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Title: Structure and cation ordering in La 2UO 6, Ce 2UO 6, LaUO 4, and CeUO 4 by first principles calculations

Abstract

In the present work, we have used density functional theory (DFT) and DFT+U to investigate the crystal structure and phase stability of four model compounds in the Ln 2O 3-UO 2-UO 3 ternary oxide system: La2UO 6, Ce 2UO 6, LaUO 4, CeUO 4, due to the highly-correlated nature of the f-electrons in uranium. We have considered both hypothetical ordered compounds and compounds in which the cations randomly occupy atomic sites in a fluorite-like lattice. We determined that ordered compounds are stable and are energetically favored compared to disordered configurations, though the ordering tendencies are weak. To model and analyze the structures of these complex oxides, we have used supercells based on a layered atomic model. In the layer model, the supercell is composed of alternating planes of anions and cations. We have considered two different ordering motifs for the cations, namely single species (isoatomic) cation layers versus mixed species cation layers. Energy differences between various ordered cationic arrangements were found to be small. This may have implications regarding radiation stability, since cationic arrangements should be able to change under irradiation with little cost in energy.

Authors:
 [1];  [1];  [2];  [2];  [3];  [1]
  1. Univ. of Tennessee, Knoxville, TN (United States)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. CNRS, Chatenay-Malabry (France)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE); USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1360025
Alternate Identifier(s):
OSTI ID: 1397519
Grant/Contract Number:
AC05-00OR22725; NA0001983; UT-TENN0112
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Computational Materials Science
Additional Journal Information:
Journal Volume: 123; Journal ID: ISSN 0927-0256
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Casillas-Trujillo, Luis, Xu, H., McMurray, Jake W., Shin, Dongwon, Baldinozzi, G., and Sickafus, Kurt. Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations. United States: N. p., 2016. Web. doi:10.1016/j.commatsci.2016.05.042.
Casillas-Trujillo, Luis, Xu, H., McMurray, Jake W., Shin, Dongwon, Baldinozzi, G., & Sickafus, Kurt. Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations. United States. doi:10.1016/j.commatsci.2016.05.042.
Casillas-Trujillo, Luis, Xu, H., McMurray, Jake W., Shin, Dongwon, Baldinozzi, G., and Sickafus, Kurt. Wed . "Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations". United States. doi:10.1016/j.commatsci.2016.05.042. https://www.osti.gov/servlets/purl/1360025.
@article{osti_1360025,
title = {Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations},
author = {Casillas-Trujillo, Luis and Xu, H. and McMurray, Jake W. and Shin, Dongwon and Baldinozzi, G. and Sickafus, Kurt},
abstractNote = {In the present work, we have used density functional theory (DFT) and DFT+U to investigate the crystal structure and phase stability of four model compounds in the Ln2O3-UO2-UO3 ternary oxide system: La2UO6, Ce2UO6, LaUO4, CeUO4, due to the highly-correlated nature of the f-electrons in uranium. We have considered both hypothetical ordered compounds and compounds in which the cations randomly occupy atomic sites in a fluorite-like lattice. We determined that ordered compounds are stable and are energetically favored compared to disordered configurations, though the ordering tendencies are weak. To model and analyze the structures of these complex oxides, we have used supercells based on a layered atomic model. In the layer model, the supercell is composed of alternating planes of anions and cations. We have considered two different ordering motifs for the cations, namely single species (isoatomic) cation layers versus mixed species cation layers. Energy differences between various ordered cationic arrangements were found to be small. This may have implications regarding radiation stability, since cationic arrangements should be able to change under irradiation with little cost in energy.},
doi = {10.1016/j.commatsci.2016.05.042},
journal = {Computational Materials Science},
number = ,
volume = 123,
