Effect of molecular intercalation on the local structure of superconducting Na_x(NH₃)_yMoSe₂ system

We have studied the local structure of layered Na_x(NH₃)_yMoSe₂ system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe₂ largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature (Θ_E of Mo-Mo distance hardly changes (∼264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of Θ_E from ∼364 K to ∼350 K. The results indicate that Na⁺ ion transported by NH₃ molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe₂ and superconductivity at low temperature.