Intercalation of alkali metals (Li, Na, and K) in molybdenum dinitride (MoN2) and titanium dinitride (TiN2) from first-principles calculations

We studied ternaries of nitrogen-rich titanium and molybdenum compounds combined with alkali metals (Li, Na, and K) as potential layered materials. LiMoN2 has already been synthesized with layered structure corresponding to intercalated 3R-MoS2, however efforts to completely deintercalate Li from LiMoN2 were unsuccessful. We studied ternaries MAN2 (A: Ti, Mo and M: Li, Na, K) having layered crystal structures, their deintercalation, and the layered binaries TiN2 and MoN2 within density-functional theory. We find that LiMoN2 and NaMoN2 have a layered structure isotypic to 2H-MoS2 with Li and Na ions filling interlayer spaces whereas LiTiN2 and NaTiN2 are isotypic to α-NaFeO2. Both KMoN2 and KTiN2 have a different kind of structure isotypic to SrTiN2 which differentiates K from Li and Na. Ternaries Li(Na)MoN2 and Li(Na)TiN2 are all metals and the alkali metal atoms are present as ions in these structures. Partially deintercalated ternaries suggest that layers can interact strongly and the material can loose its layered form. Furthermore, we find that binary MoN2 having a layered structure is not stable since monolayer MoN2 has a positive formation energy and N atoms belonging to neighboring layers interact and form N2 dimer in between Mo layers in which case formation energy becomes negative indicating that structure becomes more stable. These results can explain the decomposition of LiMoN2 during the experimental trials of complete deintercalation. In contrast, TiN2 has a negative formation energy already without interaction of N atoms belonging to neighboring layers.