DDifferent types of catalysts exert a profound influence on the HPHT synthesis of diamond, as evidenced by various theoretical models and experimental findings:

1. Transition Metals (Fe, Co, Ni, Mn, Cr) – Dual Catalytic & Solvent Roles

Transition metals are the most widely used catalysts in diamond synthesis due to their unique electronic configurations:

Electronic Structure Effects: These metals possess unfilled d-electron shells. They can temporarily capture electrons from carbon atoms to form interstitial solidsolutions and intermediate phases, thereby acting as effective "solvents" that dissolve and activate carbon into sp3 states.

Lowering Activation Energy: The unfilled d-orbitals allow these catalysts to significantly lower the activation energy required for the graphite-to-diamond phase transition, enabling the reaction to occur under far more accessible HPHT conditions.

2. Unique Effects of Nickel (Ni) Catalyst：

Supersaturation-Driven Growth: At 5.5 GPa and 1460°C, the solubility ratio of graphite to diamond in molten nickel is 1.047. When graphite reaches saturation, the solution automatically achieves a 4.7% supersaturation relative to diamond, providing the fundamental thermodynamic driving force for rapid diamond crystallization.

Lattice Matching and Structural Induction: The close-packed {111} planes of nickel match exceptionally well with the graphite net planes and diamond {111} planes. This geometric alignment attracts the 2Pz electrons of graphite, causing the hexagonal planes to wrinkle and align directly into the diamond cubic lattice.

Coordination Covalent Catalysis (MCCM Model): Molten Ni atoms in a d2 valence state can form d2sp3 hybridized octahedral non-localized covalent bonds with surrounding carbon atoms. This specific bonding forces graphite's C6 layers to wrinkle into sp3-hybridized double-layers, after which the metal atoms dissociate.

Carbide Inclusions: Structural analysis of diamonds synthesized using Co and Ni catalysts reveals metastable Co or Ni carbide inclusions as dominant internal defects. These interstitial metal atoms are trapped in the octahedral vacancies of the growing diamond crystal, physically verifying that the transition metals actively participate in both the dissolving and catalytic crystallization stages of growth.

4. Limitations of Non-Transition Metals (e.g., Cu, Pb)

Solvent without Catalysis: While metals like copper (Cu) and lead (Pb) can dissolve carbon at high temperatures, they lack the unfilled $d$-electron shells necessary to catalyze and excite the graphite structure into a tetrahedral configuration. Consequently, they cannot synthesize diamond under HPHT. This limitation strongly refutes the pure "solvent theory" and proves that an effective catalyst must possess both solvent capabilities and catalytic activation properties.