place = {United States},
year = {Wed Jul 06 00:00:00 EDT 2016},
month = {Wed Jul 06 00:00:00 EDT 2016}
}

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  • Single crystals of the new compounds Li{sub 6}[(UO{sub 2}){sub 12}(PO{sub 4}){sub 8}(P{sub 4}O{sub 13})] (1), Li{sub 5}[(UO{sub 2}){sub 13}(AsO{sub 4}){sub 9}(As{sub 2}O{sub 7})] (2), Li[(UO{sub 2}){sub 4}(AsO{sub 4}){sub 3}] (3) and Li{sub 3}[(UO{sub 2}){sub 7}(AsO{sub 4}){sub 5}O)] (4) have been prepared using high-temperature solid state reactions. The crystal structures have been solved by direct methods: 1-monoclinic, C2/m, a=26.963(3) A, b=7.063(1) A, c=19.639(1) A, beta=126.890(4){sup o}, V=2991.2(6) A{sup 3}, Z=2, R{sub 1}=0.0357 for 3248 unique reflections with |F{sub 0}|>=4sigma{sub F}; 2-triclinic, P1-bar, a=7.1410(8) A, b=13.959(1) A, c=31.925(1) A, alpha=82.850(2){sup o}, beta=88.691(2){sup o}, gamma=79.774(3){sup o}, V=3107.4(4) A{sup 3}, Z=2, R{sub 1}=0.0722 formore » 9161 unique reflections with |F{sub 0}|>=4sigma{sub F}; 3-tetragonal, I4{sub 1}/amd, a=7.160(3) A, c=33.775(9) A, V=1732(1) A{sup 3}, Z=4, R{sub 1}=0.0356 for 318 unique reflections with |F{sub 0}|>=4sigma{sub F}; 4-tetragonal, P4-bar, a=7.2160(5) A, c=14.6540(7) A, V=763.04(8) A{sup 3}, Z=1, R{sub 1}=0.0423 for 1600 unique reflections with |F{sub 0}|>=4sigma{sub F}. Structures of all the phases under consideration are based on complex 3D frameworks consisting of different types of uranium polyhedra (UO{sub 6} and UO{sub 7}) and different types of tetrahedral TO{sub 4} anions (T=P or As): PO{sub 4} and P{sub 4}O{sub 13} in 1, AsO{sub 4} and As{sub 2}O{sub 7} in 2, and single AsO{sub 4} tetrahedra in 3 and 4. In the structures of 1 and 2, UO{sub 7} pentagonal bipyramids share edges to form (UO{sub 5}){sub i}nfinity chains extended along the b axis in 1 and along the a axis in 2. The chains are linked via single TO{sub 4} tetrahedra into tubular units with external diameters of 11 A in 1 and 11.5 A in 2, and internal diameters of 4.1 A in 1 and 4.5 A in 2. The channels accommodate Li{sup +} cations. The tubular units are linked into 3D frameworks by intertubular complexes. Structures of 3 and 4 are based on 3D frameworks composed on layers united by (UO{sub 5}){sub i}nfinity infinite chains. Cation-cation interactions are observed in 2, 3, and 4. In 2, the structure contains a trimeric unit with composition [O=U(1)=O]-U(13)-[O=U(2)=O]. In the structures of 3 and 4, T-shaped dimers are observed. In all the structures, Li{sup +} cations are located in different types of cages and channels and compensate negative charges of anionic 3D frameworks. - Graphical abstract: The crystal structures of Li{sub 5}[(UO{sub 2}){sub 13}(AsO{sub 4}){sub 9}(As{sub 2}O{sub 7})] separated into tubular units and intertubular complexes.« less
  • First-principles calculations based on density-functional theory in the pseudopotential approach have been performed for the energetics and crystal structure of Y{sub 2}Si{sub 3}O{sub 3}N{sub 4} with the melilite-type structure. The calculations show the following ordering of the O/N atoms in the crystal: N atoms fully occupy the bridging site (2c) and O atoms fully occupy the terminal site (4e) WITH 2 O and 6 N atoms at the bridging 8f site. These conclusions are in good agreement with the experimental results obtained with neutron diffraction. In addition, the calculations show that there is a preferential distribution of the O andmore » N atoms at the 8f site, resulting in two different local coordinations of Y, as compared to only a single averaged crystallographic Y site. All the nitrogen ions exhibit similar electronic structure, in contrast to the oxygen ions. However, there are slightly more electrons (about 0.1 electrons per ion) for nitrogen at the 2c site than those at the 8f site, while the silicon atoms have almost the same charge distribution.« less
