Crystal-field transitions in<i>f</i>-electron oxides

Abstract

Neutron inelastic scattering has been used to measure for the first time the ground- to excited-state crystal-field transitions in ${\mathrm{PrO}}_{2}$ (130 meV), ${\mathrm{BaPrO}}_{3}$ (255 meV), and ${\mathrm{UO}}_{2}$ (\ensuremath{\sim}160 meV). Details of these neutron experiments using the epithermal neutrons from the Argonne National Laboratory spallation source are given. From the observed transitions the following values of ${V}_{4}$=${A}_{4}$〈${r}^{4}$〉 are deduced: ${\mathrm{PrO}}_{2}$ (-66 meV), ${\mathrm{BaPrO}}_{3}$ (119 meV), and ${\mathrm{UO}}_{2}$ (-385 meV). Comparisons are made with ${V}_{4}$ values deduced for metallic systems and those determined by optical techniques for dilute lanthanides in transparent hosts. In the case of ${\mathrm{UO}}_{2}$, two peaks are seen, one at 155 meV and the other at 172 meV. This structure exists both below and above the N\'eel temperature ${T}_{N}$ (30.8 K) and is discussed in terms of mechanisms that might exist in ${\mathrm{UO}}_{2}$. Several further neutron experiments are suggested now that energy transfers above \ensuremath{\sim}100 meV may be measured at small (i.e., \ensuremath{\lesssim}5 A${\r{}}^{\mathrm{\ensuremath{-}}1}$) values of the momentum transfer.