On the Raman Spectrum of Plutonium dioxide: vibrational and crystal electric field modes

TL;DR

This study combines experimental and theoretical approaches to analyze the Raman spectrum of plutonium dioxide, identifying specific electronic and vibrational modes and proposing a Raman fingerprint for its crystal structure.

Contribution

It provides a detailed assignment of high-energy Raman modes to crystal field transitions and vibrational modes, clarifying previous controversies.

Findings

High-energy modes at 2120 and 2625 cm$^{-1}$ are linked to crystal field transitions.

The T$_{2g}$ PuO stretching mode at 478 cm$^{-1}$ is confirmed as a vibrational fingerprint.

Temperature dependence suggests an electronic origin for the high-energy modes.

Abstract

The Raman spectrum of plutonium dioxide is studied both experimentally and theoretically. Particular attention has been devoted to the identification of high-energy modes at $2120$ cm$^{-1}$ and $2625$ cm$^{-1}$, the attribution of which has so far been controversial. The temperature dependence of both modes suggests an electronic origin. Crystal Field (CF) calculations reported in this work shows that these two modes can be respectively assigned to the ${\Gamma}_{1}$ $\rightarrow$ ${\Gamma}_{5}$ and ${\Gamma}_{1}$ $\rightarrow $ ${\Gamma}_{3}$ CF transitions within the $^{5}I_{4}$ manifold. These two modes, together with the only vibrational line foreseen by the group theory for the Fm${-3}$m PuO$_{2}$ symmetry the T$_{2g}$ PuO stretching mode observed at $478$ cm$^{-1}$ can thus be used as Raman fingerprint of fcc plutonium dioxide.