Discovery of Ge-based o-MAX phases with robust out-of-plane Ti-Cr chemical ordering

The discovery of out-of-plane ordered (o-MAX) phases has been largely confined to Al-containing systems, leaving a critical gap in understanding chemical ordering mechanisms across other A-element families. Meanwhile, the 413-type Ge-based MAX phases in bulk form are still lacking. Here, we report the first synthesis of bulk Ge-based 413-type MAX phases: Cr 2 Ti 2 GeC 3 , (Cr 1/3 Ti 1/3 Mo 1/3 ) 4 GeC 3 , and (CrTiVMo)GeC 3 , thereby expanding both structural complexity and chemical ordering paradigms in nanolaminate ceramics, and introducing the second series of o-MAX phases. Through combined Rietveld refinement and atomic-resolution elemental mapping, we demonstrate robust Ti-Cr out-of-plane chemical ordering in these phases, with Ti preferentially occupying the inner metal layers adjacent to Ge atoms and Cr populating the outer layers. This chemical ordering pattern is preserved even in quaternary metal compositions. In contrast, the introduced Mo and V are distributed almost randomly at both sites. First-principles calculations demonstrated the thermodynamic stability and electronic structure, and quantified the mechanical properties of Ge-based 413 phases, revealing enhanced mechanical moduli and tunable ductility through V/Mo incorporation. This work expands the o-MAX family beyond Al-based systems, establishing Ge-containing high- n phases as a platform for designing chemically ordered nanolaminates with tailored stiffness-ductility balance.