Effects of Rb Intercalation on NbSe₂: Phase Formation, Structure, and Physical Properties

Here, we report the crystal structures and properties of Rb _xNbSe₂, with 0 ≤ x ≤ 0.5. With Rb intercalation, Rb _xNbSe₂ evolves from 2H (Phase I) for 0 ≤ x ≤ 0.025, to 6R (Phase II) for x ∼ 0.2 with space group R3 m (no. 166), and finally to 2H (Phase IV) for 0.375 ≤ x ≤ 0.5 with space group P6₃/ mmc (no. 194). In addition, Phase II is found to transform to a rare 6H structure (Phase III) with space group P6₃/ mmc (no. 194) by annealing at a relatively low temperature. We show the first 6H phase in the intercalated transition-metal dichalcogenides (TMDs) family obtained through a solid-state reaction. Moreover, both 6R and 6H phases are new polymorphs in the NbSe₂ system. For the range 0.2 ≤ x ≤ 0.5 in Rb _xNbSe₂, we show metallic electronic transport behavior and a paramagnetic feature. The lack of superconductivity (SC) down to 2 K is most probably due to the decrease of hole carrier density with increasing Rb content. Through careful analysis of the structural data, we were able to assemble a phase diagram covering the range of 0 ≤ x ≤ 0.5 in Rb _xNbSe₂.