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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Electrical properties and conduction mechanism study by OLPT model of NaZnPO4 compound

Highlights: • The crystal structure was resolved using Fullproof program. • The conduction in NaZn PO{sub 4} is insured by OLPT model. • Ac conductivity of NaZn PO{sub 4} compound obeys the Joncher’s law. - Abstract: A polycrystalline sample with NaZnPO{sub 4} stoichiometry was synthesized by the conventional solid-state reaction and then identified and characterized by X-ray powder diffraction, Raman spectroscopy and impedance spectroscopy. The X-ray diffraction pattern revealed that the composite represents a single phase that crystallizes in the monoclinic system with a P2{sub 1}/n space group. All vibration modes of tetrahedral PO{sub 4} appeared in the Raman spectrum. Besides, the electrical conductivity behavior of the title compound was analysed as a function of frequency ranging from 209 Hz to 1 MHz and at a temperature interval ranging from 578 K to 728 K by means of impedance spectroscopy to elucidate the conduction mechanism in this compound. The temperature dependence of frequency exponent ′s′ showed that the conduction in NaZnPO{sub 4} is insured by the overlapping large polaron tunneling (OLPT) model. A correlation between electrical and structural properties was also depicted.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Single crystal growth and variation of thermodynamic and magnetic properties of Pr1−xLaxAlO3 (x = 0, 0.8)

Highlights: • The perovskite Pr{sub (1-x)}La{sub (x)}AlO{sub 3} was grown by two different protocols to control the P{sup 4+} concentration. • Low temperature magnetic properties in bulk single crystals are sensitive to annealing protocols. • Post growth annealing in a reducing atmosphere leads to closest Pr{sub 0.2}La{sub 0.8}AlO{sub 3} stoichiometry. • A possible new magnetic transition for single crystal PrAlO{sub 3} is observed at 6 K. - Abstract: Two approaches to using optical floating zone (OFZ) technique for single crystal growth of the perovskite Pr{sub 1−x}La{sub x}AlO{sub 3} are discussed, along with the influence on the physical properties of the resulting single crystals due to oxidizing and reducing post anneals. In method one (M1) we used pre-reduced Pr{sub 2}O{sub 3} powder starting material, while in the other method, M2, we employed an argon 5% hydrogen reducing environment during the floating zone single crystal growth. Magnetic susceptibility of the low temperature monoclinic phase of Pr{sub 0.2}La{sub 0.8}AlO{sub 3} is shown to be sensitive to the precise annealing protocol followed. For the post-growth reduced single crystal the magnetic susceptibility is flat across the R-3c to C2/m structural transition, whereas the oxidized and as-grown samples display a sharp upturn and downturn, respectively. We attribute the low temperature susceptibility differences to relative proportions of Pr{sup 3+} and Pr{sup 4+}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis and structural characterization of the vividly colored first row transition metal-containing oxyfluoride antiperovskites Sr3-xMnxMO4F and Sr3-xMnxMO4-αF1-δ (M = Al, Ga)

Highlights: • Substitution of Mn{sup 2+} demonstrates Sr{sub 3-x}A{sub x}MO{sub 4}F can tolerate divalent first row transition metals. • The colors of these materials vary from green to purple depending on anion stoichiometry. • Intense colors are likely due to small amounts of Mn{sup 3+} on the tetrahedral Al site. • Structural data is consistent with preferential occupation of eight-coordinate Sr(2) site by Mn{sup 2+}. • Large off-center displacements are observed on the underbonded ten-coordinate Sr(1) site. - Abstract: Incorporation of dopant amounts of the divalent transition metal cation Mn{sup 2+} into the oxyfluoride material Sr{sub 3-x}Mn{sub x}MO{sub 4}F (M = Al, Ga; 0 ≤ x ≤ 0.1) yields green powders which vary in hue depending on the identity of M. Treatment of these phases with post-synthesis annealing under reducing conditions produces anion deficient phases according to Sr{sub 3-x}Mn{sub x}MO{sub 4-α}F{sub 1-δ} displaying vivid purple coloration. Intense colors are likely due to small amounts of Mn{sup 3+} on the tetrahedral M site. Full structural characterization based upon powder X-ray diffraction (PXRD), neutron powder diffraction (NPD), and thermogravimetric analysis (TGA) data are presented herein, demonstrating that the Sr{sub 3-x}A{sub x}MO{sub 4}F (A = Ca, Sr, Ba; M = Al, Ga) anti-perovskite structure can readily accommodate first row transition metals.

