Peer Review #1 of "X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1) (v0.1)"

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 nonstoichiometric 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-Nb 1+x Se 2 with 0.07< x <0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe 2 . By increasing the number of additional Nb atoms in the pure 2H-NbSe 2 , a transition 2H to 3R polytype should occur in order to minimize the Nb layer -Nb extra -Nb layer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb 1+x Se 2 are thermodynamically stable in the range 0< x <0.1.