Structural dynamics of Schottky and Frenkel defects in ThO₂: a density-functional theory study

Abstract

Thorium dioxide (ThO2) is a promising alternative to mixed-oxide nuclear fuels due to its longer fuel cycle and resistance to proliferation. Understanding the thermal properties, in particular the thermal conductivity,...

Structural dynamics of Schottky and Frenkel defects in CeO2: a density-functional theory study

“…The band unfolding methodology [30,31] in Phonopy was used to obtain the phonon dispersions of the defective models projected onto the primitive stoichiometric cell for comparison. This procedure was previously used successfully for ThO 2 [20]. The density functional perturbation theory (DFPT) [32] routine in the VASP code was employed to calculate the Born effective charges and high-frequency dielectric constants required to apply a non-analytical correction to the phonon frequencies.…”

“…Ce pair aligned along the ⟨111⟩ direction with an interstitial-vacancy pair distance of 4.36 Å and is referred to as CeF⟨111⟩. Based on previous literature, the interstitial-vacancy pairs in both types of defect were placed as 2 nd nearest neighbours to prevent spontaneous recombination during optimisation [20,42].…”

“…Three configurations of the Schottky defect were considered with the two V •• O aligned along the ⟨111⟩, ⟨110⟩ and ⟨100⟩ directions with respect to the central V ′ ′ ′ ′ Ce , which we denote S⟨111⟩, S⟨110⟩ and S⟨100⟩ respectively (figure 1(D)). Based on previous literature, the vacancies were positioned as first nearest-neighbours [20]. The distance between the V ′ ′ ′ ′ Ce and V…”

“…Whilst the defect formation energies of Frenkel and Schottky defects have been calculated, their impact on the thermal conductivity has yet to be reported. In the similar fluorite-structured ThO 2 , Frenkel and Schottky defects were shown to broaden the phonon dispersion and reduce the thermal conductivity by up to 90% [20]. In insulators such as ceria, the low frequency acoustic phonons (<12 THz), which have the largest group velocities and longest lifetimes, dictate the magnitude of the heat transport [8,21].…”

Implementation strategies in phonopy and phono3py

“…Interstitial defects can be handled in the same way as the vacancy case by defining corresponding supercell differently. 44 Suppose we have one intersticial defect in the supercell model. It is treated as a usual atom in the primitive cell of the corresponding perfect supercell.…”