36 MATERIALS SCIENCE↗
DOE OSTI2018

X-ray structure and investigation of molecular motions by dielectric, vibrational and 1H NMR methods for two organic-inorganic hybrid piperazinium compounds: (C4H12N2)2[Sb2Cl10]·2H2O and (C4H12N2)2[Sb2Br10]·2H2O

Highlights: • Bis(piperazine-1,4-diium) decabromodiantimonate(III) (PSB) dihydrate has been synthesized and characterized. • The structural phase transition for PSB at ca. 50 K has been confirmed by the dielectric and {sup 1}H NMR spectroscopy results. • The molecular motion parameters of the organic cations and water molecules have been evaluated and compared for PSB and PSC. - Abstract: Bis(piperazine-1,4-diium) decabromodiantimonate(III) dihydrate, (C{sub 4}H{sub 12}N{sub 2}){sub 2}[Sb{sub 2}Br{sub 10}]·2H{sub 2}O, (abbreviated as PSB) has been synthesized and characterized by single-crystal X-ray diffraction studies. The obtained results indicate that PSB is isostructural with a recently described chloride analogue, (C{sub 4}H{sub 12}N{sub 2}){sub 2}[Sb{sub 2}Cl{sub 10}]⋅2H{sub 2}O (PSC), crystallizing with R{sub 2}MX{sub 5}·nH{sub 2}O stoichiometry and characterized by the discrete zero-dimensional anionic species. The crystal structure of PSB is described by monoclinic symmetry in a P2{sub 1}/n space group. Dielectric measurements (down to 25 K) and IR spectral analyses (measured over a broad temperature range) suggested a much more complicated phase situation below 80 K than for that of PSC. To verify and compare the molecular motions of the organic cations in both analogues, a proton magnetic resonance ({sup 1}H NMR) technique was also applied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Edaphic controls on genome size and GC content of bacteria in soil microbial communities

Nutrient limitation has been shown to reduce bacterial genome size and influence nucleotide composition; however, much of this work has been conducted in marine systems and the factors which shape soil bacterial genomic traits remain largely unknown. Here, for this work, we determined average genome size, GC content, codon usage, and amino acid content from 398 soil metagenomes across a broad geographic range and used machine-learning to determine the environmental parameters that most strongly explain the distribution of these traits. We found that genomic trait averages were most related to pH, which we suggest is primarily due to the correlation of pH with several environmental parameters, particularly soil carbon content. Low pH soils had higher carbon to nitrogen ratios (C:N) and tended to have communities with lower GC content and larger genomes, potentially a response to increased physiological stress and a requirement for metabolic diversity. Conversely, communities in high pH and low soil C:N had smaller genomes and higher GC content—indicating potential resource driven selection against AT base pairs, which have a higher C:N than GC base pairs. Similarly, we found that nutrient conservation also applied to amino acid stoichiometry, where bacteria in soils with low C:N ratios tended to code for amino acids with lower C:N. Together, these relationships point towards fundamental mechanisms that underpin genome size, and nucleotide and amino acid selection in soil bacteria.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

A Combined Variable-Temperature Neutron Diffraction and Thermogravimetric Analysis Study on a Promising Oxygen Electrode, SrCo 0.9 Nb 0.1 O 3–δ , for Reversible Solid Oxide Fuel Cells

The present study reported herein investigates the temperature–structure–stoichiometry relationship of a promising oxygen electrode SrCo 0.9 Nb 0.1 O 3-δ over a temperature (T) range from room temperature (RT) to 900 °C. The techniques employed are variable-temperature neutron diffraction (VTND) and thermogravimetric analysis (TGA). At $T$ < 75 °C, VTND reveals a tetragonal (P4/mmm) structure with a G-type magnetic ordering. Above 75 °C, the nucleus structure remains the same, while the magnetic ordering disappears. A phase transition from tetragonal ($P4/mmm$) to cubic ($Pm\overline3m$) is observed at 412 °C, where the two Co sites and three O sites in the $P4/mmm$ phase converge to one equivalent site, respectively. The phase transition temperature coincides with the peak temperature of oxygen uptake obtained by TGA. It is also observed that the Nb dopant has no preferred Co site to occupy. The oxygen vacancies are mostly located at the O3 site surrounding the Co2 site in the $P4/mmm$ structure. The intermediate-spin state of Co 3+ at the Co2 site is responsible for the observed distortions of CoO 6 octahedra, i.e., elongation of Co2O 6 octahedra and shortening of Co1O 6 octahedra along the c-axis, which is a phenomenon known as Jahn–Teller distortion. At high temperatures, large thermal displacement factor for O 2– is observed with high concentration of oxygen vacancies, providing a structural environment favorable to high O 2– conductivity in Nb-doped SrCoO 3 -based oxygen electrode materials.

