Chalcogenide MAX phases Zr2Se(B1-xSex) (x=0–0.97) and their conduction behaviors

The realization of chemical diversity of MAX phases is distinctive and important to screen out their unique physiochemical properties for prospective applications. Compared with well-known 25 M-site elements and 23 A-site elements, the option of X-site element is under-explored and restricted in few non-metal elements such as C, N and B. Herein, a chalcogen element selenium was found to be a new member to occupy X site in MAX phases which were stabilized through boron alloying. A series of Zr ₂Se(B _1-xSe _x ) phases were successfully prepared with occupancy rate x up to 0.97. The crystal structure of these chalcogenide MAX phase was comprehensively characterized and confirmed by Rietveld refinement of X-ray diffraction (XRD), atom-resolved transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques. The variation of M ₆X octahedral distortion and the difference in binding energy between Zr ₆Se and Zr ₆ B octahedra were found to contribute the phase transformation from boride MAX phase to chalcogenide MAX phase. The effect of occupancy of Se at X site on electrical conductivity of MAX phase was studied and discussed by first-principles calculations, which indicates that the deterioration of carrier motility induced by the strong binding of Zr ₆ Se octahedron in MX sublayers leads to the reduction of electrical conductivity. This work also provides a new route to tune the physiochemical properties of MAX phases through the regulation of their basic M ₆X octahedron units.