Computational Screening of Chalcogen-Terminated Inherent Multilayer MXenes and M₂AX Precursors

Sulfur-terminated single sheet (ss-)MXene was recently achieved by delamination of multilayered van der Waals bonded (vdW)-MXenes Nb₂CS₂ and Ta₂CS₂ synthesized through solid-state synthesis, rather than via the traditional way of selectively etching A-layers from the corresponding MAX phase. Inspired by this, we perform a computational screening study of vdW-MXenes M₂CCh₂ isotypical to Nb₂CS₂ and Ta₂CS₂, with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, or W and Ch = S, Se, or Te. The thermodynamic stability of each vdW-MXene M₂CCh₂ is assessed, and the dynamical stability of both vdW- and ss-MXene is considered through phonon dispersions. We predict seven stable vdW-MXenes, out of which four have not been reported previously, and one, V₂CSe₂, incorporates a new transition metal element into this family of materials. Electronic properties are presented for the vdW- and ss-forms of the stable vdW-MXenes, suggesting that the materials are either metallic, semimetallic, or semiconducting. In previous experimental reports the vdW-MXene Nb₂CS₂ is synthesized by manipulation of the corresponding M₂AX phase Nb₂SC. Therefore, we also evaluate the thermodynamic stability of the corresponding M₂AX phases, identifying 15 potentially stable phases. Six of these are experimentally reported, leaving nine new M₂AX phases for future experimental investigation.