Infrared dielectric dispersion and lattice dynamics of uranium dioxide and thorium dioxide

Abstract

Measurements at room temperature of the infrared reflectivity of single crystals of ThO2 and UO2 have been carried out and analyzed by Kramers-Kronig relations. A single strong resonance is found in each case with ωTO=279(±2) cm-1, ωLO=568(±4) cm-1, ε0-n2=15.5 for ThO2; ωTO=278(±2) cm-1, ωLO=556(±4) cm-1, ε0-n2=17.7 for UO2 (TO=transverse optic; LO=longitudinal optic). The best classical dispersionformula fit to the reflectivity yields the following parameters: for ThO2, ω0=282.7 cm-1, ε0-n2=14.85, γ̄=16.2 cm-1; for UO2, ω0=283.2 cm-1, ε0-n2=15.80, γ̄=18.5 cm-1. The low-frequency (0.3-Mc/sec) dielectric constant of ThO2 was measured to be 18.9±0.4. For UO2 the mode frequencies are in excellent agreement with those obtained by neutron spectroscopy, and can be reconciled with existing infrared absorption data by taking into account particle-size corrections which can shift the polar mode frequency by a factor ∼(ε0+2n2+2)12∼1.8 for small spherical samples. The above data are used to evaluate (Szigeti-type) effective charges (Z′Th+4=2.33, Z′U+4=2.42) and to discuss several approximate relations between the long-wave elastic and optical properties of the fluorite (CaF2-type) lattice. Absorption bands were measured in the two-phonon-combination region in UO2 and their assignment is discussed. © 1966 The American Physical Society.