Band Jahn-Teller Instability and Formation of Valence Bond Solid in a Mixed-Valent Spinel Oxide ${\mathrm{LiRh}}_2{\mathrm{O}}_4$

We have synthesized a new spinel oxide ${\mathrm{LiRh}}_2{\mathrm{O}}_4$ with a mixed-valent configuration of ${\mathrm{Rh}}^{3+}$ and ${\mathrm{Rh}}^{4+}$. At room temperature, it is a paramagnetic metal, but on cooling, a metal-insulator transition occurs and a valence bond solid state is formed below 170 K. We argue that the formation of valence bond solid is promoted by a band Jahn-Teller transition at 230 K and the resultant confinement of $t_{2g}$ holes within the $xy$ band. The band Jahn-Teller instability is also responsible for the observed enhanced thermoelectric power in the orbital-disordered phase above 230 K.

Figure 1

Powder XRD pattern of ${\mathrm{LiRh}}_2{\mathrm{O}}_4$ at 300 K (a) and the temperature evolution of cubic 311 reflection at 250 K, 190 K, and 150 K (b). The asterisks show reflections of ${\mathrm{Rh}}_2{\mathrm{O}}_3$ impurities. (a) Reflections except for those asterisked could be indexed on the basis of the cubic spinel structure with $a=8.458\AA$. The inset shows the crystal structure of ${\mathrm{LiRh}}_2{\mathrm{O}}_4$ in the cubic phase, spinel structure (left), and a $[01\overline{1}]$ electron diffraction pattern at 100 K (right).

Figure 2

Temperature dependence of resistivity (a), magnetic susceptibility (b), thermoelectric power (c), heat capacity divided by temperature (d), and the lattice constants (e) of ${\mathrm{LiRh}}_2{\mathrm{O}}_4$ polycrystalline sample. (b) The dotted line shows the susceptibility after subtracting the Curie term with a Curie constant of $1.4\times {10}^{-3}\mathrm{emu}/\mathrm{K}\mathrm{molRh}$. Inset: (a) HAXPES spectra near the Fermi level at 250, 210, and 160 K (left), and the resistivity around 170 K transition (right). (b) ${}^6\mathrm{Li}\mathrm{\text{−}}\mathrm{NMR}$ spectra at 180 and 60 K. (c) Themoelectric power up to 800 K. (d) Differential scanning calorimetry curves obtained in heating (lower) and cooling (upper) process. (e) A cubic, orthorhombic, and tetragonal unit cell of ${\mathrm{LiRh}}_2{\mathrm{O}}_4$.

Figure 3

Temperature variation of structural and electronic properties of ${\mathrm{LiRh}}_2{\mathrm{O}}_4$. A proposed model for electronic structure (DOS) evolution are also shown schematically (middle), together with the real space images of corresponding orbital states (lower). In the pictures of electronic structure, thick lines and filled regions show the total DOS and the occupied states of them, respectively. Dotted lines show partial DOS of individual bands.