substituted LiMn{sub 2}O{sub 4}, the A{sub 1g} symmetric stretching mode, the F{sup (1)}{sub 2g} and the F{sup (2)}{sub 2g} symmetric bending modes identified with the Mn-O octahedra show a red shift up to Ru 0.075 and at higher Ru concentrations, revert back to the frequency corresponding to the pristine LiMn{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Powder X-ray diffraction of altrenogest, C 21 H 26 O 2

The crystal structure of altrenogest has been solved and refined using synchrotron X-ray powder diffraction data, and optimized using density functional techniques. Altrenogest crystallizes in space group P2 1 2 1 2 1 (#19) with a = 7.286 916(16), b = 10.580 333(19), c = 22.266 08(7) Å, V = 1716.671(6) Å 3 , and Z = 4 at 295 K. Thermal expansion between 113 and 295 K is anisotropic. An O–H···O hydrogen bond links the molecules into chains along the c-axis. The powder pattern has been submitted to ICDD for inclusion in the Powder Diffraction File™ (PDF®).

36 MATERIALS SCIENCE↗
DOE OSTI2015

Eshelby twist and correlation effects in diffraction from nanocrystals

Molecular dynamics simulations were used to model the Eshelby dislocation inside Pd and Ir nanowires and to predict the powder diffraction pattern using the Debye scattering equation. We find that the ideal dislocation solution by Eshelby is in good agreement with the observed twist angle and deviatoric strain, even though it ignores both the splitting of the Eshelby dislocation into two partials and surface stress. Surface stress plays a significant role only for nanorods with small aspect ratio (∼1:1). We also find that Wilson's prediction on the diffraction peak broadening for the Eshelby dislocation is overestimated because it ignores the fact that the Eshelby twist relaxes the deviatoric strain. Moreover, the twist loosens the correlation along the nanorod, causing additional line profile broadening, which is read by diffraction as a decrease of coherent domain size when the total twist angle is bigger than 1.5°. Overall, our findings suggest a novel way to predict and analyze the dislocations as well as the resulting strain fields in the twisted nanocrystalline rods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Thermal Evolution of Internal Strain in Doped PbTe

Recent improvements in the efficiency of heat-to-electricity energy conversion in lead chalcogenide thermoelectrics involve reducing the thermal conductivity by incorporating large amounts of internal strain. The extent to which typical lead chalcogenide processing techniques (such as doping, ball milling, and densification) increase internal strain and dislocation density must be quantified to improve materials design. In this study, neutron powder diffraction is leveraged to evaluate the internal strain introduced by ball milling in doped and undoped powders. Doping with Na and/or Eu increases internal strain beyond ball milling alone, with the greatest increase from combining the two dopants. Strain recovery occurs in each powder above 400 K but can be suppressed by co-doping, indicating a strong dopant–dislocation interaction in this system. Therefore, high-temperature processing of PbTe powders should be avoided if high internal strain is desired. Low-temperature densification and/or rapid pressing techniques may be key to maintaining internal strain in the final pressed pellet. The diffraction peak asymmetry and correlated elastic softening measured in pressed PbTe pellets in past studies were not observed in the precursor powders measured here, suggesting that measurements of the Debye temperature on final pressed pellets are required to examine the influence of defect-induced elastic softening on thermal conductivity. This work provides key guidance for defect engineering to maximize internal strain and thermoelectric performance in PbTe thermoelectrics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Neutron diffraction study of quadruple perovskite SrCu{sub 3}Fe{sub 3}O{sub 12}

The magnetic structure of the quadruple perovskite SrCu{sub 3}Fe{sub 4}O{sub 12} is studied by means of neutron powder diffraction. The magnetic diffraction peaks are observed at low temperatures below 200 K. The Rietveld refinement result suggests an antiferromagnetic alignment of Fe spin magnetic moments for SrCu{sub 3}Fe{sub 4}O{sub 12} at low temperature. The refined magnetic moment at 4 K is ∼3.5 µ{sub B}, which is larger than that of that of CeCu{sub 3}Fe{sub 4}O{sub 12} (2.3 µ{sub B}). The increase in the magnetic moment is attributed to the larger Fe{sup 3+}:Fe{sup 5+} ratio for SrCu{sub 3}Fe{sub 4}O{sub 12} (∼4:1) compared with that of CeCu{sub 3}Fe{sub 4}O{sub 12} (∼3:1).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

