Resonant x-ray spectroscopy of uranium intermetallics at the $M_{4,5}$ edges of uranium

We present resonant x-ray emission spectroscopic data from the uranium intermetallics ${\mathrm{UPd}}_3$, USb, ${\mathrm{USn}}_3$, and ${\text{URu}}_2{\text{Si}}_2$ at the $M_{4,5}$ edges of uranium, and we compare the data to those from the well-localized $5f^2$ semiconductor ${\mathrm{UO}}_2$. The technique is especially sensitive to any oxidation of the surface, and this was found on the USb sample, thus preventing a good comparison with a material known to be $5f^3$. We have found a small energy shift between ${\mathrm{UO}}_2$ and ${\mathrm{UPd}}_3$, both known to have localized $5f^2$ configurations, which we ascribe to the effect of conduction electrons in ${\mathrm{UPd}}_3$. The spectra from ${\mathrm{UPd}}_3$ and ${\text{URu}}_2{\text{Si}}_2$ are similar, strongly suggesting a predominant $5f^2$ configuration for ${\text{URu}}_2{\text{Si}}_2$. The valence-band resonant inelastic x-ray scattering provides information on the U $P_3$ transitions (at about 18 eV) between the U $5f$ and U $6p$ states, as well as transitions of between 3 and 7 eV from the valence band into the unoccupied $5f$ states. These transitions are primarily involving mixed ligand states (O $2p$ or Pd,Ru $4d$) and U $5f$ states. Calculations are able to reproduce both of these low-energy transitions reasonably well.

Figure 1

Spectroscopic scheme for RXES experiments at the actinide $M_{4,5}$ edges. Transition strengths are indicated as $S$, strong; $M$, medium; and $F$, forbidden. The energy values here correspond to the NIST tables [41] and may not exactly correspond to values in the following figures due to small calibration errors. The diagram takes into account the 10 electrons of the $3d$ shell being distributed as $3d_{3/2}^4$ and $3d_{5/2}^6$ and the 14 electrons of the $4f$ shell distributed as $4f_{5/2}^6$ and $4f_{7/2}^8$.

Figure 2

High-energy resolution (HERFD) data taken at (a) $M_4$ and (b) $M_5$ edges recorded with x-ray emission spectrometer for ${\text{URu}}_2{\text{Si}}_2$ and compared to ${\mathrm{UO}}_2$, ${\mathrm{UPd}}_3$, USb, and ${\mathrm{USn}}_3$ [only (a)] reference systems.

Figure 3

Post-edge features of the HERFD spectra at the (a) U $M_4$ edge and (b) U $M_5$ edge of ${\mathrm{UO}}_2$, ${\mathrm{UPd}}_3$, USb, and ${\mathrm{USn}}_3$ [only (a)] and ${\text{URu}}_2{\text{Si}}_2$ recorded with the x-ray emission spectrometer set to the maximum of the (a) $M_{\beta }$ and (b) $M_{\alpha }$ emission lines, respectively.

Figure 4

(a) Experimental (solid lines) and reproduced (open points) U $M_4$ HERFD spectra of ${\text{URu}}_2{\text{Si}}_2$ compared to the ${\mathrm{UPd}}_3$ (taken as $5f^2$) and ${\mathrm{USn}}_3$ (taken as $5f^3$).

Figure 5

Full data from RXES experiments at $M_4$ (left) and $M_5$ (right) from ${\text{URu}}_2{\text{Si}}_2$.

Figure 6

Intensity, normalized to the incident beam intensity, as a function of energy transfer for fixed incident energies just below the $M_5$ edge for four compounds.

Figure 7

Valence-band spectra of ${\text{URu}}_2{\text{Si}}_2$, ${\mathrm{UPd}}_3$, and ${\mathrm{UO}}_2$ around the incident energy of the U $M_5$ edge. The experimental energy resolution is $\sim 1\mathrm{eV}$; see Sec. 2. Plots (a), (b), and (c) are the experimental data. Plots (d), (e), and (f) are vertical integrations of the experimental data (i.e., as a function of energy transfer). The lower plots (g), (h), and (i) are calculations of the same quantities as discussed in the text and represented by the density of states shown in Fig. 8.

Figure 8

Partial density of states of (a) ${\text{URu}}_2{\text{Si}}_2$, (b) ${\mathrm{UPd}}_3$, and (c) ${\mathrm{UO}}_2$ obtained from feff calculations. The most intense peaks are reduced for clarity (the ratio is indicated on the right side of each figure). The Fermi level (dashed line) is situated at 0 eV.