25 ENERGY STORAGE↗
DOE OSTI2012

Control of PbSe Quantum Dot Surface Chemistry and Photophysics Using an Alkylselenide Ligand

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {l_brace}111{r_brace} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Nickel vacancy behavior in the electrical conductance of nonstoichiometric nickel oxide film

Nickel vacancy behavior in electrical conductance is systematically investigated using various analysis methods on nickel oxide films deposited at different oxygen partial pressures. The results of Rutherford backscattering, x-ray diffraction, and Auger electron spectroscopy analyses demonstrate that the sputtered nickel oxide films are nickel-deficient. Through the deconvolution of Ni2p and O1s spectra in the x-ray photoelectron spectroscopy data, the number of Ni{sup 3+} ions is found to increase with the O{sub 2} ratio during the deposition. According to the vacancy model, nickel vacancies created from the non-stoichiometry are concluded to produce Ni{sup 3+} ions which lead to an increment of the conductivity of the nickel oxide films due to the increase of the hole concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Preparation and atomic structure of reconstructed (0001) InGaN surfaces

The preparation and surface structure of high quality group-III-polar (0001) InGaN layers grown by metal-organic vapor phase epitaxy have been investigated. In order to obtain a clean and well-ordered surface we studied the preparation by annealing at various temperatures under ultra high vacuum and nitrogen-rich conditions in nitrogen-plasma. We show that different InGaN surface reconstructions such as (1 Multiplication-Sign 1), (1 + 1/6), (2 Multiplication-Sign 2), and ({radical}(3) Multiplication-Sign {radical}(3))R30 Degree-Sign can be obtained as observed by low energy electron diffraction. Dependent on the annealing temperature and nitrogen supply these surfaces exhibit significant differences in stoichiometry and morphology as determined by Auger electron spectroscopy and atomic force microscopy measurements. The (1 Multiplication-Sign 1), (2 Multiplication-Sign 2), and ({radical}(3) Multiplication-Sign {radical}(3))R30 Degree-Sign superstructures are terminated by single group-III-adatoms, whereas the (1 + 1/6) exhibits a incommensurate overlayer of group-III-atoms. We show that the (2 Multiplication-Sign 2) and ({radical}(3) Multiplication-Sign {radical}(3))R30 Degree-Sign an In depletion in the first group-III layer and In or Ga adatoms in ontop position. Strain-relaxation is suggested to explain this structure formation.

36 MATERIALS SCIENCE↗
DOE OSTI2012

High quality ultrathin Bi{sub 2}Se{sub 3} films on CaF{sub 2} and CaF{sub 2}/Si by molecular beam epitaxy with a radio frequency cracker cell

We report a method to fabricate high quality Bi{sub 2}Se{sub 3} thin films using molecular beam epitaxy with a radio frequency cracker cell as an atomic selenium source. With Se-to-Bi ratios close to exact stoichiometry, optimal layer-by-layer growth of high quality Bi{sub 2}Se{sub 3} thin films with smooth surfaces has been achieved on CaF{sub 2}(111) substrates and Si(111) substrates with a thin epitaxial CaF{sub 2} buffer layer (CaF{sub 2}/Si). Transport measurements show a characteristic weak-antilocalization magnetoresistance in all the films, with the emergence of a weak-localization contribution in the ultrathin film limit. Quantum oscillations, attributed to the topological surface states have been observed, including in films grown on CaF{sub 2}/Si.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Fabrication and characterization of the gapless half-Heusler YPtSb thin films

Half-Heusler YPtSb thin films were fabricated by magnetron co-sputtering method on MgO-buffered SiO{sub 2}/Si(001) substrates. X-ray diffraction pattern and energy dispersive X-ray spectroscopy confirmed the high-quality growth and stoichiometry. The temperature dependence of the resistivity shows a semiconducting-type behavior down to low temperature. The Hall mobility was determined to be 450 cm{sup 2}/V s at 300 K, which is much higher than the bulk value ({approx}300 cm{sup 2}/V s). In-plane magnetoresistance (MR) measurements with fields applied along and perpendicular to the current direction show opposite MR signs, which suggest the possible existence of the topological surface states.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

