Diamond synthesized using iron-carbon compounds shares remarkable similarities with that produced using conventional catalysts (such as NiMnCo alloys) in terms of microscopic morphology, but also exhibits key structural differences:

1. Similarities: Similar Encapsulating Metal Film/Ring Zone

Encapsulating Structure: When synthesizing diamond with conventional NiMnCo catalysts, a metal film several micrometers thick usually encapsulates the diamond particles. Similarly, an extremely resembling iron-rich annular zone forms around diamond particles synthesized from iron-carbon compounds. After acid etching, the iron-rich ring dissolves, leaving a distinct concave annular band.

2. Differences: Outer Ring Composition and Phase Transformation Pathway

Different Outer Ring Compositions:

Conventional Catalysts: The outer side of the encapsulating metal film is typically graphite (or recrystallized graphite).

Iron-Carbon Compounds: The outer side of the iron-rich annular zone is a carbon-rich annular zone, which resists acid etching and stands out distinctly under a scanning electron microscope (SEM).

Different Carbon Source States and Phase Transformation Paths:

Conventional Catalysts: The system uses external solid graphite as the carbon source. Graphite dissolves in and diffuses through the molten metal film before transforming into diamond.

Iron-Carbon Compounds: The raw material contains no free graphite; the iron-carbon compound serves as both carbon source and catalyst. Under HPHT conditions, supersaturated carbon is released and directly aggregates to form diamond while generating a localized carbon-rich melt around it. Upon cooling, segregation occurs: part of the carbon continues growing onto the diamond, while another portion precipitates ring-wise on the outer perimeter of the iron-rich ring zone, forming the carbon-rich ring zone.

3. Insights of Micro-Morphological Comparisons into Growth Mechanisms

Previously, there was debate regarding the origin of "recrystallized graphite" on the outer side of the metal film in conventional catalysts: one view held that it transformed from the outer graphite under HPHT conditions, while another suggested it precipitated from dissolved carbon in the catalyst during cooling.

In the iron-carbon compound experiment with no external graphite involved, the presence of the outer carbon-rich ring zone can only be explained as precipitation from the localized carbon-rich melt during cooling. Therefore, by comparing their microscopic morphologies, researchers proposed that "recrystallized graphite forms during the cooling process of the metal film," shedding light on the cooling phase transformation mechanism in diamond synthesis.