`A COMPLETELY NOVEL MECHANISM OF METAL-NONMETAL TRANSITION IN EXPANDED SELENIUM'
F.Yonezawa, H.Ohtani and T.Yamaguchi
Journal of Optoelectronics and Advanced Materials 7(4) 1707-1715 (2005)
The purpose of the present article is to review our discovery concerning a completely new
mechanism of metal-nonmetal (M-NM) transition in expanded Se. Several mechanisms were
previously proposed for M-NM transitions which includes the Bloch-Wilson transition, the
Mott-Hubbard transition and the Anderson transition. In each of these cases, a transition
takes place from metal to nonmetal when the system is expanded. This behavior is brought
about by the narrowing of bands on the increase of volume, which leads to the opening-up of
bands and the appearance of an energy gap at the Fermi level. On the other hand, a transition
occurs in expanded Se from nonmetal to metal on expansion, which is perfectly against the
common knowledge. In expanded Se, chains are fragmented into small sizes of 10, and
consequently some of bonds are stretched. We show that the energy splitting
Es~*-s~ o Es~* - Es~ corresponding to these stretched bonds becomes smaller, and eventually the
gap between the valence band (composed of the lone-pair orbitals) and the conduction band
(composed of the anti-bonding * ~s orbitals relevant to the stretched bonds) closes and the
system transforms from nonmetal to metal.
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2005/11/01