Van der Waals Substitutional Solid Solutions of (Nb_xTa_1-x)₂CSe₂ at the M-Site

The rapid rise of M₂CSe₂ (M = Nb, Ta) MXenes with a single type of selenium termination has brought new opportunities due to their layered nature and van der Waals interlayer coupling. However, their solid solutions remain unclear. Here, we report the molten salt synthesis and characterization of (Nb_xTa_1-x)₂CSe₂ solid solutions at the M-site. With the assistance of molten salts acting as a liquid medium for both the reaction bed and mass transport, continuous solid solutions with a well-defined hexagonal shape can be synthesized from elemental powders. As evidenced by electron microscopy and Raman spectroscopy, Nb and Ta at the M-site are distributed randomly in the substitutional solid solutions. The solid solutions exhibit a metal-like behavior. The Nb/Ta composition dependence of both electrical resistivity and temperature coefficient shows a "W" shape. Intriguingly, the composition dependence of lattice parameter a also follows the same shape. In the whole Nb/Ta composition range, the work functions can be tuned from 5.10 eV (Nb₂CSe₂) to 5.35 eV (Ta₂CSe₂). The friction coefficients are approximately 0.2, indicating van der Waals interlayer coupling in the solid solutions.