Direct observation of a pressure-induced metal-insulator transition in LiV2O4 by optical studies

The metal-insulator (MI) transition in LiV2O4 has been studied by optical measurements in infrared regions at low temperatures and under high pressures. At 40 K, the metal phase under ambient pressure changes gradually into the insulating state under pressures above 7 GPa, where the newly opened 2 gap is estimated to be ∼0.4 eV. At pressures higher than 8 GPa, the precursor of the structural phase transition is observed as a redshift of the transverse optical phonon peak in the far-infrared (FIR) region with decreasing temperatures. The behavior of this pressure-induced MI transition is observed not only at low temperatures but also at room temperature. At 300 K, a decrease in conduction carriers with pressure is also detected as a gradual suppression in the Drude response of the FIR region below 0.1 eV, with a tento 20-fold decrease in the optical conductivity. Extensive investigation of the optical response revealed a successive transformation of the electronic state in a region between the metal and the insulator phases in the pressure-temperature phase diagram. The redshift is only observed near the boundary to the insulator phase (boundary 2), where a complete homogeneous structural change appears to occur. On the contrary, an inhomogeneous structural change is realized in the intermediate phase between the metal and insulator phases, between boundaries 1 and 2, accompanied by a topographic localization or a short-range alternation of the valence.