Hard x-ray photoelectron spectroscopy of the metal-insulator transition in LiRh${}_2$O${}_4$

We have applied hard x-ray photoelectron spectroscopy to LiRh${}_2$O${}_4$ to study its bulk electronic structures. LiRh${}_2$O${}_4$ shows the metal-insulator transition (MIT) at 170 K when the temperature is decreased via an intermediate phase, which appears below 230 K and is ascribed to the band Jahn-Teller instability. Clear spectral changes are observed across the MIT for both the valence band near the Fermi level ($E_F$) and the Rh 3$d$ core state. Their slight spectral changes are also observed across 230 K, induced by the orbital ordering and the slight change of the screening by conduction electrons. The linear polarization dependence of the spectra was measured for the O 2$s$ and valence-band states. The contribution of the Rh 5$s$ state hybridized with the O 2$p$ states in the valence band is semiquantitatively evaluated.

Figure 1

Valence-band HAXPES spectra of LiRh${}_2$O${}_4$ taken at $h$ν $=$ 8180 eV and 250 K. $P_L=1.0$ and $P_L=-0.82$ spectra were obtained by horizontally and vertically polarized x rays, respectively. The $P_L=-1.0$ spectrum is derived from the two spectra for $P_L=1.0$ and $P_L=-0.82[M1]$.

Figure 2

Horizontally polarized ($P_L=1.0$) and vertically polarized ($P_L=-1.0$) valence-band HAXPES spectra of LiRh${}_2$O${}_4$ taken at 250 K.

Figure 3

(a) Valence-band HAXPES spectra of LiRh${}_2$O${}_4$ excited by the horizontally polarized light ($P_L=1.0$) taken at 250, 210, and 160 K. The spectra are normalized by the integrated intensity up to 3.5 eV. (b) The same spectra expanded up to 0.5 eV.

Figure 4

(a) Rh 3$d$ HAXPES spectra ($P_L=1.0$) of LiRh${}_2$O${}_4$ at 250 K. (b) Rh $3d_{5/2}$ HAXPES spectra ($P_L=1.0$) taken at 250, 210, and 160 K.

Figure 5

Rh $3d_{5/2}$ HAXPES data (open circles) of LiRh${}_2$O${}_4$ ($P_L=1.0$), at 250, 210, and 160 K. Lower curves show the individual components (solid line with $+$: Rh${}^{3+}$ component; dotted line: Rh${}^{4+}$ component; solid line: well-screened component), whose sum is given by the solid curves through the experimental data.