-The local coordination numbers of As2Se3 glass were determined by a combination of anomalous x-ray scattering experiments, reverse Monte Carlo calculations, and 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'.

We have investigated the intermediate range structure of amorphous Cu 2 GeTe 3 based on ab initio molecular dynamics simulations. The highest population of ring size is three, which makes the triangle structure. This ring consists of mainly Cu 2 Te. Rings may also consist of CuCuCu, Cu 2 Ge, and CuGeTe, where approximately 88% of Cu atoms in the system are related with the three-membered ring. The second highest population of ring size is five. Three-and five-membered rings in the amorphous phase originate from six-membered ring in the crystalline phase. This situation can enhance the phase transition between crystalline and amorphous phases. In the phase change process, Cu atoms may diffuse in the amorphous state with changing bonds. The diffusion coefficient of Cu D Cu is estimated to be approximately 0.12 × 10 −9 m 2 s −1 . Such high diffusion coefficient of Cu atoms is contributed from only 10% of Cu atoms in the amorphous phase.

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