Direct observation of a pressure-induced metal-insulator transition in LiV${}_2$O${}_4$ by optical studies

The metal-insulator (MI) transition in LiV${}_2$O${}_4$ 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 $2\Delta$ gap is estimated to be $\sim$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 ten- to 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.

Figure 1

Pressure dependence on the reflectivity (a) and the optical conductivity (b) of LiV${}_2$O${}_4$ at 40 K.

Figure 2

Pressure dependence on the reflectivity (a) and the optical conductivity (b) of LiV${}_2$O${}_4$ at 300 K.

Figure 3

Pressure-temperature phase diagram of LiV${}_2$O${}_4$. Three regions (metal—filled triangles; intermediate—filled circles; and insulator—filled stars) with two boundaries (boundary 1 and 2) are determined by the optical response. The HF state is located in the bottom left-hand-side area (shaded) and is estimated from the NMR experiment (Refs. 48 and 49). The characteristic points from the other experiments are also plotted in the figure [EXAFS (Ref. 47)—open circles; XRD (Ref. 43)—open triangles; and resistivity (Ref. 41)—open diamonds).

Figure 4

Phonon softening phenomenon observed in the optical spectra at 11, 13, and 16 GPa.

Figure 5

Kramers-Kronig analysis of the reflectivity spectrum (a) to derive the imaginary and real parts of the dielectric function ${\epsilon }_1$, ${\epsilon }_2$ [(b), (c), respectively] at 40 K and 16 GPa. The estimated ${\omega }_{\mathrm{TO}}$, ${\omega }_{\mathrm{LO}}$, ${\epsilon }_0$, and ${\epsilon }_{\infty }$ from fitting results (dashed lines) are also indicated with experimental results (solid lines) in figures.

Figure 6

Changes of ${\omega }_{\mathrm{TO}}$ as a function of the temperature under 0 (ambient pressure), 8, 13, and 16 GPa.

Figure 7

Pressure dependence on the reflectivity of the fractured (a) and the polished (b) surfaces in MIR region at 40 K. The dashed line drawn at 0.1 eV is a guide for the intensity plot in Fig. 8.

Figure 8

Pressure dependence on the intensity of the reflectivity at $E=0.1$ eV on the fractured and the polished surfaces. See also the caption of Fig. 7.