Commensurate stacking phase transitions in an intercalated transition metal dichalcogenide

TL;DR

This paper reports the discovery of a new intercalated TMDC with unique two-step stacking phase transitions, revealing complex structural behavior, superconductivity, and topological features, advancing material engineering possibilities.

Contribution

It introduces Pb intercalated TMDCs with a novel 124-phase and characterizes their unique stacking phase transitions and associated properties.

Findings

Two-step first-order structural phase transition at 230 K

Observation of bulk superconductivity at 1.8 K

Presence of topological nodal lines in band structure

Abstract

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(Ta$_{1+x}$Se$_2$)$_2$, the first 124-phase, is reported. Pb(Ta$_{1+x}$Se$_2$)$_2$ 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 phase with ABC stacking and symmetry $R3m$ to an intermediate phase with AB stacking and $P3m1$, and finally to a low temperature phase with again symmetry $R3m$, 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 temperature, bulk superconductivity with $T_\textrm{c}\approx$ 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. Our results demonstrate the possibility to realize higher order metal intercalated phases of TMDCs, advance our knowledge of polymorphic transitions and may inspire stacking order engineering in TMDCs and beyond.