Does the $8-N$ bonding rule break down in As$_2$Se$_3$ glass?

TL;DR

This study investigates the local atomic structure of As$_2$Se$_3$ glass, revealing that the traditional 8-N bonding rule fails around arsenic atoms due to the formation of As-As wrong bond chains, supported by experiments and simulations.

Contribution

It combines experimental and computational methods to demonstrate the breakdown of the 8-N bonding rule in As$_2$Se$_3$ glass and identifies the formation of As-As wrong bond chains as the cause.

Findings

The 8-N bonding rule breaks down around As atoms by 7-26%.

As atoms form As-As wrong bond chains instead of expected configurations.

Experimental and theoretical results are in agreement.

Abstract

The local coordination numbers of As$_2$Se$_3$ glass were determined by a combination of anomalous x-ray scattering experiments, reverse Monte Carlo calculations, and {\it ab initio} molecular dynamics simulations. The well-known `8-$N$ bonding rule' proposed by Mott breaks down around the As atoms, exceeding the rule by 7--26%. An experimental prediction based on mean-field theory agrees with the present experimental and theoretical results. The fourfold coordinated As atoms likely form As-As wrong bond chains rather than ethan-like configurations, which is identified as the origin for the breakdown of the `8-$N$ bonding rule'.