Photoinduced phase transitions (PIPTs) are typically described within Landau theory for group-subgroup systems. However, transitions between phases with unrelated symmetries—such as the diamond-to-graphite transformation from the cubic Fd-3m space group to the hexagonal $\mathrm{P}{6}_3/\mathrm{mmc}$ space group—lie beyond its scope. Here, we uncover a two-step nucleation mechanism in carbon: the system first passes through a metastable intermediate, which evolves into graphite under weak excitation or into an amorphous state under strong excitation. Real-time mapping of carriers and bond distortions reveals that directional redistribution of photoexcited carriers generates atomic driving forces, breaking C-C bonds along specific directions and steering the lattice toward graphite. In contrast, Si and Ge lack stable π bonds, leading only to amorphization under the same photoexcitation. These findings not only establish a unified framework for understanding PIPT between distinct symmetry groups but also resolve longstanding questions about the distinct phase transition pathways in diamond-based materials.

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• Phase transitions
• Ultrafast phenomena
• Carbon-based materials
• Semiconductors
• Time-dependent DFT