Reversible pressure-driven nanoscale phase separation in ${\text{Rb}}_4{\text{C}}_{60}$

We present Raman spectra for ${\text{Rb}}_4{\text{C}}_{60}$ at pressures up to 13 GPa. Above 1 GPa we observe a phase separation of ${\text{Rb}}_4{\text{C}}_{60}$ into ${\text{Rb}}_6{\text{C}}_{60}$ and ${\text{Rb}}_3{\text{C}}_{60}$, identified by a splitting of the $A_g\left(2\right)$ mode and the appearance of characteristic Raman modes of the daughter phases. Remarkably, the phase separation is completely reversible and the sample returns to a homogeneous ${\text{Rb}}_4{\text{C}}_{60}$ phase after pressure release. The reversibility of the phase separation suggests that the structural change is associated with small displacements of Rb ions in the unit cell, giving local differences in the charge state of the ${\text{C}}_{60}$ molecules. The pressure dependence of the Raman shifts agrees well with earlier measurements. Our observation of a reversible phase separation explains several peculiar results reported earlier for ${\text{Rb}}_4{\text{C}}_{60}$.

Figure 1

(Color online) Raman spectra of a ${\text{Rb}}_4{\text{C}}_{60}$ sample at the pressures indicated. Linear backgrounds have been removed for clarity. The break at $1300–1350{\text{cm}}^{-1}$ is inserted to remove the Raman peak of diamond at $1333{\text{cm}}^{-1}$.

Figure 2

(Color online) (a) Raman shifts of the $A_g\left(2\right)$ modes as a function of pressure. Inset shows the intensity ratio of the fitted $A_g\left(2\right)$ peaks from ${\text{Rb}}_4{\text{C}}_{60}$ and ${\text{Rb}}_6{\text{C}}_{60}$. (b) Raman shifts of low-frequency modes as functions of pressure [corresponding pressure dependences for the ${\text{H}}_g\left(7\right)$ and ${\text{H}}_g\left(8\right)$ modes are 3.8 and $3.5{\text{cm}}^{-1}/\text{GPa}$, respectively]. In both figures, dots indicate ${\text{Rb}}_6{\text{C}}_{60}$ and triangles indicate “${\text{Rb}}_4{\text{C}}_{60}$.”

Figure 3

The $A_g\left(2\right)$ Raman mode for ${\text{Rb}}_4{\text{C}}_{60}$ with (left) increasing and (right) decreasing pressure; the corresponding pressure values are given in the figure.

Figure 4

(Color online) Raman spectra of our samples from $200–850{\text{cm}}^{-1}$ at the pressure indicated. Stars, triangles, and circles indicate assignment of modes to ${\text{Rb}}_6{\text{C}}_{60}$, ${\text{Rb}}_4{\text{C}}_{60}$, and ${\text{Rb}}_3{\text{C}}_{60}$, respectively.