The crystal structure of UC<sub>2</sub>

Bowman, A. L.; Arnold, G.; Witteman, W. G.; Wallace, T.C.; Nereson, N. G.

doi:10.1107/s0365110x66003670

Cited by 63 publications

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“…The optimized lattice constant of 5.332 Å is in fair agreement with the experimental value 5.410 Å at 1820°C. 11 The computed U−C and C−C distance (in the CC units) are 2.325 Å and 1.443 Å (experimental value 2.330 Å vs 1.460 Å at 1820°C), respectively. The U−C distance of 2.325 Å is shorter than seen in many structures in the Inorganic Crystal Structure Database (ICSD).…”

Section: Resultsmentioning

confidence: 99%

Rotational Rehybridization and the High Temperature Phase of UC₂

et al. 2012

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The screened hybrid approximation (HSE) of density functional theory (DFT) is used to examine the structural, optical, and electronic properties of the high temperature phase, cubic UC(2). This phase contains C(2) units with a computed C-C distance of 1.443 Å which is in the range of a CC double bond; U is formally 4+, C(2) 4-. The closed shell paramagnetic state (NM) was found to lie lowest. Cubic UC(2) is found to be a semiconductor with a narrow gap, 0.4 eV. Interestingly, the C(2) units connecting two uranium sites can rotate freely up to an angle of 30°, indicating a hindered rotational solid. Ab-initio molecular dynamic simulations (HSE) show that the rotation of C(2) units in the low temperature phase (tetragonal UC(2)) occurs above 2000 K, in good agreement with experiment. The computed energy barrier for the phase transition from tetragonal UC(2) to cubic UC(2) is around 1.30 eV per UC(2). What is fascinating about this system is that at high temperature, the phase transformation to the cubic phase is associated with a rehybridization of the C atoms from sp to sp(3).

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Section: Resultsmentioning

confidence: 99%

Rotational Rehybridization and the High Temperature Phase of UC₂

et al. 2012

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“…This phase is labeled non-quenchable as attempts to preserve its structure via sudden temperature drops for characterization at ambient conditions have been unsuccessful [7]. Remarkably, these three experiments by Bowman et al [9] (investigated using neutrons), Wilson [10] (X-ray diffraction, XRD) and Bredig [7] (XRD) originating in the early 1960's reveal conflicting results which have not been clearly resolved with modern diffraction techniques. Despite the UC2 crystal structure being the basis for nuclear reactor-relevant thermodynamics calculations [11], [12] there has not been, to the best of our knowledge, a more recent study to unambiguously determine the crystal structure of the cubic UC2 δ-phase.…”

Section: Introductionmentioning

confidence: 99%

In situ synthesis and characterization of uranium carbide using high temperature neutron diffraction

Reiche

Vogel

Tang

2016

Journal of Nuclear Materials

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We investigated the formation of UCx from UO2+x and graphite in situ using neutron diffraction at high temperatures with particular focus on resolving the conflicting reports on the crystal structure of non-quenchable cubic UC2. The agents were UO2 nanopowder, which closely imitates nano grains observed in spent reactor fuels, and graphite powder. In situ neutron diffraction revealed the onset of the UO2 + 2C  UC + CO2 reaction at 1440˚C, with its completion at 1500˚C. Upon further heating, carbon diffuses into the uranium carbide forming C2 groups at the octahedral sites. This resulting high temperature cubic UC2 phase is similar to the NaCl-type structure as proposed by Bowman et al. Our novel experimental data provide insights into the mechanism and kinetics of formation of UC as well as characteristics of the high

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“…Er3Pd2oPö adopts the CraCö-type structure (Pearson symbol cF116) [1], which is built up of fragments of columns of cuboctahedra (figure, top), two tetragonal antiprisms, and cubes (figure, middle). The Er atoms substitute for the transition metals in sites 4a and 8c.…”

Section: Discussionmentioning

confidence: 99%

Crystal structure of erbium palladium phosphor (3:20:6), Er3Pd20P6

Zelinska¹,

Pivan²,

Oryshchyn³

et al. 2006

Zeitschrift Für Kristallographie - New Crystal Structures

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Er1 Er2Abstract Er3P2flPd6, cubic, Fm3m (no. 225), a = 12.111(1) Ä, V= 1776.4 Ä 3 , Ζ = 4, Rgt(F) = 0.030, wR Kf (F) = 0.032, 7 1 =293 K. Source of materialErbium, red phosphorus, and palladium were of 99.99 wt.-% purity. A mixture of the starting components with nominal composition Er3Pd2oP6 was pressed into a small pellet (1 g) and melted in an arc furnace under a Ti/Zr-gettered argon atmosphere at normal pressure. The alloy was heat-treated at 1070 Κ for 20 d in evacuated fused silica ampules, followed by quenching in cold water. After the sample was crushed, bar-like air-stable single crystals with metallic luster were extracted and were for the structure determination. Energy-dispersive analysis of several crystals in a scanning electron microscope confirmed the presence of erbium, palladium, and phosphorus as the only components in atomic percentages of Er:Pd:P = 11:68:21 (with estimated standard deviations of 2%). DiscussionEr3Pd2oPö adopts the CraCö-type structure (Pearson symbol cF116) [1], which is built up of fragments of columns of cuboctahedra (figure, top), two tetragonal antiprisms, and cubes (figure, middle). The Er atoms substitute for the transition metals in sites 4a and 8c. The Ρ atoms have tetragonal antiprismatic coordination (figure, bottom). The shortest interatomic distance found (

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The crystal structure of UC₂

Cited by 63 publications

References 0 publications

Rotational Rehybridization and the High Temperature Phase of UC₂

Rotational Rehybridization and the High Temperature Phase of UC₂

In situ synthesis and characterization of uranium carbide using high temperature neutron diffraction

Crystal structure of erbium palladium phosphor (3:20:6), Er3Pd20P6

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The crystal structure of UC2

Cited by 63 publications

References 0 publications

Rotational Rehybridization and the High Temperature Phase of UC2

Rotational Rehybridization and the High Temperature Phase of UC2

In situ synthesis and characterization of uranium carbide using high temperature neutron diffraction

Crystal structure of erbium palladium phosphor (3:20:6), Er3Pd20P6

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The crystal structure of UC₂

Rotational Rehybridization and the High Temperature Phase of UC₂

Rotational Rehybridization and the High Temperature Phase of UC₂