The situation of the nonmetal-metal transition in liquid Se under volume expansion is perfectly the other way around compared to the cases of the metal-nonmetal transitions studied so far.
Liquid metals like a mercury and Alkali metals transform from a metal to a nonmetal under the thermal expansion by the increase of the temperature, while liquid Se transforms from a nonmetal to a metal under this thermal expansion. From the results of the structural experiments, we give attention to the shortening of the chain length by the fragmentation of a long chain and introduce a finite chain model with the effect of a chain end.
The relationship between the structural changes and the electronic properties is investigated using the model structure for liquid Se by means of the ab-initio calculations. The results obtained from our calculations are as follows;
The mechanism with a planar zigzag structure is proposed as the mechanism of the metalization in liquid Se under volume expasion. We compare the results for the planar zigzag structure and the helical structure with the different rotation angles from each other.
As a results, we find that the infinite and finite planar zigzag structures are energetically unstable compared with the helical structure, and that the electronic property of the planar zigzag structure transforms from a metal to a nonmetal when the structure changes from a infinite chain to a finite chain under volume expansion. Therefore, the possibility of the planar zigzag structure in liquid Se as the mechanism for the nonmetal-metal transition is overruled.
We make it sure that the breaking of the some bonds of the helical chains changes the wave functions of the terminal atoms of the chains and reduces the splitting between the bonding and anti-bonding states corresponding to the broken bonds in volume expansion, and that the energetically smallest states become relatively lowered from the conduction anti-bonding bands. The band width is decreased by the increase of the interchain distance under volume expansion, however, the anti-bonding states overlap with the valence lone-pair band because of the dramatically lowness of these state. The energy gap finally becomes disappeared. It becomes clear that liquid Se transforms from a nonmetal to a metal because the chemical potential falls in this energy overlap region and the Fermi level exists there. In this metallic state, the local three-fold structure is formed between the terminal atom of the chain and a neighbour chain, and the electron conducts in the interchain because of the interchain overlap of the wave functions. This mechanism of a nonmetal-metal transition is a new type of mechanism.
The transition pressures from a nonmetal to a metal are different for the solid and liquid states under volume compression by pressure. From the results of the structural experiments, we introduce a infinite chain model with the structural disorder in the interchain interactions.
The relationship between the structural changes and the electronic properties is investigated using the model structures for liquid calcogens by means of the ab-initio calculations. The results obtained from our calculations are as follows;
A system with the structural disorder transforms to a metal under smaller decrease of the interchain distance from the crystal because of the lowing of the anti-bonding states corresponding to long bonds. This shows that liquid chalcogen transforms to a metal under pressure lower than crystal.
This thesis investigates the relationship between the structural changes and the electronic properties in liquid chalcogen with giving attention to the interchain interactions, and give a theoretical explanation for the mechanisms of metalizations observed in liquid chalcogens under high temperature and under high pressure. From the analysis of our results, we propose a completely new mechanism of nonmetal-metal transition in liquid Se. This adds a novel member to the list of mechanisms for metal-nonmetal transitions studied so far.