Structural characterization of the hard fullerite phase obtained at $13\mathrm{GPa}$ and $830\mathrm{K}$

A detailed characterization of two samples synthesized at $13\mathrm{GPa}$ and $830\mathrm{K}$ with heating times of $20\min$ and $1\mathrm{h}$, respectively, was performed using x-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The sample heated for $20\min$ was amorphous and exhibited a high hardness of $45\mathrm{GPa}$, while the second sample appeared to be softer and crystalline. A slightly distorted fcc cubic packing was found by XRD and HRTEM. Both samples are proved to consist of unbroken ${\mathrm{C}}_{60}$ molecules. The crystalline sample was also studied by in situ XRD during compression up to $26\mathrm{GPa}$ using synchrotron radiation. The bulk modulus value of $217\mathrm{GPa}$ obtained from compression experiments is in good agreement with the measured hardness of the sample $\left(37\mathrm{GPa}\right)$. The Raman spectroscopy shows an unusual combination of features: some additional peaks compared to untreated ${\mathrm{C}}_{60}$ and no shift for the $A_g\left(2\right)$ mode.

Figure 1

XRD pattern recorded from the samples obtained by heat treatment of ${\mathrm{C}}_{60}$ at $13\mathrm{GPa}$ and $830\mathrm{K}$ for $1\mathrm{h}$ (top, sample 2) and for $20\min$ (bottom, sample 1, $\times 500$). Inset shows XRD pattern recorded previously from the sample of ${\mathrm{C}}_{60}$ at $13\mathrm{GPa}$ and room temperature in diamond anvil cells (Ref. 12). The pattern given in the inset gives good approximation to the state of sample 2 prior heating. The wavelengths of the x-ray beam were $0.3738\mathrm{\AA }$ for the samples 1 and 2, and $0.7\mathrm{\AA }$ for the pattern shown in the inset.

Figure 2

HRTEM image of fcc structured ${\mathrm{C}}_{60}$ with microtwins and stacking faults parallel to (111). The inset shows a SAED pattern with streaks along the ${\left[111\right]}^{*}$ direction.

Figure 3

Raman spectra recorded from the pristine ${\mathrm{C}}_{60}$ (curve 1), sample 1 (curve 2), sample 2 at low laser power (3), and Sample 2 at higher laser power, which resulted in appearance of peaks from pristine ${\mathrm{C}}_{60}$.

Figure 4

Typical load-displacement curves for samples 1 and 2 (heated at $13\mathrm{GPa}$ and $830\mathrm{K}$ for 20 and $60\min$, respectively) compared to pristine ${\mathrm{C}}_{60}$.

Figure 5

In situ XRD patterns recorded for sample 2 during pressurizing up to $26\mathrm{GPa}$ using synchrotron radiation.

Figure 6

Experimental points from in situ compression experiments (unit cell volume vs pressure) fitted with Birch-Murnaghan equation ($K=217$, $V_0=2371$).