Type-II Red Phosphorus: Wavy Packing of Twisted Pentagonal Tubes

TL;DR

This study uncovers the complex structure of type-II red phosphorus, revealing a large triclinic unit cell with twisted tubular motifs, using advanced microscopy and diffraction techniques to deepen understanding of phosphorus polymorphs.

Contribution

It introduces a new structural motif in phosphorus, characterized by a large triclinic unit cell and twisted tubular building blocks, advancing knowledge of phosphorus polymorphs.

Findings

Identified the largest unit cell for elemental phosphorus to date.

Confirmed twisted wavy tubular motifs as fundamental building blocks.

Provided detailed structural insights using multiple advanced techniques.

Abstract

Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 {\AA}^3, the largest unit cell for elemental phosphorus crystals up to now, which contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems.