skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: High pressure behaviour of uranium dicarbide (UC{sub 2}): Ab-initio study

Abstract

The structural stability of uranium dicarbide has been examined under hydrostatic compression employing evolutionary structure search algorithm implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with ab-initio electronic band structure calculation method. The ab-initio total energy calculations involved for this purpose have been carried out within both generalized gradient approximations (GGA) and GGA + U approximations. Our calculations under GGA approximation predict the high pressure structural sequence of tetragonal → monoclinic → orthorhombic for this material with transition pressures of ∼8 GPa and 42 GPa, respectively. The same transition sequence is predicted by calculations within GGA + U also with transition pressures placed at ∼24 GPa and ∼50 GPa, respectively. Further, on the basis of comparison of zero pressure equilibrium volume and equation of state with available experimental data, we find that GGA + U approximation with U = 2.5 eV describes this material better than the simple GGA approximation. The theoretically predicted high pressure structural phase transitions are in disagreement with the only high experimental study by Dancausse et al. [J. Alloys. Compd. 191, 309 (1993)] on this compound which reports a tetragonal to hexagonal phase transition at a pressure of ∼17.6 GPa. Interestingly, during lowest enthalpy structure search using USPEX, we do not see any hexagonalmore » phase to be closer to the predicted monoclinic phase even within 0.2 eV/f. unit. More experiments with varying carbon contents in UC{sub 2} sample are required to resolve this discrepancy. The existence of these high pressure phases predicted by static lattice calculations has been further substantiated by analyzing the elastic and lattice dynamic stability of these structures in the pressure regimes of their structural stability. Additionally, various thermo-physical quantities such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Gruneisen parameter, and heat capacity at ambient conditions have been determined from these calculations and compared with the available experimental data.« less

Authors:
; ; ;  [1]
  1. Applied Physics Division, Bhabha Atomic Research Centre, Mumbai 400085 (India)
Publication Date:
OSTI Identifier:
22598891
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 120; Journal Issue: 8; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ALGORITHMS; ALLOYS; APPROXIMATIONS; CARBON; COMPARATIVE EVALUATIONS; DEBYE TEMPERATURE; ENTHALPY; EQUATIONS OF STATE; MONOCLINIC LATTICES; ORTHORHOMBIC LATTICES; PHASE TRANSFORMATIONS; PRESSURE RANGE GIGA PA; PRESSURE RANGE MEGA PA 10-100; SPECIFIC HEAT; STABILITY; THERMAL EXPANSION; URANIUM; URANIUM CARBIDES

