Structure of glassy and liquid GeSe2

The partial structure factors of bulk-quenched glassy GeSe 2 were measured by using the method of isotopic substitution in neutron diffraction to enable the first detailed comparison at the partial pair distribution function level of a covalently bonded network system in both its glassy and liquid phases. The results show that the basic building block of the glass is the Ge(Se 1/2 ) 4 tetrahedron in which 34(5)% of the Ge atoms reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5)% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) Å, respectively, which is consistent with the existence of these features in the liquid phase of GeSe 2 . Like for the liquid, concentration fluctuations in the glass are found to extend over distances characteristic of the intermediate-range atomic ordering as manifested by the appearance of a first sharp diffraction peak at 1.00(2) Å −1 in the Bhatia–Thornton concentration–concentration partial structure factor. A comparison is made between the measured partial structure factors and recent first principles molecular dynamics simulations for the glassy and liquid phases. It is found that the most significant disagreement between experiment and simulation occurs with respect to the Ge–Ge correlations and that the simulated results for the glass are too liquid-like, reflecting the use of a quench time greatly in excess of that achieved experimentally.