Packing structure of chains and rings in an expanded liquid Se80Te20 mixture near the semiconductor to metal transition

X-ray scattering measurements of an expanded liquid Se80Te20 mixture in the temperature range between 300 and 1000 0C and at pressures of 10, 200, and 1600 bar have been carried out. The reverse Monte Carlo simulation and Voronoi-Delaunay void analyses have been applied to clarify the relationship between the voids and chain geometries in the intermediate scale near the semiconductor to metal (SC-M) transition at 800 0C under 200 bar accompanied by a shrinkage of molar volume. The structure of the liquid mixture can be envisaged in terms of a packing of covalently bonded chains and interstitial voids. The thermal expansion leads to a decrease in the number of chains around a chain and is compensated for by empty spaces (voids). The packing density of helical chains decreases and so voids supported by chains increase with rising temperature. At high temperature the shortening of chains and frequent transfer of lone pair electrons on Se (or Te) atoms cause modification of the helical chain to ring and zigzag chain conformations. The stacking of zigzag chains, joined to layers above 800 0C, leads to the formation of metallic domains separated by voids. The shrinkage of molar volume near the SC-M transition arises through the progressive filling of the voids around chains with (Se, Te)5,6,7,8 rings.