A new study on the catalytic mechanism of Fe-based alloys in diamond formation by Mossbauer spectroscopy

Mossbauer spectroscopy has been used to systemically study the catalytic me chanism of Fe-based alloys in diamond formation at high temperature-high pr essure (HTHP) for the first time. Mossbauer spectra reveal the magnetic sta te of the 3d electrons of a Fe atom in the Fe-based alloy catalyst during d iamond formation at HTHP. During carburization at lower temperatures than t hat required for diamond formation and diamond formation in the diamond-sta bility region using Fe-based alloys as a catalyst, both the quadrupole spli tting QS and the isomer shift IS change from negative to positive, especial ly reaching a state in which they are zero. It was indicated that the state of the 3d-shell electrons of the iron atom changes greatly during carburiz ation and diamond formation and that the incomplete 3d sub-bands of Fe atom s in the catalyst alloys could be filled up in proper order by electrons of interstitial carbon atoms. During diamond formation, the Impaired 3d-shell electrons of an iron atom in the Fe-based alloy absorb and interact with 2 P(z) electrons of the carbon atoms. There exist a Fe-C bonding and an elect ron charge transfer stage. The 2P(z) electrons of the carbon atoms could be dragged into the metal atoms in the catalyst alloy and would make a transi tion of triangular (sp(2)pi) hybridization of valence electrons to tetrahed ral (sp(3)) hybridization of valence electrons (a transition of sp(2)pi bon ds of graphite to sp(3) bonds of diamond), resulting in a transition of gra phite structure to diamond. Although the conclusion of this study is strict ly applicable only to Fe-based alloy catalysts, it could be considered more general because of the chemical similarities between the transition elemen ts used as solvent catalysts for diamond synthesis.