Evidence for oxygen holes due to $d\text{−}p$ rehybridization in thermoelectric ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$

Soft-x-ray photoemission and absorption spectroscopies are employed to investigate the electronic structures of ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$. Similar to the layered cobaltates such as ${\text{Na}}_{1-x}{\text{CoO}}_2$, a valence-band satellite feature (VBS) occurs at higher binding energy to the $\text{O}2p$ band. We find that the VBS resonates at the $\text{O}1s$ edge. Additionally, core absorption shows clear $x$ dependence in the $\text{O}1s$ edge rather than in the $\text{Rh}3p$ edge. These results indicate that the holes in the initial state mainly have $\text{O}2p$ character presumably due to $d\text{−}p$ rehybridizations affected by ${\text{Sr}}^{2+}$ vacancy potentials. The resultant inhomogeneous charge texture may have impact on the thermoelectric transport properties at low $x$.

Figure 1

(Color online) ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$, an analog of ${\text{Na}}_{1-x}{\text{CoO}}_2$. (a) Electronic structure. The band-theoretical DOSs of the cobaltates and the rhodates exhibit $t_{2g}$ and $\text{O}2p$ bands, but the valence-band PES spectra additionally exhibit VBSs. (b) Resistivity and (c) TE power of ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$ as functions of temperature. The crystal structure of ${\text{SrRh}}_2{\text{O}}_4$ is shown in the inset of (b).

Figure 2

(Color online) $\text{Rh}3p\text{−}4d$ resonant PES of ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$ $\left(x=0.22\right)$. (a) Valence-band spectra recorded across the $\text{Rh}3p_{3/2}$ edge. (b) CIS spectra and $\text{Rh}3p_{3/2}$ XAS. CIS spectra are normalized to the intensity at 486.4 eV and have arbitrary offsets. The labels (i–x) on the spectra in (b) correspond to the photon energies indicated by bars on the $\text{Rh}3p_{3/2}$ XAS in (c).

Figure 3

(Color online) Valence-band spectra of ${\text{Sr}}_{1-x}{\text{Rh}}_2{\text{O}}_4$ $\left(x=0.22\right)$ recorded across the $\text{O}1s$ edge. (a) Valence-band spectra recorded in the vicinity of the $\text{O}1s$ edge. Inset shows a schematic of the final state of an $\text{O}1s2p2p$ Auger-electron emission. (b) Difference spectra to the 523.0-eV spectrum. The labels (A–H) on the spectra correspond to the photon energies indicated on the $\text{O}1s$ XAS in the inset in (b). The dotted vertical lines indicate the 11.7 eV VBS, and the bars indicate the $\text{O}1s2p2p$ Auger peaks. The $\text{O}1s2p2p$ Auger peak positions are plotted in the inset in (b) in which the dotted line indicates the constant kinetic energy.

Figure 4

(Color online) (a) $\text{Rh}3p_{3/2}$ and (b) $\text{O}1s$ XAS. The arrows in (b) indicate the change with increasing $x$.