<p>Metastable carbides and chalcogenides are attractive candidates for wide and promising applications. However, their inherent instability leads to synthetic difficulty and poor durability. Thus, the development of facile strategies for the controllable synthesis and stabilization of metastable carbides is still a great challenge. Here, taking metastable <i>ɛ</i>-Fe₂C as a case study, potassium ions (K⁺) are theoretically predicted and experimentally reported to control the synthesis of metastable <i>ɛ</i>-Fe₂C from an Fe₂N precursor by increasing the surface carbon chemical potential (<i>μ</i>_C). The controllable synthesis and improved stability are attributed to the better-matched denitriding and carburizing rates and the impeded spillover of carbon atoms in metastable <i>ɛ</i>-Fe₂C with high carbon contents due to the enhanced surface <i>μ</i>_C. In addition, this strategy is suitable for synthesizing metastable <i>γ</i>’-MoC, MoN, 1<i>T</i>-MoS₂, 1<i>T</i>-MoSe₂, 1<i>T</i>-MoSe_2<i>x</i>Te_{2(1−<i>x</i>)}, and 1<i>T</i>-Mo₁₋_<i>x</i>W_<i>x</i>Se₂, highlighting the universality of the methodology. Impressively, gram-level scalable metastable <i>ɛ</i>-Fe₂C remains stable for more than 398 days in air. Furthermore, <i>ɛ</i>-Fe₂C exhibits remarkable olefin selectivity and durability for more than 36 h of continuous testing. This work not only demonstrates a facile, easily scalable, and general strategy for accessing various metastable carbides and chalcogenides but also addresses the synthetic difficulty and poor durability challenge of metastable materials.</p>