Large resistivity reduction in mixed-valent ${\mathrm{CsAuBr}}_3$ under pressure

We report on high-pressure $p\le 45$ GPa resistivity measurements on the perovskite-related mixed-valent compound ${\mathrm{CsAuBr}}_3$. The compounds high-pressure resistivity can be classified into three regions: For low pressures ($p<10$ GPa) an insulator to metal transition is observed; between $p=10$ GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; and above $p=14$ GPa a semiconducting state is observed. From this pressure up to the highest pressure of $p=45$ GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than six orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show—by an analysis of the electronic band structure evolution of this material—that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving toward one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.

Figure 1

(a) Crystal structure of ${\mathrm{CsAuBr}}_3$. (b) Raman modes of the gold octahedra. (c) Raman spectra for pressures ranging from $p=2$ GPa to 12 GPa. For higher pressures, no peaks in the Raman spectra were observed. The Raman spectra after the released pressure, at 2 GPa is shown on top, indicating full reversibility. The spectra at 8.9, 10, and 12 GPa are multiplied by a factor of 4 and 20, respectively, for better comparability.

Figure 2

Evolution of the resistivity of ${\mathrm{CsAuBr}}_3$ at 300 K in a pressure range up to $p=45$ GPa. The highlighted region A corresponds to an insulator to metal transition. In region B an electronic gap is reopened. In region C corresponds to a robust semiconducting state at high pressures.

Figure 3

Temperature-dependent resistivity of ${\mathrm{CsAuBr}}_3$ in a temperature range between $T=300$ K and 1.5 K for pressures of $p=5.1$, 6.8, 7.8, 8.5, 10.2, 11.6, 12.6, 18.0, 38.0 GPa. The anomalies in the resistivity are marked with $T_1$ and $T_2$.

Figure 4

Pressure evolution of structural parameters of ${\mathrm{CsAuBr}}_3$, comparing experimental data of reference [13] and theoretical predictions.

Figure 5

Calculated pressure evolution of the density of states at the Fermi level of ${\mathrm{CsAuBr}}_3$.

Figure 6

Orbital weights of the two inequivalent Au ions in the unit cell of ${\mathrm{CsAuBr}}_3$.

Figure 7

Brillouin zone, marked in gray, and the path chosen, marked in green, for the electronic band structure calculations in this work.

Figure 8

Electronic bands structures and densities of states of ${\mathrm{CsAuBr}}_3$ at selected pressures. Panels (a) to (c) are calculated using $I4/mmm$ space group for one ${\mathrm{CsAuBr}}_3$ formula unit and (d) with $Pm\overline{3}m$ space group for one ${\mathrm{CsAuBr}}_3$ formula unit.