Infrared-active Phonon Modes in Single-crystal Thorium Dioxide and Uranium Dioxide
Document Type
Article
Publication Date
3-2020
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.
Source Publication
Journal of Applied Physics
Recommended Citation
Knight, S., Korlacki, R., Dugan, C., Petrosky, J. C., Mock, A., Dowben, P. A., Matthew Mann, J., Kimani, M. M., & Schubert, M. (2020). Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide. Journal of Applied Physics, 127(12), 125103. https://doi.org/10.1063/1.5143724
Comments
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Funding notes: This work was supported in part by the National Science Foundation (NSF) under Award No. DMR 1420645 (Nebraska Materials Research Science and Engineering Center) and under Award No. DMR 1808715. This work was supported in part by the Air Force Office of Scientific Research (AFOSR) under Award No. FA9550-18-1-0360 and by the Defense Threat Reduction Agency (Grant No. HDTRA1-14-1-0041) and the Domestic Nuclear Detection Office of the Department of Homeland Security (Grant No. HSHQDC14X00089).