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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Pr(FeSb3)4 by Materials Project

PrFe4Sb12 crystallizes in the cubic Im-3 space group. The structure is three-dimensional. Pr3+ is bonded to twelve equivalent Sb+1.25- atoms to form PrSb12 cuboctahedra that share faces with eight equivalent FeSb6 octahedra. All Pr–Sb bond lengths are 3.42 Å. Fe3+ is bonded to six equivalent Sb+1.25- atoms to form FeSb6 octahedra that share corners with six equivalent FeSb6 octahedra and faces with two equivalent PrSb12 cuboctahedra. The corner-sharing octahedral tilt angles are 52°. All Fe–Sb bond lengths are 2.56 Å. Sb+1.25- is bonded in a 2-coordinate geometry to one Pr3+, two equivalent Fe3+, and two equivalent Sb+1.25- atoms. There are one shorter (2.99 Å) and one longer (3.04 Å) Sb–Sb bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Pr(As3Ru)4 by Materials Project

PrRu4As12 crystallizes in the cubic Im-3 space group. The structure is three-dimensional. Pr3+ is bonded to twelve equivalent As+1.67- atoms to form PrAs12 cuboctahedra that share faces with eight equivalent RuAs6 octahedra. All Pr–As bond lengths are 3.27 Å. Ru+4.25+ is bonded to six equivalent As+1.67- atoms to form RuAs6 octahedra that share corners with six equivalent RuAs6 octahedra and faces with two equivalent PrAs12 cuboctahedra. The corner-sharing octahedral tilt angles are 59°. All Ru–As bond lengths are 2.47 Å. As+1.67- is bonded in a 5-coordinate geometry to one Pr3+, two equivalent Ru+4.25+, and two equivalent As+1.67- atoms. There are one shorter (2.58 Å) and one longer (2.59 Å) As–As bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Pr(FeAs3)4 by Materials Project

PrFe4As12 crystallizes in the cubic Im-3 space group. The structure is three-dimensional. Pr3+ is bonded to twelve equivalent As+1.25- atoms to form PrAs12 cuboctahedra that share faces with eight equivalent FeAs6 octahedra. All Pr–As bond lengths are 3.16 Å. Fe3+ is bonded to six equivalent As+1.25- atoms to form FeAs6 octahedra that share corners with six equivalent FeAs6 octahedra and faces with two equivalent PrAs12 cuboctahedra. The corner-sharing octahedral tilt angles are 55°. All Fe–As bond lengths are 2.37 Å. As+1.25- is bonded in a 2-coordinate geometry to one Pr3+, two equivalent Fe3+, and two equivalent As+1.25- atoms. There are one shorter (2.63 Å) and one longer (2.64 Å) As–As bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2020

CFD-DEM and PR-DNS studies of low-temperature densely packed beds

Over the past few decades, granular media is gaining attention as a viable option for heat transfer fluids (HTFs). Several research efforts are studying the use of particle-based heat transfer fluids in a wide variety of applications. With this motivation, the current work focusses on analyzing the different heat transfer mechanisms in low-temperature mono-sized densely packed granular media. To study the heat transfer behavior of granular media at different scales, the current work employs a two-way coupled computational strategy. The motion of particles is solved using the Discrete Element Method (DEM) and the interstitial air is solved using a Finite-Volume (CFD) approach. The Open-Source library CFDEM Coupling® is used in the current study to join the Finite Volume PISO solver of OpenFOAM® and the DEM solver of LIGGGHTS®. Typically, particle-particle contact conduction and particle-air convection are the most popular closure models. But recent research identified a different heat transfer phenomenon in packed beds that cannot be identified by conduction or convection models. While closure models were developed to implement this on a CFD-DEM framework, they did not capture the effect of intra-particulate thermal gradients on this phenomenon. Hence the current work also employs Particle-Resolved Direct Numerical Simulations (PR-DNS) to gain valuable insights allowing for the modification of existing models. A new closure model is then proposed here and is implemented in the CFD-DEM framework. This model provides key insights into the different heat transfer mechanism of packed beds.