A neutron diffraction demonstration of long-range magnetic order in the quasicrystal approximant DyCd 6

We have used neutron powder diffraction to demonstrate the existence of long-range antiferromagnetic order of Ising-like Dy moments in the DyCd 6 quasicrystal approximant phase. This cubic compound undergoes a slight distortion to a monoclinic cell at low temperatures. The Néel temperature is 18.0(2) K and the magnetic order of the Dy sublattice may be described in the parent cubic Im 3¯ structure using a combination of two propagation vectors, k 1 = [0 0 0] and k 2 = [1/2 0 1/2], yielding ‘anti-I’ order. Furthermore, when referred to the monoclinic C 2/ c cell, the magnetic structure may be described by a single propagation vector: k = [1 0 0].

36 MATERIALS SCIENCE↗
DOE OSTI2018

Low-Frequency Phonon Driven Negative Thermal Expansion in Cubic GaFe(CN) 6 Prussian Blue Analogues

Here, the understanding of the negative thermal expansion (NTE) mechanism is vital not only for the development of new NTE compounds but also for effectively controlling thermal expansion. Here, we report an interesting isotropic NTE property in cubic GaFe(CN) 6 Prussian blue analogues (α l = –3.95 × 10 –6 K –1 , 100–475 K), which is a new example to understand the complex NTE mechanism. A combined study of synchrotron X-ray diffraction, X-ray total scattering, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations shows that the NTE of GaFe(CN)6 originates from the low-frequency phonons (< ~100 cm –1 ), which are directly related to the transverse vibrations of the atomic –Ga–N≡C–Fe– chains. Both the Ga–N and Fe–C chemical bonds are much softer to bend than to stretch. The direct evidence that transverse vibrational contribution to the NTE of GaFe(CN) 6 is dominated by N, instead of C atoms, is illustrated. It is interesting to find that the polyhedra of GaFe(CN) 6 are not rigid, which is a starting assumption in some models describing the NTE properties of other systems. The NTE mechanism can be vividly described by the “guitar-string” effect, which would be the common feature for the NTE property of many open-framework functional materials, such as Prussian blue analogues, oxides, cyanides, metal–organic frameworks, and zeolites.

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

Magnetic structure of the mixed antiferromagnet NdMn 0.8 Fe 0.2 O 3

The magnetic structure of the mixed antiferromagnet NdMn 0.8 Fe 0.2 O 3 was resolved. Neutron powder diffraction data definitively resolve the Mn sublattice with a magnetic propagation vector k=(000) and with the magnetic structure (A x , F y , G z ) for 1.6 K N (≈ 59 K). The Nd sublattice has a (0, f y , 0) contribution in the same temperature interval. The Mn sublattice undergoes a spin-reorientation transition at T 1 ≈ 13 K while the Nd magnetic moment abruptly increases at this temperature. Powder x-ray diffraction shows a strong magnetoelastic effect at T N but no additional structural phase transitions from 3 to 300 K. Density functional theory calculations confirm the magnetic structure of the undoped NdMnO 3 as part of our analysis. Taken together, these results show that the magnetic structure of the Mn sublattice in NdMn 0.8 Fe 0.2 O 3 is a combination of the Mn and Fe parent compounds, but the magnetic ordering of the Nd sublattice spans a broader temperature interval than in the case of NdMnO 3 and NdFeO 3 . Lastly, this result is a consequence of the fact that the Nd ions do not order independently, but via polarization from the Mn/Fe sublattice.

