Uncovering the Phase and Surface Stability of New Aluminoferrite C₄AM (M = Sc-Zn): Ab Initio Calculations

Securing heavy metals within cement clinker phases is a sustainable strategy for solid waste upcycling, yet the atomic-scale mechanisms governing the stability and surface properties of these phases remain elusive. This study systematically investigates the phase stability and surface properties of transition metal-bearing tetracalcium aluminoferrite solid solutions Ca₂AlMO₅ (C₄AM, M = Sc-Zn) using DFT + U calculations. Electronic strong correlation proves essential for correctly describing the insulating behavior and spin states of these minerals. Notably, a spin-state transition of Fe³⁺ and Co³⁺ in the octahedral field induces lattice contraction and reverses the energetic stability between I-type and P-type unit cells. Thermodynamic analysis reveals that while all C₄AM phases possess negative formation energies, only Cr, Mn, Fe, Ni, and Co are absolutely stable against decomposition into their respective binary oxides. Unstable systems exemplified by Cu and Zn are destabilized by substantial antibonding orbital occupancy at the E_F. Furthermore, Mn and Cr exhibit anomalous surface anisotropy, preferring cleavage along the (110) plane over the (001) plane to release Jahn-Teller strain. These findings provide critical theoretical insights for the long-term sequestration of heavy metals in cementitious materials.