Change in the topology of the glass forming liquid GeSe₂ with increasing temperature

The change in topology with increasing temperature of liquid GeSe₂ was studied by using neutron diffraction to measure the Bhatia-Thornton number-number partial structure factor, S_NN(Q), where Q is the scattering vector. As the temperature is raised from 800 to 1100 °C there is a broadening but little other change in the peaks describing the local coordination environment. There is, however, a collapse of the intermediate range atomic ordering of the network associated with the Ge-Ge correlations as manifest by a substantial reduction in the height of the first sharp diffraction peak and a shift in its position from 0.99(1) to 1.05(1) Å^-1. The changes in the structure with increasing temperature mimic those observed as the composition of liquid GeSe₂ is changed to the GeSe stoichiometry by the addition of germanium. The significance of the results for recent Car-Parinello type ab-initio molecular dynamics simulations is briefly discussed.