  • A new caesium uranyl molybdate belonging to the M{sub 6}U{sub 2}Mo{sub 4}O{sub 21} family has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. Contrary to the other alkali uranyl molybdates of this family (A=Na, K, Rb) where molybdenum atoms adopt only tetrahedral coordination and which can be formulated A{sub 6}[(UO{sub 2}){sub 2}O(MoO{sub 4}){sub 4}], the caesium compound Cs{sub 6}U{sub 2}Mo{sub 4}O{sub 21} should be written Cs{sub 6}[(UO{sub 2}){sub 2}(MoO{sub 4}){sub 3}(MoO{sub 5})] with molybdenum atoms in tetrahedral and square pyramidal environments. Cs{sub 6}[(UO{sub 2}){sub 2}(MoO{sub 4}){sub 3}(MoO{sub 5})] crystallizes in the triclinic symmetry withmore » space group P1-bar and a=10.4275(14) A, b=15.075(2) A, c=17.806(2) A, {alpha}=70.72(1){sup o}, {beta}=80.38(1){sup o} and {gamma}=86.39(1){sup o}, V=2604.7(6) A{sup 3}, Z=4, {rho}{sub mes}=5.02(2) g/cm{sup 3} and {rho}{sub cal}=5.08(3) g/cm{sup 3}. A full-matrix least-squares refinement on the basis of F{sup 2} yielded R{sub 1}=0.0464 and wR{sub 2}=0.0950 for 596 parameters with 6964 independent reflections with I{>=}2{sigma}(I) collected on a BRUKER AXS diffractometer with Mo(K{alpha}) radiation and a CCD detector. The crystal structure of Cs compound is characterized by {sub {infinity}}{sup 1}[(UO{sub 2}){sub 2}(MoO{sub 4}){sub 3}(MoO{sub 5})]{sup 6-} parallels chains built from U{sub 2}O{sub 13} dimeric units, MoO{sub 4} tetrahedra and MoO{sub 5} square pyramids, whereas, Na, K and Rb compounds are characterized by {sub {infinity}}{sup 1}[(UO{sub 2}){sub 2}O(MoO{sub 4}){sub 4}]{sup 6-} parallel chains formulated simply of U{sub 2}O{sub 13} units and MoO{sub 4} tetrahedra. Infrared spectroscopy measurements using powdered samples synthesized by solid-state reaction, confirm the structural results. The thermal stability and the electrical conductivity are also studied. The four compounds decompose at low temperature (between 540 and 610 {sup o}C). -- Graphical abstract: The staking of {sub {infinity}}{sup 1}[(UO{sub 2}){sub 2}(MoO{sub 4}){sub 3}(MoO{sub 5})]{sup 6-} infinite uranyl molybdate ribbons in the Cs{sub 6}[(UO{sub 2}){sub 2}(MoO{sub 4}){sub 3}(MoO{sub 5})] structure. Display Omitted Highlights: {yields} Cs{sub 6}U{sub 2}Mo{sub 4}O{sub 2} a new compound with bidimensional crystal structure, characterized by infinite uranyl molybdate chains. {yields} Crystal structure similar to these of the compounds containing Na, K, Rb. {yields} Molybdenum atoms surrounded by five oxygen atoms to form an original and strongly distorted MoO{sub 5} environment. {yields} The chains arrangement illustrates the key role of the alkaline ionic radius, in the crystal structure distortion for Cs compound.« less
  • Trace amounts of H/sub 2/O and limited exposure to air of reaction mixtures of UCl/sub 4/ and 12-crown-4, 15-crown-5, benzo-15-crown-5, 18-crown-6, or dibenzo-18-crown-6 in 1:3 mixtures of CH/sub 3/OH and CH/sub 3/CN resulted in the hydrolysis and oxidation of UCl/sub 4/ to (UO/sub 2/Cl/sub 4/)/sup 2/minus//. In the presence of these crown ethers, it has been possible to isolate intermediate products via crystallization of crown complexes of the (UO/sub 2/Cl/sub 4/)/sup 2/minus// ion, the (UCl/sub 6/)/sup 2/minus// ion, and (UO/sub 2/Cl/sub 2/(OH/sub 2/)/sub 3/). The neutral moiety crystallizes as a hydrogen-bonded crown ether complex; however, crown ether complexation of amore » counterion, either an ammonium ion formed during the oxidation of U(IV) or a Na/sup +/ ion leached from glass reaction vessels, resulted in novel crystalline complexes of the ionic species. ((NH/sub 4/)(15-crown-5)/sub 2/)/sub 2/(UO/sub 2/Cl/sub 4/) /times/ 2CH/sub 3/CN, ((NH/sub 4/)(benzo-15-crown-5)/sub 2/)/sub 2/(UCl/sub 6/) /times/ 4CH/sub 3/CN, and ((NH/sub 4/)(dibenzo-18-crown-6))/sub 2/(UO/sub 2/Cl/sub 4/) /times/ 2CH/sub 3/CN have been structurally characterized by single-crystal X-ray diffraction techniques. The results of all the crystal studies are presented in detail. The ammonium ions interact with the crown ethers via hydrogen-bonding and electrostatic interactions. 15-Crown-5 and benzo-15-crown-5 form 2:1 sandwich cations, allowing no H/sub 4/N/sup +//hor ellipsis/(UO/sub 2/Cl/sub 4/)/sup 2/minus// interaction. The dibenzo-18-crown-6 complexed ammonium ions are 1:1 and form bifurcated hydrogen bonds with the chlorine atoms in the (UO/sub 2/Cl/sub 4/)/sup /minus// anion. The formation of (Na(12-crown-4)/sub 2//sub 2/(UO/sub 2/Cl/sub 4/) /times/ 2OHMe and (UO/sub 2/Cl/sub 2/(OH)/sub 2/)/sub 3/) /times/ 18-crown-6 /times/ H/sub 2/O /times/ OHMe has been confirmed by preliminary single-crystal X-ray diffraction studies.« less