V-V dimerization effects on bulk-sensitive hard x-ray photoemission spectra for Magnéli phase vanadium oxides

Highly bulk-sensitive hard x-ray photoemission has been performed for the Magnéli phase ${\text{V}}_5{\text{O}}_9$ and ${\text{V}}_6{\text{O}}_{11}$, which are composed of ${\text{V}}_2{\text{O}}_3$ and ${\text{VO}}_2$ from chemical point of view. The valence-band spectra near the Fermi level are rather similar between ${\text{V}}_5{\text{O}}_9$ and ${\text{V}}_6{\text{O}}_{11}$ in both metallic and insulating phases. Although the core-level spectra in the metallic phase are similar between ${\text{V}}_5{\text{O}}_9$ and ${\text{V}}_6{\text{O}}_{11}$, the core-level spectra in the insulating phase are noticeably different. Our results are understood by considering the contribution of the V-V dimerization effects, which are strong in ${\text{V}}_6{\text{O}}_{11}$ and negligible in ${\text{V}}_5{\text{O}}_9$. Comparison of the core-level spectra of ${\text{V}}_5{\text{O}}_9$ and ${\text{V}}_6{\text{O}}_{11}$ with those of ${\text{VO}}_2$ has revealed that the mechanism of the metal-insulator transitions for the former two compounds is different from that for ${\text{VO}}_2$.

Figure 1

(Color online). Schematic crystal structure of ${\text{V}}_5{\text{O}}_9$ (left) and ${\text{V}}_6{\text{O}}_{11}$ (right). Top two panels are viewed along the $a_{prut}$. Large open and small filled circles mark vanadium and oxygen atoms, respectively. The hexagonal arrangement of O atoms does not show the real position of the O atoms but only stands for the O octahedron. Bottom two panels are schematic views of two adjacent V chains along the $a_{prut}$ axis.

Figure 2

(Color online). Valence-band spectra of ${\text{V}}_5{\text{O}}_9$ (spectra with open ○ mark) and ${\text{V}}_6{\text{O}}_{11}$ (spectra with filled ● mark). Upper (lower) two spectra are in the metallic: M (insulating: I) phase. The solid lines show the smoothed data.

Figure 3

(Color online). (a) $\text{V}1s$ and (b) $\text{V}2p$ core-level spectra of ${\text{V}}_5{\text{O}}_9$, ${\text{V}}_6{\text{O}}_{11}$, and ${\text{VO}}_2$. Spectra in the metallic (insulating) phase are indicated by full (dashed) lines in both (a) and (b).

Figure 4

(Color online). (a) $\text{V}1s$ and (b) $\text{V}2p_{3/2}$ spectra in the insulating phase of ${\text{V}}_6{\text{O}}_{11}$ (open circles) can be reproduced by the sum of ${\text{V}}_5{\text{O}}_9$ (dotted line) and ${\text{VO}}_2$ (hatched area) data.