Structure of glassy and liquid GeSe2

Abstract

The partial structure factors of bulk-quenched glassy GeSe2were measured by using the method of isotopic substitution in neutron diffractionto enable the first detailed comparison at the partial pair distribution functionlevel of a covalently bonded network system in both its glassy and liquidphases. The results show that the basic building block of the glass isthe Ge(Se1/2)4 tetrahedron in which 34(5)% of the Ge atoms reside inedge-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 inhomopolar bonds at distances of 2.42(2) and 2.32(2) Å, respectively, which isconsistent with the existence of these features in the liquid phase of GeSe2.Like for the liquid, concentration fluctuations in the glass are found to extend overdistances characteristic of the intermediate-range atomic ordering as manifestedby the appearance of a first sharp diffraction peak at 1.00(2) Å−1in the Bhatia–Thornton concentration–concentration partial structure factor. Acomparison is made between the measured partial structure factors andrecent first principles molecular dynamics simulations for the glassy andliquid phases. It is found that the most significant disagreement betweenexperiment and simulation occurs with respect to the Ge–Ge correlations andthat the simulated results for the glass are too liquid-like, reflecting theuse of a quench time greatly in excess of that achieved experimentally.