Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide

Abstract

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium dioxide (UO2), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO2 and UO2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO2, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational mode pair is detected within the reststrahlen band for both ThO2 and UO2. Elevated temperature measurements were performed for ThO2 in order to study the mechanisms by which the phonon parameters evolve with increased heat. The observed change in the TO resonant frequency is in excellent agreement with previous density functional calculations, which only consider volume expansion of the crystal lattice. This suggests that the temperature-dependent change in the TO frequency is primarily due to volume expansion. The change in the main mode pair’s broadening parameters is nearly linear within the temperature range of this study, which indicates the intrinsic anharmonic scattering (via cubic anharmonicities) as the main decay mechanism.

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

“…The calculated frequencies of the F 1u TO and LO modes at 8.5 and 17.1 THz compare well to single-crystal measurements performed by Knight et al 68 The defects break the symmetry of stoichiometric ThO 2 and result in some of the zone-boundary modes being "folded" to the G-point. This in turn leads to additional IR-and Ramanactive modes in the defective systems, which are oen associated with localised vibrations of atoms around the defect sites.…”

Interband Transitions And Critical Points Of Single Crystal Thoria Compared To Urania

“…Single crystals of UO2 were grown by hydrothermal synthesis [12,13,[16][17][18][19][20][21][22], as described in [12,13,22]. Further hydrothermal growth information is detailed in [16].…”

“…Absorption of UO2 thin films, by optical transmission has also been measured [14], while the optical properties for both ThO2 and UO2 have been previously calculated [15] using the Heyd-Scuseria-Ernzerhof (HSE) functional. The challenges of growing uranium oxide singlecrystals have been addressed with the fabrication of high-quality, single crystal actinide oxide samples using a hydrothermal synthesis growth technique [12,13,[16][17][18][19][20][21][22]. This growth process has produced bulk single crystals of near-stoichiometric UO2 [12,13,17,[20][21][22] and ThO2 [13,18,19,22].…”

“…The challenges of growing uranium oxide singlecrystals have been addressed with the fabrication of high-quality, single crystal actinide oxide samples using a hydrothermal synthesis growth technique [12,13,[16][17][18][19][20][21][22]. This growth process has produced bulk single crystals of near-stoichiometric UO2 [12,13,17,[20][21][22] and ThO2 [13,18,19,22]. These improved single crystals make higher quality assessments of the UO2 and ThO2 optical and semiconductor properties possible, as defect contributions to the complex dielectric function are much suppressed [13].…”

Thermal Energy Transport in Oxide Nuclear Fuel

“…If the melt growth is eschewed because of these assorted shortcomings, the next most common growth techniques dissolve the feedstock under conditions well below the melting point and then crystallize out the feedstock from the solution. Chemical vapor deposition (CVD) or chemical transport (UO2 [228,229,230,231], UxTh1-xO2 [232], and U1-xPuxO2 [233]), flux (CeO2 [234,235,236,237,238,239,240,241], ThO2 [220,223,234,235,236,237,238,242,243,244,245,246,247], UO2 [217,248,249], UxTh1-xO2 [249], and PuO2 [242,250,251,252,253]), and hydrothermal or solvothermal synthesis (CeO2 [254], ThO2 [29,67,179,255,256,257,258,259,260,…”