Absence of van der Waals Gap in Ternary Thorium Nitride ThNF and ThNCl

Abstract

Two kinds of ternary thorium nitride compounds, ThNF and ThNCl, are synthesized. Via the refinement of X-ray diffraction patterns, the accurate crystal structure of the two compounds is solved. Although ThNF and ThNCl share a similar structure with MNX (M = Ti, Zr, Hf; X = Cl, Br) compounds, the interaction between adjacent ThNF and ThNCl layers is not a van der Waals gap. For ThNF, the strong electronegativity of F ions leads to the bonding of Th to the F both in the nearest neighbor layer and the next nearest neighbor layer, which results in the absence of a van der Waals gap between ThNF layers. However, for ThNCl, the reason for the absence of a van der Waals gap could be attributed to the large Th–Cl bond length due to the partially covalent Th–Cl bond as well as the flat ThN layer. It is the absence of van der Waals gap that results in the failure of intercalating cations into ThNF and ThNCl. Our result reveals the reason for unsuccessful intercalation in ThNF and ThNCl, thereby providing a deeper understanding for the interlayer interaction in ternary layer structures in metal nitride halides.

Investigation on the interlayer coupling and bonding in layered nitride-halides ThNF and ThNCl

“…2 , ThNF and ThNCl are direct and indirect semiconductors with band gaps of 2.3 eV at K point, and 2.8 eV from Γ point to valence band maximum between Q and Z , respectively. As a comparison, the experimental data showed that the band gaps are ∼3.0 eV for ThNF 17 and 3.79 eV for ThNCl, 17 respectively. The theoretical band gaps are comparable with, but considerably smaller than experimental data.…”

“…Here we find that in ThNF, the valence of each element is Q (Th) = +2.3, Q (N) = −1.43, Q (F) = −0.87; and in ThNCl, Q (Th) = +2.38, Q (N) = −1.62, Q (Cl) = −0.76, in agreement with the experimental data by the X-ray photoemission spectra (XPS) very well. 17 …”

“…Therefore, the van der Waals coupling in ZrNCl plays more important roles than in ThNCl and ThNF, leading to easy intercalation behavior in the ZrNCl and hard intercalation behavior in the ThNF and ThNCl, as observed in the experiment. 17 …”

“…Here we nd that in ThNF, the valence of each element is Q(Th) ¼ +2.3, Q(N) ¼ À1.43, Q(F) ¼ À0.87; and in ThNCl, Q(Th) ¼ +2.38, Q(N) ¼ À1.62, Q(Cl) ¼ À0.76, in agreement with the experimental data by the X-ray photoemission spectra (XPS) very well. 17 The charge analysis of ThNCl and ThNF, as well as ZrNCl, gives rise to their electron localization function and deformation charge density, as shown in Fig. 3.…”

“…Such attempts have been performed recently. 17 Wang et al found ThNF and ThNCl could not be intercalated by lithium or organic molecules like other nitride-halides ZrNCl and HfNCl. This arises a question that what happen of interlayer coupling and chemical bonding in these two compounds.…”