The electrical and optical properties of Ta{sub x}Si{sub y}N{sub z} thin films deposited by reactive magnetron sputtering from individual Ta and Si targets were studied in order to investigate the effects of nitrogen and silicon contents on both properties and their correlation to the film microstructure. Three sets of fcc-Ta{sub x}Si{sub y}N{sub z} thin films were prepared: sub-stoichiometric Ta{sub x}Si{sub y}N{sub 0.44}, nearly stoichiometric Ta{sub x}Si{sub y}N{sub 0.5}, and over-stoichiometric Ta{sub x}Si{sub y}N{sub 0.56}. The optical properties were investigated by near-normal-incidence reflectivity and ellipsometric measurements in the optical energy range from 0.375 eV to 6.8 eV, while the d.c. electrical resistivity was measured in the van der Pauw configuration from 20 K to 300 K. The optical and electrical measurements were interpreted using the standard Drude-Lorentz model and the so-called grain boundary scattering model, respectively. The electronic properties were closely correlated with the compositional and structural modifications of the Ta{sub x}Si{sub y}N{sub z} films due to variations in the stoichiometry of the fcc-TaN{sub z} system and the addition of Si atoms. According to the nitrogen and silicon contents, fcc-Ta{sub x}Si{sub y}N{sub z} films can exhibit room temperature resistivity values ranging from 10{sup 2} {mu}{Omega} cm to about 6 Multiplication-Sign 10{sup 4} {mu}{Omega} cm. The interpretation of the experimental temperature-dependent resistivity data within the Grain Boundary Scattering model, combined with the results from optical investigations, showed that the mean electron transmission probability G and the free carriers concentration, N, are the main parameters that control the transport properties of these films. The results indicated that the correlation between electrical and optical measurements with the chemical composition and the nanostructure of the Ta{sub x}Si{sub y}N{sub z} thin films provides a pertinent and consistent description of the evolution of the Ta-Si-N system from a solid solution to a nanocomposite material due to the addition of Si atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Electric field-tunable Ba{sub x}Sr{sub 1-x}TiO{sub 3} films with high figures of merit grown by molecular beam epitaxy

We report on the dielectric properties of Ba{sub x}Sr{sub 1-x}TiO{sub 3} (BST) films grown by molecular beam epitaxy on epitaxial Pt bottom electrodes. Paraelectric films (x Less-Than-Or-Equivalent-To 0.5) exhibit dielectric losses that are similar to those of BST single crystals and ceramics. Films with device quality factors greater than 1000 and electric field tunabilities exceeding 1:5 are demonstrated. The results provide evidence for the importance of stoichiometry control and the use of a non-energetic deposition technique for achieving high figures of merit of tunable devices with BST thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

X-ray diffraction and extended X-ray absorption fine structure study of epitaxial mixed ternary bixbyite Pr{sub x}Y{sub 2-x}O{sub 3} (x = 0-2) films on Si (111)

Ternary single crystalline bixbyite Pr{sub x}Y{sub 2-x}O{sub 3} films over the full stoichiometry range (x = 0-2) have been epitaxially grown on Si (111) with tailored electronic and crystallographic structure. In this work, we present a detailed study of their local atomic environment by extended X-ray absorption fine structure at both Y K and Pr L{sub III} edges, in combination with complementary high resolution x-ray diffraction measurements. The local structure exhibits systematic variations as a function of the film composition. The cation coordination in the second and third coordination shells changes with composition and is equal to the average concentration, implying that the Pr{sub x}Y{sub 2-x}O{sub 3} films are indeed fully mixed and have a local bixbyite structure with random atomic-scale ordering. A clear deviation from the virtual crystal approximation for the cation-oxygen bond lengths is detected. This demonstrates that the observed Vegard's law for the lattice variation as a function of composition is based microscopically on a more complex scheme related to local structural distortions which accommodate the different cation-oxygen bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ultrafast switching in wetting properties of TiO{sub 2}/YSZ/Si(001) epitaxial heterostructures induced by laser irradiation

