X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb_1+xSe₂ (0 < x < 0.1)

Abstract

Single crystals of 2H and 3R niobium diselenide were grown by a chemical transport reaction. The current determinations by single crystals X-ray diffraction reveal a non-stoichiometric composition. The structures are built from Se—Nb—Se slabs with Nb in trigonal prismatic coordination whereas the extra or additional Nb atoms are located in the octahedral holes between the slabs giving rise to the formula 2H and 3R-Nb1+xSe2 with 0.07 &lt; x &lt; 0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe2. By increasing the number of additional Nb atoms in the pure 2H-NbSe2, a transition 2H to 3R polytype should occur in order to minimize the Nblayer—Nbextra—Nblayer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb1+xSe2 are thermodynamically stable in the range 0 &lt; x &lt; 0.1.

Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films

“…Our STEM images disclose that epitaxial films of Nb 1+x Se 2 near a global x ~0.29 exhibit a nanoscale separation into 180°-stacked layers with a large number of Nb intercalants, perhaps several tens of percentage occupancy (considering the average x value), and 0°-stacked layers whose number of Nb intercalants falls close to the detection limit. Bulk studies have shown that the polytype of pristine NbSe 2 is 2H a with 180°s tacking, and that a few percent of excess Nb should stabilize the 3R polytype with 0°stacking [24][25][26][27] . Here, the situation seems to be reversed, with the 180°-stacked layers hosting a much greater number of Nb intercalants than the 0°-stacked layers.…”

“…Early studies showed that TMDCs, especially with group-5 metal atoms, i.e., M = V, Nb, or Ta, undergo complex transformations of stacking polytypes upon excess M stoichiometry 23,24 . For example, bulk samples of Nb 1+x S 2 and Nb 1+x Se 2 undergo a transition from a 2H a polytype with 180°stacking of layers to a 3R polytype with 0°stacking when x crosses a threshold around 0.03-0.07, presumably because the 3R polytype better accommodates the increasing number of Nb intercalants in its interstitial voids [24][25][26][27] . With larger values of x, polycrystalline samples with 2H a , 2H b , and 3R polytypes and Nb intercalants have been stabilized 24 , but less is known about this regime.…”

“…In essence, the cooperative behavior of stacking and intercalation is already realized "naturally" in the rich chemistry and polytypism of M 1+x X 2 , but the mechanistic details remain hidden, and the behavior remains to be exploited for applications. To determine the precise atomic position and chemical state of the intercalants in a mixed-phase sample with various stacking configurations, cross-sectional scanning transmission electron microscopy (STEM) with sub-angstrom resolution is ideal [28][29][30][31][32][33][34][35][36] and avoids the problem of interlayer averaging present in other techniques, such as x-ray diffraction [24][25][26][27] or Raman spectroscopy 37,38 . To exploit the cooperative behavior of stacking and intercalation for technological purposes, it is useful to reproduce the previously synthesized M 1+x X 2 compounds as thin films, which offer large surface areas, precise thickness control, amenability to lithography and plentiful pathways for properties engineering 32,[39][40][41][42] .…”