42 ENGINEERING↗
DOE OSTI2022

High‐Performance and Durable Fuel Cells using Co/Sr‐Free Fluorite‐Based Mixed Conducting (Pr,Ce)O 2‐δ Cathode

Abstract A major challenge to overcome in demonstrating solid oxide fuel cells (SOFCs) to be suitable as efficient and environmentally friendly energy conversion devices capable of addressing pressing clean energy and environmental needs is to surmount chemical and thermo‐mechanical instabilities in their operational phase. To date, perovskite‐based mixed conducting cathodes, which include inherent Co and Sr elements for enhanced reactivity and conductivity, have been intensively studied. These Co/Sr‐based oxides, however, exhibit severe thermochemical expansion and suffer from Sr surface segregation, ultimately degrading the electrode performance. Here, high‐performance and durable SOFCs are demonstrated by employing a Co/Sr‐free fluorite‐based mixed conducting (Pr,Ce)O 2‐δ (PCO) cathode eminently compatible with fluorite‐based solid electrolytes. The nanocolumnar PCO electrode developed in this study provides not only a remarkable low level of electrode resistance (e.g., ≈0.05 Ω cm 2 at 600 °C) but also exceptional long‐term stability (e.g., a degradation rate 15 times slower compared to the state‐of‐the‐art La 0.6 Sr 0.4 CoO 3‐δ perovskite). The competitive peak power densities of an anode‐supported single cell with the PCO cathode are also successively achieved, recording a value of 0.92 W cm −2 at 600 °C. These findings herald the development of new Co/Sr‐free electrodes for SOFCs at intermediate temperatures.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

Tailored (La 0.2 Pr 0.2 Nd 0.2 Tb 0.2 Dy 0.2 ) 2 Ce 2 O 7 as a Highly Active and Stable Nanocatalyst for the Oxygen Evolution Reaction

Abstract Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self‐assembly (EISA) can be used to synthesize highly porous and high surface area cerate‐based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd 2 Ce 2 O 7 , with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd 3 + cation substitution for Ce in the CeO 2 fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE 1.5 Ce 1.5 O 5 +x phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally‐complex fluorite– (La 0.2 Pr 0.2 Nd 0.2 Tb 0.2 Dy 0.2 ) 2 Ce 2 O 7 is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm −2 in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalyst design.

Chemistry↗
DOE OSTI2020

Syntheses and Crystal Structures of Rare-Earth Oxyapatites Ca 2 RE 8 (SiO 4 ) 6 O 2 (RE = Pr, Tb, Ho, Tm)

Four different rare-earth oxyapatites of Ca 2 RE 8 (SiO 4 ) 6 O 2 (RE = Pr, Tb, Ho, Tm) were synthesized using a solution-based method followed by drying, calcination, and high-temperature sintering in air. X-ray powder diffraction and Raman spectroscopy were performed on the synthesized oxyapatites. Here, the RE oxyapatites crystallize in the hexagonal space group P6 3 /m with similar unit cell parameters, increasing linearly with larger RE cations. The unit cell volumes increase linearly whereas the densities decrease nonlinearly with larger RE cations. Raman spectra showed intense bands of the symmetric bending and stretching modes of SiO 4 at ~ 400 and 860 cm -1 regions, respectively. The bands generally shifted to higher frequencies with smaller RE cations in the structures.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Chemical and environmental stability of monazite-cheralite solid solutions Ln 1-2 x Ca x Th x PO 4 (Ln = Pr, Nd; x = 0–0.15): A thermodynamic study

Monazite-cheralite ceramics are a promising waste form for actinides. To elucidate the long-term behavior of this matrix in aqueous solutions, this study measured thermodynamic data for Th-rhabdophanes Ln 1-2 x Ca x Th x PO 4 ·nH 2 O (with Ln = Pr, Nd; x = 0–0.15) and the associated anhydrous monazite-cheralites Ln 1-2 x Ca x Th x PO 4 . Further, solubility experiments at 298K and high temperature oxide melt solution calorimetry were combined for calculation of ΔG$^°_f$, ΔH$^°_f$ and S$^°_m$ of Th-rhabdophanes and associated monazite-cheralites. Standard solubility constants were employed in a geochemical simulation using the PHREEQC software, the results of which confirmed the high chemical stability of the monazite-cheralite phases and supported their use as a specific conditioning matrix for the long-term immobilization of actinides.

