33 / 2023-04-03 18:53:37

On the high-pressure phase of cold compressed bulk graphite and graphene nanoplatelets

Graphite,X-ray Diffraction,Raman spectroscopy,ab-initio

高压物理与材料科学 > 相变与机制

The versatility of elemental carbon to form structures made up of sp-, sp²-, and sp³-type bonding results in a rich variety of allotropes, both in the bulk, as well as in the nanoscale [1]. The stable three-dimensional phase of carbon at ambient conditions is the hexagonal graphite phase (HG), made up of planar sheets of six-membered sp²-bonded carbon rings with AB-type (Bernal) stacking along the hexagonal c-axis. Application of joint pressure and temperature transforms this phase into the sp³-bearing diamond polymorphs, with profound industrial applications [2].

At room temperature, compression of bulk graphite leads to a new modification with increased electrical resistivity, optical transparency, and high hardness [3-8]. These characteristics indicated that the high-pressure phase contains sp³-type carbon bonding, similar to cubic diamond (CD). Several candidates emerged from the theoretical side as a means of identifying this high-pressure modification, with a monoclinic phase (dubbed M-carbon) proposed to show the best agreement with the available experimental results [9,10]. It should be pointed out, however, that specific experimental observations, such as the absence of a clear sp³-derived Raman signal, as well as the reversibility of the starting HG phase upon decompression, appear to contradict the general consensus of a purely sp³-bearing high-pressure carbon phase under cold compression. Thus, the high-pressure structure of cold compressed graphite still remains an open question.

Similar to the bulk graphite studies, recent investigations on graphene samples of varying layer numbers indicate the direct transformation of the starting HG to a semiconducting phase under compression. This high-pressure graphene modification was inferred to adopt a hexagonal diamond-like (HD) structure. The pressure-induced HG→HD formation at room temperature has been reported for bulk graphite as well, but was eventually dismissed. The HD (lonsdaleite) represents a metastable carbon modification, acting as an "intermediate" phase between HG and CD. The existence of HD as a discrete material, however, has been debated in recent literature. For the sake of completeness, we should also mention that compression of glassy carbon at room temperature has been proposed to yield disordered diamond-like modifications; this suggestion, however, has been also questioned.

In order to explore this matter further, we have investigated the high-pressure behavior of bulk natural graphite and graphite nanoplatelets GNPs at room temperature by means of high-pressure XRD and Raman spectroscopic probes. In both cases, we were able to detect the same pressure-induced structural transition. The measured data are consistent with a metastable modification with an average orthorhombic symmetry, encompassing nevertheless some degree of (local) structural disorder. We have termed the discovered structure as high-pressure orthorhombic graphite (HP-OG). Both our XRD and Raman investigations indicate that HP-OG is composed of a sp²/sp³-type carbon bonding mixture, thus lying in-between the sp²-bearing HG and the sp³-type diamond phases. Such an intermediate carbon configuration has been already considered from a theoretical perspective as a necessary precursor for the formation of diamond from graphite, strikingly assigned to orthorhombic symmetry. Taken together, there might be a strong possibility that our metastable HP-OG phase is actually this predicted "bridging" transient state, linking structurally the sp²-bearing and the sp³-type carbon polymorphs.



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