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

Abstract

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 G_D 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.