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

Depicting the roles of CuO secondary phase and heat treatment in driving the magnetic and magnetocaloric features of Pr 2/3 Sr 1/3 MnO 3 manganite

In this work, we report a detailed experimental study regarding the impact of copper oxide (CuO) secondary phase and heat treatments on the structural, magnetic, and magnetocaloric properties of the near-room temperature Pr 2/3 Sr 1/3 MnO 3 (PSMO) magnetic refrigerant. Our investigations are carried out by using structural and microstructural analyses, alongside magnetization measurements. Here, the analysis of X-ray diffraction data of PSMO(95%)-CuO(5%) (PSMO-CuO) samples shows the coexistence of both CuO and PSMO phases. Further, the microstructural analysis of PSMO-CuO reveals that the addition of CuO significantly enhances the grains size. On the other hand, the added secondary phase markedly reduces the Curie temperature (T c ) from 293 K for PSMO to about 273 K for PSMO-CuO composite while increasing the magnetocaloric effect. This decrease in T c is associated with a significant change from 162° to about 156° in the Mn-OII-Mn bond angle respectively. Moreover, performed investigations regarding the role of heat treatments unveil that the observed changes in the structural and magnetic features are mainly driven by the secondary phase that modify grains size and double-exchange interactions in the PSMO compound. Interestingly, our findings demonstrate that the Curie temperature of the PSMO and accordingly its magnetocaloric effect can be tailored by adding small amounts of CuO without need to substitution on cation sites. In the light of obtained results, a multilayered refrigerant composed of PSMO and PSMO-CuO is proposed to cover the magnetic cooling temperature range close to room-temperature. The resulting entropy change remains practically constant between 273 K and 293 K. Such a behavior is highly appreciated from a practical point of view, particularly in cases where the cooling process is carried out by using the AMR and Ericsson cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Origin for retained activity in Pr 2 NiO 4 while undergoing substantial phase transformation in a long-term solid oxide cell operation

Pr 2 NiO 4 (PNO) is a known active oxygen electrode for solid oxide cells but undergoes phase transformation at high temperatures. An in-situ synchrotron study on PNO electrodes show that phase transformation reaches nearly 100% in a long-term operation. Such significant phase transformation is expected to play a detrimental role on the cell performance. However, PNO retains the activity in an enduring operation. The origin for this dichotomy has remained obscure. Here, the aim of this article is to investigate the origin for retained activity and performance stability in PNO electrode. High-resolution transmission electron microcopy analysis shows the presence of a large number of nanoclusters (~ 5 nm) through the entire electrode bulk, which are localized in the 20–60 nm region. In-situ synchrotron studies show that those nanoclusters are nickelates that account for the retained activity during phase transformation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Evidence against the wobbling nature of low-spin bands in 135 Pr

The electromagnetic character of the ΔI=1 transitions connecting the 1- to 0-phonon and the 2- to 1-phonon wobbling bands should be dominated by an E2 component, due to the collective motion of the entire nuclear charge. In the present work it is shown, based on combined angular correlation and linear polarization measurements, that the mixing ratios of all analyzed connecting transitions between low-lying bands in 135 Pr interpreted as 0-, 1-, and 2-phonon wobbling bands, have absolute values smaller than one. This indicates predominant M1 magnetic character, which is incompatible with the proposed wobbling nature. All experimental observables are instead in good agreement with quasiparticle-plus-triaxial-rotor model calculations, which describe the bands as resulting from a rapid re-alignment of the total angular momentum from the short to the intermediate nuclear axis

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Is a Homologous Series Member Equal to or Greater than the Sum of its Subunits? A Focus on a Few Fe-based BaNiSn3 subunits: LnFeGe3 (Ln = La, Pr)

