The lattice stiffening transition in UO₂single crystals

Abstract

The effective Debye temperatures ([Formula: see text]) of the surface region of UO single crystals, prepared by the hydrothermal synthesis technique, were obtained from temperature-dependent x-ray photoemission in the temperature range of 300 K-623 K. A lattice stiffening transition, characterized by different regions of different effective Debye temperature, 500 ± 59 K below 475 K and 165 ± 21 K above 475 K is identified. A comparison of the temperature dependence of the effective UO Debye temperature, with the changes in the lattice expansion coefficient for UO, support strong lattice-phonon interaction arising from the Jahn-Teller distortion.

The Debye Temperature of a Single Crystal Thorium–Uranium Dioxide Alloy

“…. The room temperature uranium binding energies (also denoted as E F − E ) occur at 391.3 and 380.3 eV for U 4 f 7/2 and U 4 f 5/2 , respectively and are consistent with the reported binding energies of 390.8 and 380.8 eV and 390.9 and 380.1 eV …”

“…These are also observed in our data in Figure a. The presence of these 6.7 eV peaks can be explained by assignment of the U(4 f 13 –5 f 2 5/2 5 f 1 7/2 )O(2 p 5 ) in the final state . In Figure a, the U 4 f peaks have a higher intensity on the tail of the high‐energy side than the Th 4 f peaks, revealing the photoelectrons from the U 4 f distributions are scattered into the lower part of the spectrum due to electron inelastic scattering .…”

“…Furthermore, the temperature dependence of the satellite peaks for U 4 f 5/2 in Figure a are shifted above the primary peak at 6.7 eV until 623 K at which the satellite peaks are at 4 and 10 eV. This is above the previously published UO 2 's lattice stiffening transition at 475 K . Although there are no other literature values in alloys, γ‐UO 3 has two satellite peaks approximately 3.9 and 9.5 eV from the fundamental U 4 f peaks .…”

“…Using the same methods as the present research, the Debye temperature of single crystal ThO 2 has previously been reported from experiment as 268 K and 468 ± 32 K . For comparison, the Debye temperature of single crystal UO 2 has been reported as 500 ± 59 K at temperatures below 475 K and 165 ± 21 K at temperatures greater than 475 K . For comparison, ThO 2 's Debye temperature using experimentally determined elastic constants, was calculated at 297 and 370 K; note these are not single crystal samples .…”

“…The presence of these 6.7 eV peaks can be explained by assignment of the U(4 f 13 –5 f 2 5/2 5 f 1 7/2 )O(2 p 5 ) in the final state . In Figure a, the U 4 f peaks have a higher intensity on the tail of the high‐energy side than the Th 4 f peaks, revealing the photoelectrons from the U 4 f distributions are scattered into the lower part of the spectrum due to electron inelastic scattering . When the temperature increases, as shown in Figures a and 2a, lattice vibrations increase further lowering the photoelectron's energy and shifting the peak.…”