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 degrees 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) Angstrom(-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.