X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 &lt; x &lt; 0.1)

Mohamed Sidoumou,Mohammed Kars,Adrian Gómez-Herrero,Roisnel Thierry,Soumia Merazka

doi:10.7717/peerj-ichem.2

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 < x < 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 < x < 0.1.

Highlights

NbSe2 belongs to the transition metal dichalcogenides TMDC’s with chemical formula TX2 (T = Nb, Ta; X = S, Se), which have recently renewed interest because of their quasi 2D nature similar to graphene making them very promising in novel electronic devices applications (Vogel & Robinson, 2015)
The system NbSe2 has been the subject of many investigations since it exhibits incommensurate charge density waves (CDW) and superconductivity phenomenon above 4K (Wilson, Disalvo & Mahajan, 1975)
The role of vacancy and Nb-Nb interactions on the non-stoichiometry and stacking mode in NbSe2 has been examined. Both 2H and 3R polytypes may co-exist in the range 0

Summary

Introduction

NbSe2 belongs to the transition metal dichalcogenides TMDC’s with chemical formula TX2 (T = Nb, Ta; X = S, Se), which have recently renewed interest because of their quasi 2D nature similar to graphene making them very promising in novel electronic devices applications (Vogel & Robinson, 2015). The system NbSe2 has been the subject of many investigations since it exhibits incommensurate charge density waves (CDW) and superconductivity phenomenon above 4K (Wilson, Disalvo & Mahajan, 1975). The structure consists in hexagonally arranged Se-Nb-Se sandwiches with metal atoms (Nb) that are located between two layers of chalcogen atoms (Se) in a trigonal prismatic coordination. The stacking along the c axis gives rise to several polytypes usually described as 1T–NbSe2, 2H- NbSe2, 3R–NbSe2. X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1).

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Conclusion