Structural and Semiconductor-Metal Transition in liquid Se

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In recent years, many investigations on liquid chalcogens such as Se and Te have been done. It has been reported that the liquids undergo the semiconductor to meal (S-M) transition at high temperatures, being accompanied by structural transformation. Much of the interest stems from the apparent persistence of two-fold covalent bonding across the S-M transition.The studies have been focused on the links between the structural aspects of liquid chalcogens and their electronic properties. The extended X-ray absorption fine structure (EXAFS), conductivity and Hall coefficient measurements have been carried out for liquid Te-Se (1-Te-Se) mixtures and liquid As-Te (1-As-Te) mixture. The first-principle molecular-dynamics simulation has been also performed since it provides the microscopic mechanism of the S-M transition.EXAFS data analysis for 1-Te-Se mixtures reveals that the two-fold chains in the metallic 1-Te-Se shortened by thermal agitation are composed of short and long covalent bonds … More and the long bonds vanish in the semiconducting state. The network structure of the 1-As-Te mixtures with three-fold coordinated Te atoms and two-fold coordinated Te atoms is transformed into the two-fold chain structure. The network-chain transformation is accompanied with the semiconductor to metal transition which is demonstrated by the results for conductivity and Hall coefficient. The radial distribution functions including bond-angle and dihedral-angle distribution for 1-Se are wide temperature and pressure range up to critical point have been determined by the tight-binding molecular dynamic simulation. It has been found that the chain-like structure persists even in metallic state though the average chain length decreased and the dihedral angle has a flatter distribution.The dangling bonds of Te or the two-fold coordinated as atoms modified by the network-chain transformation cause large fluctuations of the charge distribution in the shortened chains, which results in a frequent inter- and intra-charge transfer. This is our scenario for the source of the S-M transition in the liquid chalcogen mixtures composed of covalently bonded chains. Less