High pressure behaviour of uranium dicarbide (UC{sub 2}): Abinitio 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 abinitio electronic band structure calculation method. The abinitio 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 »
 Authors:
 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 10100; SPECIFIC HEAT; STABILITY; THERMAL EXPANSION; URANIUM; URANIUM CARBIDES
Citation Formats
Sahoo, B. D., Email: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., and Kaushik, T. C. High pressure behaviour of uranium dicarbide (UC{sub 2}): Abinitio study. United States: N. p., 2016.
Web. doi:10.1063/1.4961497.
Sahoo, B. D., Email: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., & Kaushik, T. C. High pressure behaviour of uranium dicarbide (UC{sub 2}): Abinitio study. United States. doi:10.1063/1.4961497.
Sahoo, B. D., Email: bdsahoo@barc.gov.in, Mukherjee, D., Joshi, K. D., and Kaushik, T. C. 2016.
"High pressure behaviour of uranium dicarbide (UC{sub 2}): Abinitio study". United States.
doi:10.1063/1.4961497.
@article{osti_22598891,
title = {High pressure behaviour of uranium dicarbide (UC{sub 2}): Abinitio study},
author = {Sahoo, B. D., Email: 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 abinitio electronic band structure calculation method. The abinitio 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 thermophysical 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 = 2016,
month = 8
}

Dissociation Pressure of Uranium Dicarbide
Vapor transport of uranium from UC _{1.86} in a graphite crucible was measured from 1930 to 2365 deg K by the Knudsen technique. In order to treat the data, unusual assumptions were made concerning the thermal properties of carbides which hydrolyze to yield hydrocarbons. The gaseous uranium species is assumed to be monatomic. With the aid of these assumptions, the data were treated by the thirdlaw as well as by the secondlaw method. Agreement between the two is considered good, and the DELTA H _{298} deg obtained for the process UC _{1.86}(s) = U(g) + 1.86 C (graphite) is 140.15more »