36 MATERIALS SCIENCE↗
DOE OSTI2014

τ-MnAl with high coercivity and saturation magnetization

In this paper, high purity τ-Mn{sub 54}Al{sub 46} and Mn{sub 54−x}Al{sub 46}C{sub x}alloys were successfully prepared using conventional arc-melting, melt-spinning, and heat treatment process. The magnetic and the structural properties were examined using x-ray diffraction (XRD), powder neutron diffraction and magnetic measurements. A room temperature saturation magnetization of 650.5 kAm{sup -1}, coercivity of 0.5 T, and a maximum energy product of (BH){sub max} = 24.7 kJm{sup -3} were achieved for the pure Mn{sub 54}Al{sub 46} powders without carbon doping. The carbon substituted Mn{sub 54−x}Al{sub 46}C{sub x}, however, reveals a lower Curie temperature but similar saturation magnetization as compared to the carbon-free sample. The electronic structure of MnAl shows that the Mn atom possesses a magnetic moment of 2.454 μ{sub B} which results from strong hybridization between Mn-Al and Mn-Mn. We also investigated the volume and c/a ratio dependence of the magnetic moments of Mn and Al. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion, leads to a large magnetic moment for the Mn atom. The Mn magnetic moment can reach a value of 2.9 μ{sub B} at a volume expansion rate of ΔV/V ≈ 20%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Epitaxial Growth of Cadmium Selenide Films on Silicon with a Silicon Carbide Buffer Layer

An epitaxial cubic 350-nm-thick cadmium selenide has been grown on silicon for the first time by the method of evaporation and condensation in a quasi-closed volume. It is revealed that, in this method, the optimum substrate temperature is 590°C, the evaporator temperature is 660°C, and the growth time is 2 s. To avoid silicon etching by selenium with formation of amorphous SiSe{sub 2}, a high-quality ~100-nm-thick buffer silicon carbide layer has been synthesized on the silicon surface by substituting atoms. The powder diffraction pattern and the Raman spectrum unambiguously correspond to cubic cadmium selenide crystal. The ellipsometric, Raman, and electron diffraction analyses demonstrate high structural perfection of the cadmium selenide layer and the absence of a polycrystalline phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Earth Abundant High Temperature Materials for Radioisotope Power Conversion System

Advances in high temperature thermoelectric materials were made and published. Two major developments are out lined in this final report. Alkaline earth Zintl phases represent the starting point of a series of compounds selected for development due to their high abundance as the supply chain and as the availability of certain rare earths (i.e. Yb) are limited and can be difficult to obtain. A new Zintl phase, Ca 14 MnSb 11-x Sn x , where Sb is replaced by Sn resulting in changes to the band structure and properties. The structures are investigated by powder diffraction and samples consolidated into fully dense pellets for thermoelectric measurements. The band structure of a reasonable structure, Ca 14 MgSb 7 Sn 4 , is calculated in order to better understand the properties. These Sn containing phases show improved thermoelectric properties over Ca 14 MgSb 11 by increasing the Seebeck while decreasing resistivity without significant impact to the already low thermal conductivity. In addition, a composite phase of Yb 14 MgSb 11 was investigated. Composite phases have been shown to improve both the thermoelectric efficiency and mechanical properties of materials. Here we demonstrate an improved thermoelectric figure of merit, power factor, and mechanical properties for the high-temperature p-type Zintl phase Yb 14 MgSb 11 . Composites with 0, 1, 2, 3, 4, 6, 8 volume % 6 – 10 µm reduced Fe powder were prepared via a fast, scalable mechanical milling and spark plasma sintering procedure. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that Fe is not incorporated into the Yb 14 MgSb 11 structure. First order reversal curves and scanning electron microscopy images show that the Fe inclusions are larger and closer together with increasing Fe content. Thermogravimetric and differential scanning calorimetry show that the composites are stable up to 1273 K. The elastic constants of the 8 volume % Fe composite were measured by resonant ultrasound spectroscopy and show that Yb 14 MgSb 11 becomes stiffer with increasing Fe volume % and SEM after indentations show crack arresting occurs at the Fe interface. Thermoelectric properties on dense pellets are measured from 300 K – 1273 K. The thermoelectric power factor (PF= $\frac{S^2}{ρ}$) increases with increasing Fe content, with the 8 volume % Fe resulting in 40% higher PF than pristine Yb 14 MgSb 11 . The increase in PF is attributed to a systematic reduction in electrical resistivity. Peak thermoelectric figure of merit (zT= ($\frac{S^2T}{κρ}$) is observed at 3 volume % Fe, an 11% improvement in zT compared to Yb 14 MgSb 11 . Yb 14 MgSb 11 composites with Fe are compatible with Ce 0.9 Fe 3.5 Co 0.5 Sb 12 for thermoelectric generator couple segmentation.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