The homologous series Lnn+1MnGe3n+1 (Ln = Ce, Pr, M = Fe, Co) has proven to be a fruitful platform for tuning physical properties as a function of subunit stacking. In this study, we investigate the crystal growth, structure, and physical properties of PrFeGe3, aiming to deconstruct Pr4Fe3Ge10 (n = 3) into its subunits, including the BaNiSn3 and CeNiSi2 structure types. The electrical resistivity and magnetic susceptibility of PrFeGe3 were measured. The magnetic properties reveal PrFeGe3 to be antiferromagnetic (5 K) with an effective moment of eff = 4.38 B/F.U., larger than the spin-only moment from Pr3+ (3.58 B). Due to the large magnetic moment observed, the oxidation state of the elements was determined using spectroscopic methods including X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. The neutral oxidation state of iron determined from X-ray photoelectron spectroscopy supports itinerant magnetic behavior of iron. Annealing, in tandem with differential scanning calorimetry, revealed PrFeGe3 to be a precursor for the formation of new homologous series members.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Flux-Assisted Boron Chalcogen Mixture (BCM) Method for Synthesizing Mixed Chalcogenide Semiconductors (AkRE 2 Si 2 Se x S 8– x and CaRE 2 Si 2 Se 8 ) ( Ak = Ca and Sr; RE = La, Ce, Pr, Nd, and Sm): Investigation of Their Magnetic and Optical Properties

We report a detailed structural analysis of a series of ten quaternary rare-earth-containing seleno-thiosilicates AkRE 2 Si 2 Se x S 8-x and selenosilicates, CaRE 2 Si 2 Se 8 (Ak = Ca and Sr; RE = La, Ce, Pr, Nd, and Sm). Single crystals were obtained by using the flux-assisted boron chalcogen mixture (BCM) method and single crystal X-ray diffraction was used to determine their structures. All members of the AkRE 2 Si 2 Se x S 8-x and CaRE 2 Si 2 Se 8 series crystallize in the space group R$\bar{3}$c (space group number 167) of the trigonal crystal system. The single-crystal X-ray diffraction analysis revealed a strong preference for Se/S atoms to occupy one vs. the other of the two available sites. Polycrystalline samples were used for magnetic susceptibility and UV–visible diffuse reflectance measurements. Magnetic measurements show that CaCe₂Si₂Se₁.₇₃S₆.₂₇ and CaNd₂Si₂Se₂.₅S₅.₅ are paramagnetic with negative Weiss constants (θ = –60.1 and –26.2). Diffuse reflectance analysis gives optical band gaps of 2.7(1) eV (CaLa₂Si₂Se₂.₃₈S₅.₆₂), 2.2(1) eV (CaCe₂Si₂Se₁.₇₃S₆.₂₇), 2.5(1) eV (CaNd₂Si₂Se₂.₅S₅.₅), and 2.0(1) eV (CaCe₂Si₂Se₈), consistent with density functional theory calculations. By partially or fully replacing S sites with Se it was possible to achieve band gap tuning. Photoluminescence behavior was also investigated for CaCe 2 Si 2 Se 1.73 S 6.27 via irradiation with 375 nm ultraviolet light.

BCM method↗
DOE OSTI2020

Tuning Irreversible Magnetoresistance in Pr 0.67 Sr 0.33 MnO 3 Film via Octahedral Rotation

Herein, the oxygen octahedral rotation around the out-of-plane axis is explored to study its effect on metastable status, magnetic cluster glass in manganite. The antiphase rotation around the out-of-plane axis (TiO 6 α Ο α Ο c – ) of SrTiO 3 enhances the Mn–O bond anisotropy along in-plane and out-of-plane directions and weakens the ferromagnetic interactions in a 12 nm Pr 0.67 Sr 0.33 MnO 3 film on the (001) SrTiO 3 substrate, which together promote the formation of magnetic cluster-glassiness and enlarges the irreversible magnetoresistance (MR) effect with enhanced relaxation time of charge carriers. The effect of TiO 6 α Ο α Ο c – in the SrTiO 3 substrate on material properties is obvious with a large irreversible MR effect for thin films, which fades away with the increase in film thickness. At 10 K, the irreversible MR is 0.91 for the 12 nm film and 0.22 for the 30 nm film. This work extends current understanding on interfacial coupling to metastable status and could help explore other systems in the perovskite structure with octahedral rotation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Conformally coated scaffold design using water-tolerant Pr 1.8 Ba 0.2 NiO 4.1 for protonic ceramic electrochemical cells with 5,000-h electrolysis stability

