Raman spectrum characteristic and structural evolution of high rank coals-cryptocrystalline graphite

Raman spectroscopy can reflect the order degree and structural defects of carbon materials, and can be used to characterize the structural changes during the evolution of high rank coal-cryptocrystalline graphite, and analyze the relationship between Raman parameters of coal with different deformation and metamorphic degrees and the spacing of surface network(d₀₀₂). The results showed that: (1) there was stepped variation between the G peak position and d₀₀₂, but the graphite and high rank coal could be distinguished well. The S₂ peak, the full width at half maximum of D₁ and G had a good linear relationship with d_002; (2) the peak difference between D₁ and G and the full width at half maximum ratio decreased with the decrease of d₀₀₂; the peak difference between S₂ and S₄ increased first and then decreased with the decrease of d₀₀₂, while the intensity ratio and area ratio increased gradually; (3) the relationship of interlayer spacing(d₀₀₂) and Raman parameters figure shows the structure of two obvious evolutionary step, namely anthracite to ultra-anthracite (R_max>6.5 %, P(D₁-G)<235 cm^-1, P(S₂-S₄)> 525 cm^-1 and half peak width ratio dropped significantly, L_a/L_c reduce 2-3 times), semi-graphite to graphite evolutionary stages (P(D₁-G), I_D1/I_G, A_D1/A_G significantly reduced; L_a, L_c increase rapidly). The Raman spectral characteristics of coal structure at different evolution stages can be well reflected by taking d₀₀₂ as the scale.