Evolution of Structural Properties in Fe Intercalated 2H-NbSe2: Phase Transformation Induced by Strong Host–Guest Interaction

In this article, we report the synthesis and structural characterization of polycrystalline FexNbSe2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.33, 0.4, and 0.5) and single crystals of FexNbSe2 (x = 0.14, 0.225, and 0.37). The intercalation limit of Fe in NbSe2 is found to be 50 mol %, above which elemental iron remains as an impurity phase. Single crystal diffraction studies show three distinct phase transitions with Fe intercalation in FexNbSe2. The phase evolves from a mixture phase (2H + 4H) for the pristine NbSe2 to Fe intercalated 2H phase for 0 < x < 0.20, which transforms to the 2H(I) phase with a 2ao × 2ao × 1co ordered superstructure for 0.2 ≤ x ≤ 0.25 and finally to the 2H(II) phase with a 3ao×3ao × 1co ordered superstructure for 0.33 ≤ x ≤ 0.5 in FexNbSe2 at the reaction temperature of 900 °C. During the phase evolution process, the polytype is retained but the sub-polytype changes; i.e., the crystal symmetry and atom coordinates change. Intercalation of Fe induces a more covalent character in the phases by interacting with the adjacent NbSe2 layers, which is evident from the XPS and theoretical study. From the theoretical study on pristine NbSe2 and representative intercalated compositions, Fe0.25NbSe2 and Fe0.33NbSe2, it is observed that the percentage of metal–metal bonding contribution toward the total bonding energy is higher for the intercalated compounds as compared to the parent NbSe2. The density of state analysis shows a significant overlap of 3d states of Fe with the NbSe2 layer resulting in the increase in covalent character, which also supports the observed shifting of Nb 3d and Se 3d peaks in XPS toward a higher energy with intercalation. A plausible explanation for the stabilization of higher Fe intercalation in FexNbSe2 has been proposed from both theoretical and experimental studies, and a phase diagram detailing the phase evolution with Fe concentration is constructed (0 ≤ x ≤ 0.5).