Protonic ceramic electrochemical cells (PCECs) have potential as long-duration energy storage systems. However, their operational stability is limited under industrially relevant conditions due to the intrinsic chemical instability of doped barium cerate-based electrolytes and oxygen electrodes against H 2 O, as well as the poor electrode–electrolyte interfacial contact. Here, in this study, we present a conformally coated scaffold (CCS) design to comprehensively address these issues. A porous proton-conducting scaffold is constructed and conformally coated with Pr 1.8 Ba 0.2 NiO 4.1 electrocatalyst, which has high chemical stability against H 2 O, triple conductivity and hydration capability, and protects vulnerable electrolytes from H 2 O. The CCS structure consolidates the electrode–electrolyte interfacial bonding to enable fast proton transfer in the percolated network. This design enables PCECs to reach electrolysis stability for 5,000 h at −1.5 A cm−2 and 600 °C in 40% H 2 O. This work provides a general strategy to stabilize PCECs and offers guidance for designing resilient and stable solid-state energy storage systems.

Protonic ceramic electrochemical cell↗
DOE OSTI2024

Magnetic structure and properties of the compositionally complex perovskite (Y 0.2 La 0.2 Pr 0.2 Nd 0.2 Tb 0.2 )MnO 3

Large configurational disorder in compositionally complex ceramics can lead to unique functional properties that deviate from traditional rules of alloy mixing. In recent years, compositionally complex oxides (CCOs) have shown intriguing magnetic behavior including long-range order, enhanced magnetic exchange couplings, and mixed phase magnetic structures. This work focuses on how large local spin disorder affects magnetic ordering in a CCO. Specifically, we investigated the A-site alloyed perovskite, (Y 0.2 La 0.2 Pr 0.2 Nd 0.2 Tb 0.2 )MnO 3 , or (5A)MnO 3 , using a combination of bulk magnetometry, synchrotron X-ray diffraction, and temperature-dependent neutron diffraction. The five A-site ions have an average spin and ionic radius nearly equal to that of Nd 3+ ions, which minimizes structural distortions and allows for an understanding of the local spin disorder effects through a direct comparison with NdMnO 3 . Our magnetometry data show that (5A)MnO 3 exhibits two distinct phase transitions associated with the A-site and B-site sublattices, as seen in NdMnO 3 , as well as the presence of domain pinning and exchange bias at low temperature, suggesting a mixed phase magnetic ground state, as seen in other magnetic CCOs. Neutron powder diffraction shows clear long-range antiferromagnetic ordering below 67 K and refines to a Pn'ma' magnetic structure at low temperature, in excellent agreement with the well-studied behavior of NdMnO 3 . The two most notable differences in (5A)MnO 3 magnetism apparent from our data are a slight suppression of the B-site ordering temperature, which is explained by a smaller Mn–O–Mn bond angle in (5A)MnO 3 than NdMnO 3 , and the presence of a magnetic susceptibility transition above the B-site ordering, which could indicate the formation of a cluster glass but requires further study. Finally, this work demonstrates a general method of isolated investigation of size and spin disorder in CCOs and motivates future work using local structure probes to better understand the effects of nanoscale clustering and local spin disorder in magnetic CCOs.

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

Ab initio study on fcc Pr with correlation matrix renormalization theory

Here, we studied fcc praseodymium (Pr) with the ab initio correlation matrix renormalization theory (CMRT) explicitly calculating the Coulomb interactions among basis orbitals and without using any adjustable parameters to work for strongly correlated electron systems. We calculated its total energy in a paramagnetic ground state and studied the role of the correlated 4f electrons in the system. Good agreement was obtained between CMRT and experiments in the pressure volume dependence of the fcc phase. We also compared the CMRT results against other theoretical methods including local density approximation + dynamical mean-field theory and showed consistent results among them. Moreover, we found the normalized local charge fluctuation of the 4f electrons as the function of volume exhibits a clear slope change at the volume collapse region, indicating a switch in their correlation nature there.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