Oxides flex their bonds

The interplay between the electronic properties and atoms of a crystal is the origin of many fascinating phenomena such as superconductivity. Physicists from the RIKEN Advanced Science Institute, Wako, the University of Tokyo and Osaka University have now discovered another intriguing phenomenon in the newly synthesized oxide compound LiRh2O4, which could lead to a more efficient generation of electricity from heat.

LiRh2O4 belongs to one of the most common families of oxides, the spinels. Spinels are an attractive playground for physicists: the unique geometry of their atomic lattice can make a mutually convenient arrangement of electrons and ions difficult. At room temperature, the rhodium ions in LiRh2O4 are forced to assume a state of mixed valency, Rh3.5+, whereas the electrons are distributed indiscriminately between the different orbital electronic states.

Led by Hidenori Takagi, the researchers studied how this uneasy arrangement in LiRh2O4 plays out at lower temperatures1, and they have discovered several electronic and structural rearrangements occurring at different temperatures. At 230 K (-43.15 °C), the crystal suddenly expands in one direction and contracts in another, a structural change attributed to the ‘Jahn-Teller effect’. A common occurrence in oxides, this effect explains how the crystal distorts itself to gain energy by lifting the equivalence between the different orbital electronic states, in this case favoring the yz and zx orbitals over the xy ones.

Surprisingly, at the temperature of 170 K (-103.15 °C) the electrical resistivity suddenly increases, and the material changes from a metal to an insulator. This transition indicates that the Rh3.5+ ions have separated into Rh3+ and Rh4+ ions. Intriguingly, it is the Jahn-Teller effect that dictates this transition because it is the electrons from the Rh4+ ions occupying the xy orbitals that are exposed by the crystal distortion. “The Jahn-Teller effect really is the master of the physics here,” notes Takagi.

The implications of this discovery may extend well beyond the interest of physicists because, owing to the indiscriminateness of electrons, the thermoelectric power of LiRh2O4 is enhanced dramatically when the Jahn-Teller effects are about to occur. A large figure for thermoelectric power is the key to efficient generation of electricity from heat. “Our study provides an important clue for the exploration of high-performance thermoelectrics and therefore bridges basic and applied physics,” says Takagi. The restructuring seen in this spinel compound may well prove an important template for more efficient electricity generation.

Reference

1. Okamoto, Y., Niitaka, S., Uchida, M., Waki, T., Takigawa, M., Nakatsu, Y. , Sekiyama, A., Suga, S., Arita, R. & Takagi, H. Band Jahn-Teller instability and formation of valence bond solid in a mxed-valent spinel oxide LiRh2O4. Physical Review Letters 101, 086404 (2008).

The corresponding author for this highlight is based at the RIKEN Magnetic Materials Laboratory