Crystal-field transitions inf-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 ${\mathrm{UO}}_2$ (∼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 Néel 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 ∼100 meV may be measured at small (i.e., ≲5 A${̊}^{\mathrm{-}1}$) values of the momentum transfer.