We have demonstrated dark hydrophilicity of single crystalline rutile TiO{sub 2}(100) thin films, in which rapid switching from a hydrophobic to a hydrophilic surface was achieved using nanosecond excimer laser irradiation. The TiO{sub 2}/YSZ/Si(001) single crystalline heterostructures were grown by pulsed laser deposition and were subsequently irradiated by a single pulse of a KrF excimer laser at several energies. The wettability of water on the surfaces of the samples was evaluated. The samples were hydrophobic prior to laser annealing and turned hydrophilic after laser annealing. Superhydrophilic surfaces were obtained at higher laser energy densities (e.g., 0.32 J.cm{sup -2}). The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies on the surface, which are surmised to be responsible for the observed superhydrophilic behavior. According to the AFM images, surface smoothening was greater in films that were annealed at higher laser energy densities. The samples exhibited hydrophobic behavior after being placed in ambient atmosphere. The origin of laser induced wetting behavior was qualitatively understood to stem from an increase of point defects near the surface, which lowered the film/water interfacial energy. This type of rapid hydrophobic/hydrophilic switching may be used to facilitate fabrication of electronic and photonic devices with novel properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Characterization of plasma chemistry and ion energy in cathodic arc plasma from Ti-Si cathodes of different compositions

Arc plasma from Ti-Si compound cathodes with up to 25 at. % Si was characterized in a DC arc system with respect to chemistry and charge-state-resolved ion energy. The plasma ion composition showed a lower Si content, diverging up to 12 at. % compared to the cathode composition, yet concurrently deposited films were in accordance with the cathode stoichiometry. Significant contribution to film growth from neutrals is inferred besides ions, since the contribution from macroparticles, estimated by scanning electron microscopy, cannot alone account for the compositional difference between cathode, plasma, and film. The average ion charge states for Ti and Si were higher than reference data for elemental cathodes. This result is likely related to TiSi{sub x} phases of higher cohesive energies in the compound cathodes and higher effective electron temperature in plasma formation. The ion energy distributions extended up to {approx}200 and {approx}130 eV for Ti and Si, respectively, with corresponding average energies of {approx}60 and {approx}30 eV. These averages were, however, not dependent on Si content in the cathode, except for 25 at. % Si where the average energies were increased up to 72 eV for Ti and 47 eV for Si.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electron beam physical vapor deposition of thin ruby films for remote temperature sensing

Thermographic phosphors (TGPs) possessing temperature-dependent photoluminescence properties have a wide range of uses in thermometry due to their remote access and large temperature sensitivity range. However, in most cases, phosphors are synthesized in powder form, which prevents their use in high resolution micro and nanoscale thermal microscopy. In the present study, we investigate the use of electron beam physical vapor deposition to fabricate thin films of chromium-doped aluminum oxide (Cr-Al{sub 2}O{sub 3}, ruby) thermographic phosphors. Although as-deposited films were amorphous and exhibited weak photoluminescence, the films regained the stoichiometry and {alpha}-Al{sub 2}O{sub 3} crystal structure of the combustion synthesized source powder after thermal annealing. As a consequence, the annealed films exhibit both strong photoluminescence and a temperature-dependent lifetime that decreases from 2.9 ms at 298 K to 2.1 ms at 370 K. Ruby films were also deposited on multiple substrates. To ensure a continuous film with smooth surface morphology and strong photoluminescence, we use a sapphire substrate, which is thermal expansion coefficient and lattice matched to the film. These thin ruby films can potentially be used as remote temperature sensors for probing the local temperatures of micro and nanoscale structures.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Reordering between tetrahedral and octahedral sites in ultrathin magnetite films grown on MgO(001)

Magnetite ultrathin films were grown using different deposition rates and substrate temperatures. The structure of these films was studied using (grazing incidence) x-ray diffraction, while their surface structure was characterized by low energy electron diffraction. In addition to that, we performed x-ray photoelectron spectroscopy and magneto optic Kerr effect measurements to probe the stoichiometry of the films as well as their magnetic properties. The diffraction peaks of the inverse spinel structure, which originate exclusively from Fe ions on tetrahedral sites are strongly affected by the preparation conditions, while the octahedral sites remain almost unchanged. With both decreasing deposition rate as well as decreasing substrate temperature, the integrated intensity of the diffraction peaks originating exclusively from Fe on tetrahedral sites is decreasing. We propose that the ions usually occupying tetrahedral sites in magnetite are relocated to octahedral vacancies. Ferrimagnetic behaviour is only observed for well ordered magnetite films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