A computational study on the superionic behaviour of ThO₂

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Title: A computational study on the superionic behaviour of ThO₂
Author(s): Ghosh, PS
Arya, A
Dey, GK
Kuganathan, N
Grimes, RW
Item Type: Journal Article
Abstract: This study reports the density functional theory (DFT) and classical molecular dynamics (MD) study of the lattice dynamical, mechanical and anionic transport behaviours of ThO2 in the superionic state. DFT calculations of phonon frequencies were performed at different levels of approximation as a function of isotropic dilation (ε) in the lattice parameter. With the expansion of the lattice parameter, there is a softening of B1u and Eu phonon modes at the X symmetry point of the Brillouin zone. As a result of the nonlinear decrease at the X point, the B1u and Eu phonon modes cross each other at ε = 0.03, which is associated with a sharp increase in the narrow peak of the phonon density of states, signifying a higher occupation and hence a higher coupling of these modes at high temperatures. The mode crossing also indicates anionic conductivity in the 〈001〉 direction leading to occupation of interstitial sites. Moreover, MD and nudged elastic band calculated diffusion barriers indicate that 〈001〉 is the easy direction for anion migration in the normal and superionic states. With a further increase in the lattice parameter, the B1u mode continues to soften and becomes imaginary at a strain (ε) of 0.036 corresponding to a temperature of 3430 K. The calculated temperature variation of single crystal elastic constants shows that the fluorite phase of ThO2 remains elastically stable up to the superionic regime, though the B1u phonon mode is imaginary in that state. This leads to anionic disorder at elevated temperatures. Tracking of anion positions in the superionic state as a function of time in MD simulations suggests a hopping model in which the oxygen ions migrate from one tetrahedral site to another via octahedral interstitial sites.
Publication Date: 24-Oct-2016
Date of Acceptance: 21-Oct-2016
URI: http://hdl.handle.net/10044/1/57585
DOI: https://dx.doi.org/10.1039/c6cp05794b
ISSN: 1463-9076
Publisher: Royal Society of Chemistry
Start Page: 31494
End Page: 31504
Journal / Book Title: Physical Chemistry Chemical Physics
Volume: 18
Issue: 46
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/K00817X/1
Copyright Statement: © the Owner Societies 2016.
Keywords: Science & Technology
Physical Sciences
Chemistry, Physical
Physics, Atomic, Molecular & Chemical
Chemistry
Physics
NEUTRON-SCATTERING TECHNIQUES
FLUORITE-TYPE CRYSTALS
MINIMUM ENERGY PATHS
ELASTIC BAND METHOD
MOLECULAR-DYNAMICS
URANIUM-DIOXIDE
MIXED OXIDES
THERMAL-CONDUCTIVITY
THORIUM-DIOXIDE
TRANSPORT-PROPERTIES
Science & Technology
Physical Sciences
Chemistry, Physical
Physics, Atomic, Molecular & Chemical
Chemistry
Physics
NEUTRON-SCATTERING TECHNIQUES
FLUORITE-TYPE CRYSTALS
MINIMUM ENERGY PATHS
ELASTIC BAND METHOD
MOLECULAR-DYNAMICS
URANIUM-DIOXIDE
MIXED OXIDES
THERMAL-CONDUCTIVITY
THORIUM-DIOXIDE
TRANSPORT-PROPERTIES
02 Physical Sciences
03 Chemical Sciences
Chemical Physics
Publication Status: Published
Appears in Collections:Faculty of Engineering
Materials
Faculty of Natural Sciences



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