Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide

Intercalation and stacking‐order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb‐intercalated TMDCs–Pb(Ta1+xSe2)2, the first 124‐phase, is reported. Pb(Ta1+xSe2)2 exhibits a unique two‐step first‐order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high‐temperature (high‐T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low‐temperature (low‐T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low‐T, bulk superconductivity with Tc ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first‐principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher‐order metal‐intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking‐order engineering in TMDCs and beyond. Two‐step first‐order structural phase transitions at around 230 K in Pb(Ta1+xSe2)2 are solely associated with the stacking degree of freedom, evolving from a high‐temperature phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low temperature phase again with R3m symmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0].