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 Zr2Se(B1-xSex) 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 M6X octahedral distortion and the difference in binding energy between Zr6Se and Zr6B 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 Zr6Se 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 M6X octahedron units.