Unravelling the structure of amorphous red phosphorus

Researchers from the Department of Chemistry have created accurate models for the structure of amorphous red phosphorus, addressing a long-standing puzzle in inorganic chemistry.

Phosphorus is one of the most “creative” chemical elements in terms of the different structures it can take. White phosphorus consists of small, tetrahedral molecules; black phosphorus forms layered sheets; violet phosphorus contains complex cages that are arranged into tubes. The most common commercial form of phosphorus, however, is amorphous: it lacks long-range structural (crystal-like) order, and its atomic-scale building units connect in intricate ways to make up its three-dimensional structure.

The structure of amorphous red phosphorus has long been unclear, just because it is so challenging to study: there are many different experimental tools for studying the structure of amorphous solids, but each of them gives only partial insight. This lack of structural understanding is particularly unfortunate as red phosphorus is widely used in industrial applications, and is also an emerging anode material for future sodium-ion batteries.

Realistic structural models of amorphous phosphorus.

In a publication highlighted as a Hot Paper in Angewandte Chemie, a team from Oxford Chemistry have now described computer simulations, based on quantum-mechanical and machine-learning (ML) methods, that give new insight into amorphous red phosphorus. Using advanced, highly accurate, ML-accelerated simulations, the authors were able to create realistic structural models (see the images above of three different models) – and to study their microscopic properties, such as the chemical bonding between the atoms.

First author of the study, Yuxing Zhou, is currently a senior DPhil student in the Deringer research group. Yuxing’s doctoral research is supported by a prestigious China Scholarship Council-University of Oxford scholarship, and is focused on understanding complex inorganic materials with advanced ML-driven simulations. The study was co-authored by Professor Stephen Elliott, who reported foundational experimental studies of red phosphorus in the 1980s already, and currently holds a Visiting Professorship in the Physical and Theoretical Chemistry Laboratory at Oxford.

In earlier work, published in 2022, Yuxing and Masters student William Kirkpatrick had shown that their approach for modelling a-P can explain the material’s intricate behaviour under pressure. The new Angewandte Chemie paper now provides more detailed insight by combining ML-driven simulations with a quantum-mechanical description of the chemical bonding in phosphorus allotropes. The authors expect that their openly available research data can be readily used by others.

Professor Volker Deringer, senior author of the study, leads a research group in the Inorganic Chemistry Laboratory. His team develops ML approaches for atomic-scale materials modelling and applies them to research questions in chemistry – from the fundamental structural nature of the amorphous state to the computational design of new inorganic materials. In 2022, he was awarded the Royal Society of Chemistry’s Harrison-Meldola Memorial Prize for “innovative contributions to the modelling and understanding of amorphous materials”.

This research was supported by a generous award of high-performance computing resources on the CSD3 system, via the UK’s Engineering and Physical Sciences Research Council.

The published work is openly available at https://doi.org/10.1002/anie.202216658.