Local structure and paramagnetic properties of the nanostructured carbonaceous material shungite

Abstract

Using a scanning electron microscopy, elemental analysis, electron paramagnetic resonance, and Raman scattering methods, two types of the shungite materials (Sh-II from Zazhogino deposit and shungite from a commercial filter (ShF)), with different carbon content and porosity, are studied in this work. It was established by scanning electron microscopy data that the structure of the shungite samples is formed by a micron-size agglomeration of carbon and silicon dioxide clusters. It is found from the Raman data that carbon fraction is formed from sp2-hybridized clusters, size of which increases from 9 up to 12 nm after annealing of the samples. High conductivity of shungite is found to belong to the carbon nanoclusters of different sizes. Big clusters give the conduction electron spin resonance signal with a Dysonian line shape with variable g-factor and line width. The careful search of the nature of two other narrow electron paramagnetic resonance signals in shungite, which used to be prescribed to fullerene-like molecules, is fulfilled. Here, it is shown that the oxygen-deficient E'γ centers are responsible for these signals. A strong correlation is revealed between the concentration of Е'γ centers and the line width of conduction electron spin resonance signal, which occurs under annealing process of the samples at T = 570 K. The correlation reasons are a spin-spin coupling between two spin subsystems and time dependent of the Е'γ concentration during annealing process.