Graphene can be made auxetic through the introduction of vacancy defects. This results in the thinnest negative Poisson's ratio material at ambient conditions known so far, an effect achieved via a nanoscale de-wrinkling mechanism that mimics the behavior at the macroscale exhibited by a crumpled sheet of paper when stretched.

Molecular dynamics (MD) simulations and extended x-ray absorption fine structure (EXAFS) investigations of the structure of lead-silicate glasses, xPbO(1 − x)SiO2, have been undertaken to elucidate the problem of partially contradicting experimental findings reported in the literature about basic structural units and their interconnection. The MD simulations were performed in a wide range of compositions, x = 0.1–0.9. The atoms were assumed to interact by a two-body Born–Mayer–Huggins interaction potential. The EXAFS measurements were performed for x = 0.3, 0.5 and 0.7, and also for pure crystalline (red) PbO at the L3-edge of Pb. The absorption spectra were analysed within the GNXAS approach.Our EXAFS and MD results are in good agreement, and support some previous suggestions that: (1) the PbO4 groups are the dominant structural units in lead-silicate glasses for any concentration and (2) at lower PbO concentrations the co-existence of the PbO4 and PbO3 groups is possible.The medium-range ordering in the simulated glasses has also been investigated in detail. The connectivity of the SiO4 tetrahedra network breaks at about x = 0.45, whereas the Pb structural units form a continuous (mainly edge-sharing) network even at relatively low PbO concentrations (x > 0.2). The cation–anion ring statistics is also discussed.

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