Valence alternation pairs-the example of selenium

Lattice relaxation is an essential property of negative U defects because the Coulomb contribution to the correlation energy in Eq. 

Selenium is normally two-fold coordinated, Se, , and of its four valence p electrons, two are bonding electrons and the other two are lone pair states which make up the top of the valence band. A simple molecular orbital model of the bonding is shown in Fig. 4.6. The neutral selenium dangling bond, SCj , has one bonding electron, and 

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