Order-disorder phase transition driven by interlayer sliding in lead iodides

Abstract

A variety of phase transitions have been found in two-dimensional layered materials, but some of their atomic-scale mechanisms are hard to clearly understand. Here, we report the discovery of a phase transition whose mechanism is identified as interlayer sliding in lead iodides, a layered material widely used to synthesize lead halide perovskites. The low-temperature crystal structure of lead iodides is found not 2H polytype as known before, but non-centrosymmetric 4H polytype. This undergoes the order-disorder phase transition characterized by the abrupt spectral broadening of valence bands, taken by angle-resolved photoemission, at the critical temperature of 120 K. It is accompanied by drastic changes in simultaneously taken photocurrent and photoluminescence. The transmission electron microscopy is used to reveal that lead iodide layers stacked in the form of 4H polytype at low temperatures irregularly slide over each other above 120 K, which can be explained by the low energy barrier of only 10.6 meV/atom estimated by first principles calculations. Our findings suggest that interlayer sliding is a key mechanism of the phase transitions in layered materials, which can significantly affect optoelectronic and optical characteristics.

Crystal and Electronic Structures of Epitaxial Atomic Thick Lead Iodide

“…This value is larger than the band gap of bulk PbI 2 . In bulk PbI 2 , a band gap of approximately 2.4 eV was observed by ARPES …”

Dualistic insulator states in 1T-TaS2 crystals

“…This feature, along with a distinct interlayer distance difference between LC-and 1T-TaS 2 crystals, implies that the periodic laddering stack structure is unlikely to be the structural origin for the heating-triggered surface intermediate Mottinsulator phase 22,44 . However, the possibility of an interlayer atomic sliding confined to the surface region (5-8 layers), such as the irregular sliding observed in layered bulk PbI 2 45 , cannot be ruled out. Furthermore, we performed density functional theory (DFT) calculations to gain a comprehensive understanding of electronic structures in the above STS and ARPES experiments.…”

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

“…Transmission electron microscopy (TEM) provides a means of directly imaging the atomic structure of 2D materials with atomic scale spatial resolution. For example, cross-sectional STEM was employed to reveal interlayer sliding as the mechanism of an order–disorder phase transition in lead iodides (PbI 2 ) Figure a,c show high-angle annular dark-field scanning TEM (HAADF-STEM) images above versus below the transition temperature, and Figure b,d show corresponding fits to the locations of Pb atoms compared to red circles that denote their expected position in the 4H-PbI 2 phase, clearly showing the change in the stacking order from interlayer sliding, whereas the intralayer atomic structure remains the same .…”

“…(b, d) Corresponding fits to the locations of Pb atoms below (b) and above (d) the transition temperature, compared to red circles indicating their expected position in the 4H-PbI 2 phase, showing the interlayer sliding. Reproduced with permission from ref . Copyright 2023 Springer Nature.…”

“…For example, cross-sectional STEM was employed to reveal interlayer sliding as the mechanism of an order–disorder phase transition in lead iodides (PbI 2 ) Figure a,c show high-angle annular dark-field scanning TEM (HAADF-STEM) images above versus below the transition temperature, and Figure b,d show corresponding fits to the locations of Pb atoms compared to red circles that denote their expected position in the 4H-PbI 2 phase, clearly showing the change in the stacking order from interlayer sliding, whereas the intralayer atomic structure remains the same . The stacking order may also be visualized with TEM in plan view, normal to the surface, by directly viewing the vertical alignment of atoms between layers, for instance, to identify the stacking sequence in TMD. ,, Atomic imaging provides a direct visualization of the stacking order and is a vital element of 2D material characterization.…”