sp2-to-sp3 transitions in graphite during cold-compression

Pressure-induced sp2-to-sp3 transitions in graphite have been studied for decades by experiments and simulations. In general, pressures of 15-18 GPa are needed to initiate structural transitions in graphite at room temperature, and the high-pressure phases are usually unquenchable, as evidenced by in situ resistivity and optical transmittance measurements, X-ray diffraction (XRD), and inelastic X-ray scattering (IXS). However, our in situ Raman results show that the onset transition pressure can be as low as 9.7 GPa when using the methanol-ethanol-water (MEW) mixture as the pressure-transmitting medium (PTM), indicated by an additional GD Raman peak caused by the sp3 bonding between adjacent graphite layers. Moreover, using a combination of XRD, Raman, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), we show that a small amount of sp3 bonds associated with a unique feature of cross stacking are present in the recovered samples. Our findings will be useful to understand the intricate structural transitions in graphite-like materials under compression.