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 Fe x NbSe 2 ( x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.33, 0.4, and 0.5) and single crystals of Fe x NbSe 2 ( x = 0.14, 0.225, and 0.37). The intercalation limit of Fe in NbSe 2 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 Fe x NbSe 2 . The phase evolves from a mixture phase (2H + 4H) for the pristine NbSe 2 to Fe intercalated 2H phase for 0 < x < 0.20, which transforms to the 2H(I) phase with a 2 a o × 2 a o × 1 c o ordered superstructure for 0.2 ≤ x ≤ 0.25 and finally to the 2H(II) phase with a × 1 c o ordered superstructure for 0.33 ≤ x ≤ 0.5 in Fe x NbSe 2 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 NbSe 2 layers, which is evident from the XPS and theoretical study. From the theoretical study on pristine NbSe 2 and representative intercalated compositions, Fe 0.25 NbSe 2 and Fe 0.33 NbSe 2, 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 NbSe 2 . The density of state analysis shows a significant overlap of 3 d states of Fe with the NbSe 2 layer resulting in the increase in covalent character, which also supports the observed shifting of Nb 3 d and Se 3 d peaks in XPS toward a higher energy with intercalation. A plausible explanation for the stabilization of higher Fe intercalation in Fe x NbSe 2 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).