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

Abstract

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 2a o × 2a o × 1c o ordered superstructure for 0.2 ≤ x ≤ 0.25 and finally to the 2H(II) phase with a a a 3 3

Bridging Structure, Magnetism, and Disorder in Iron-Intercalated Niobium Diselenide, FexNbSe2, below x = 0.25

“…Single-crystal X-ray diffraction (SCXRD) data were obtained for single crystals at x = 0.14, 0.19, and 0.23 (Figure a–d), each yielding a hexagonal structure with the centrosymmetric space group P 63/ mmc . The out-of-plane lattice constant c was found to be 12.6011(9), 12.6174(8), and 12.6498(8) Å for x = 0.14, 0.19, and 0.23, respectively, which are all slightly larger than that for 2 H -NbSe 2 ( c 0 = 12.547(3) Å), suggesting a very minor expansion upon Fe intercalation . The in-plane lattice constant a , b for the x = 0.23 crystal was found to be 6.9151(4) Å, which is close to twice that of native 2 H -NbSe 2 ( a 0 = 3.4425(5) Å), and, similarly, the a , b lattice constant for the x = 0.19 sample was determined to be 6.9046(3) Å.…”

“…These peak positions are consistent with the Raman frequency range of Fe–Se stretching vibration modes observed in other crystalline systems. 17 , 32 These features could also stem from Brillouin zone-folding effects, which arise when a structure of higher crystal symmetry is superimposed with that of a lower crystal symmetry, potentially allowing phonons at high-symmetry points of one BZ to map onto different high-symmetry points of the other. 25 Therefore, for a Fe 0.23 NbSe 2 structure with a 2 a 0 × 2 a 0 intercalant supercell, zone-edge optical phonons located at the M point of the host lattice Brillouin zone ( M NbSe 2 ) can now be folded onto the zone-center Γ point of the superlattice BZ (Γ SL ).…”

“…The out-of-plane lattice constant c was found to be 12.6011 (9), 12.6174(8), and 12.6498(8) Å for x = 0.14, 0.19, and 0.23, respectively, which are all slightly larger than that for 2H-NbSe 2 (c 0 = 12.547(3) Å), suggesting a very minor expansion upon Fe intercalation. 17 The in-plane lattice constant a, b for the x = 0.23 crystal was found to be 6.9151(4) Å, which is close to twice that of native 2H-NbSe 2 (a 0 = 3.4425(5) Å), and, similarly, the a, b lattice constant for the x = 0.19 sample was determined to be 6.9046(3) Å. For each of these two stoichiometries, the Fe intercalants occupy octahedral interstitial sites between the NbSe 2 layers (Wyckoff position 2a), forming an ordered 2a 0 × 2a 0 superlattice (Figure 1c).…”