Citation Formats

Sahoo, B. D., E-mail: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., and Kaushik, T. C. High pressure behaviour of uranium dicarbide (UC{sub 2}): Ab-initio study. United States: N. p., 2016. Web. doi:10.1063/1.4961497.
Sahoo, B. D., E-mail: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., & Kaushik, T. C. High pressure behaviour of uranium dicarbide (UC{sub 2}): Ab-initio study. United States. doi:10.1063/1.4961497.
Sahoo, B. D., E-mail: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., and Kaushik, T. C. Sun . "High pressure behaviour of uranium dicarbide (UC{sub 2}): Ab-initio study". United States. doi:10.1063/1.4961497.
@article{osti_22598891,
title = {High pressure behaviour of uranium dicarbide (UC{sub 2}): Ab-initio study},
author = {Sahoo, B. D., E-mail: bdsahoo@barc.gov.in and Mukherjee, D. and Joshi, K. D. and Kaushik, T. C.},
abstractNote = {The structural stability of uranium dicarbide has been examined under hydrostatic compression employing evolutionary structure search algorithm implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with ab-initio electronic band structure calculation method. The ab-initio total energy calculations involved for this purpose have been carried out within both generalized gradient approximations (GGA) and GGA + U approximations. Our calculations under GGA approximation predict the high pressure structural sequence of tetragonal → monoclinic → orthorhombic for this material with transition pressures of ∼8 GPa and 42 GPa, respectively. The same transition sequence is predicted by calculations within GGA + U also with transition pressures placed at ∼24 GPa and ∼50 GPa, respectively. Further, on the basis of comparison of zero pressure equilibrium volume and equation of state with available experimental data, we find that GGA + U approximation with U = 2.5 eV describes this material better than the simple GGA approximation. The theoretically predicted high pressure structural phase transitions are in disagreement with the only high experimental study by Dancausse et al. [J. Alloys. Compd. 191, 309 (1993)] on this compound which reports a tetragonal to hexagonal phase transition at a pressure of ∼17.6 GPa. Interestingly, during lowest enthalpy structure search using USPEX, we do not see any hexagonal phase to be closer to the predicted monoclinic phase even within 0.2 eV/f. unit. More experiments with varying carbon contents in UC{sub 2} sample are required to resolve this discrepancy. The existence of these high pressure phases predicted by static lattice calculations has been further substantiated by analyzing the elastic and lattice dynamic stability of these structures in the pressure regimes of their structural stability. Additionally, various thermo-physical quantities such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Gruneisen parameter, and heat capacity at ambient conditions have been determined from these calculations and compared with the available experimental data.},
doi = {10.1063/1.4961497},
journal = {Journal of Applied Physics},
number = 8,
volume = 120,
place = {United States},
year = {Sun Aug 28 00:00:00 EDT 2016},
month = {Sun Aug 28 00:00:00 EDT 2016}
}
  • Gaseous carbides of uranium, UC, UC/sub 2/, UC/sub 3/, UC/sub 4/, UC/sub 5/, and UC/sub 6/ have been observed in a mass spectrometric investigation of the Knudsen cell effusate from a thorium--uranium--rhodium--graphite system at high temperatures. Partial pressures of the carbide molecules were measured as a function of temperature in the 2300--2700/sup 0/ K range. Second and third law methods were employed to determine enthalpy changes for the reactions of the type U(g)+nC(graphite) =UC/sub n/(g), n=1 to 6, and for additional reactions with graphite involving two gaseous carbide species and various homogeneous gas phase reactions. The experimental enthalpies when combinedmore » with thermodynamic data taken from literature yielded the following atomization energies, ..delta..H/sup 0//sub at,298/, and standard heats of formation, ..delta..H/sup 0//sub f,298/:« less
  • Sm{sub 2}O{sub 3} was compressed at room temperature up to 44.0 GPa and then decompressed back to ambient pressure. In situ X-ray diffraction was used to monitor the structural changes in the sample. A cubic to hexagonal phase transformation was observed in Sm{sub 2}O{sub 3} for the first time. After decompression back to ambient pressure, the hexagonal phase was not quenchable and transformed to a monoclinic phase. Ab initio Density-Functional-Theory (DFT) calculations were performed to obtain theoretical data for comparison with the experimental results and elucidation of the transformation mechanism. A possible phase transformation mechanism that is consistent with themore » experimental results and theoretical calculations is proposed.« less
  • DFT calculations have been carried out for Cu{sub 4}Bi{sub 5}S{sub 10} and Bi{sub 2}S{sub 3} to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi{sub 2}S{sub 3} and the results are compared to published experimental data. Bi{sup 3+} in Cu{sub 4}Bi{sub 5}S{sub 10} is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi{sub 2}S{sub 3} are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lonemore » pairs as lobes of non-shared charge. These lobes are related to an effective Bi s-Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s-p hybridization is seen for the symmetric site in Cu{sub 4}Bi{sub 5}S{sub 10}, whereas Bi s-p hybridization coexists with a much reduced lone pair in Bi{sub 2}S{sub 3} at high pressure. - Graphical abstract: The article includes charge density maps used to analyze the charge distribution around bismuth in sulfides. This map shows the orientation of a lone electron pair.« less
  • The elastic constants of NaZr{sub 2}(PO{sub 4}){sub 3} were computed as a function of pressure through Density Functional Theory calculations. The behavior of elastic constants show that the rhombohedral (R-3c) NaZr{sub 2}(PO{sub 4}){sub 3} becomes unstable above 8 GPa and is driven by softening of C{sub 44} through one of the Born stability criteria. High pressure equation of state and enthalpy show further that the ambient rhombohedral (R-3c)) NaZr{sub 2}(PO{sub 4}){sub 3} transforms first to another rhombohedral (R3) phase and subsequently to LiZr{sub 2}(PO{sub 4}){sub 3}-type orthorhombic phase at pressures above 6 and 8 GPa respectively which are in agreement with recentmore » X-ray diffraction study.« less