X-ray diffraction under grazing incidence conditions

Material properties frequently relate to structures at or near surfaces, particularly in thin films. As a result, it is essential to understand these structures at the molecular and atomistic levels. The most accurate and widely used techniques for characterizing crystallographic order are based on X-ray diffraction. When dealing with thin films or interfaces, standard approaches, such as single crystal or powder diffraction, are not suitable. However, X-ray diffraction under grazing incidence conditions can provide the required information. Here, in this Primer, grazing incidence X-ray diffraction (GIXD) is comprehensively introduced, starting from basic considerations on X-ray diffraction at crystals with reduced dimensionality and the optical properties of X-rays, followed by a more in-depth description of an experimental performance, including X-ray sources, goniometers and detectors. Experimental errors, limitations and reproducibility are discussed. Various applications, from highly ordered inorganic single crystal surfaces to weakly ordered polymer thin films, are presented to illustrate the potential of GIXD. Data visualizations, representations and evaluation strategies are summarized, based on the example of anthracene thin films. The Primer compiles information relevant to perform high-quality GIXD experiments, evaluate data and interpret results, to extend knowledge about X-ray diffraction from surfaces, interfaces and thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nucleation and growth of polar clusters with in-phase tilts into a long-range ferroelectric matrix in a sodium niobate based complex relaxor

In this study, we have investigated the temperature dependence of atomic ordering at multiple length scales in a lead-free sodium niobate-based relaxor, i.e., 0.75 NaNbO 3 -0.25 Ba 0.9⁢ Ca 0.1⁢ TiO 3 (NN-25BCT) via synchrotron x-ray diffraction, Raman spectroscopy, and pair distribution function analysis. High-resolution synchrotron x-ray powder diffraction (SXRD) measurements reveal a ferroelectric phase transition in the relaxor ferroelectric NN-25BCT below the Vogel-Fulcher freezing temperature (𝑇 VF ≈ 270 K). In addition, SXRD analysis demonstrates the competition between in-phase octahedral tilting and ferroelectric order at the long-range scale using mode crystallography. On the other hand, Raman spectroscopic analysis provides evidence of polar ordering for 𝑇 > 𝑇 VF (with tetragonal symmetry) persisting up to the Burns temperature (𝑇 B ). Furthermore, pair distribution function (PDF) analysis reveals the presence of a polar antiferrodistortive tetragonal phase with 𝑃⁢4⁢𝑏𝑚 space group at short ranges throughout the studied temperatures (i.e., 110 K ≤ 𝑇 ≤500 K), irrespective of nonpolar long-range ordering above 𝑇 VF . Therefore, our measurements provide direct evidence for the presence of polar ordering at short ranges and their gradual transformation into long-range polar ordering using an integrated multiscale structural analysis. In conclusion, as a result of a transition from relaxor to a ferroelectric phase in the vicinity of room temperature, NN-25BCT can be exploited for applications in pyroelectric detectors, electrocaloric devices, and multilayered ceramic capacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2012

{sup 3}He alternative technologies: 1. Application of neutron image plate detector for powder diffractometry

Neutron image plate was used for recording powder diffraction pattern from a polycrystalline Fe rod, mounted on the High Q Diffractometer at Dhruva reactor and using neutron beam of {lambda}= 0.783 A and {Phi}= 3 x 10{sup 5} n/cm{sup 2}/sec. The Bragg peaks were sharp and well resolved. The overall resolution ({Delta}2{theta}/2{theta}) value was found be {approx}0.025.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