TOPICAL REVIEW: Temperature driven semiconductor metal transition and structural changes in liquid As2Se3

Intensive experimental investigations have been performed on a typical liquid semiconductor As₂Se₃ over a wide temperature range under pressure, such as electrical conductivity, optical absorption coefficient, density, x-ray absorption fine structure, and x-ray diffraction measurements. With increasing temperature, it was found that liquid As₂Se₃ undergoes a semiconductor-metal transition at about 1000 °C, which is clearly seen in the electrical conductivity and optical absorption measurements. This transition is accompanied by significant structural and thermodynamical changes. The volume of the system apparently contracts near the transition temperature. The structural changes are observed especially in the intermediate-range order or the prepeak in the total structure factor characteristic to this glass-forming liquid. X-ray absorption fine structure (XAFS) and x-ray scattering results strongly suggest a formation of new As-As bonds in addition to the original As-Se bonds in the high-temperature metallic region. These experimental results are carefully discussed by comparing to theoretical results from an ab initio molecular dynamics (MD) simulation. Some new insights can be realized from this computational work. For example, a small portion of homopolar bonds already exist in the semiconducting region, and twofold coordinated As atoms existing in a chain structure with twofold coordinated Se atoms play a very important role for the metallization of this liquid. Some limitations of the MD simulation are also pointed out.