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  • p2x receptors Thus the Fe ion in the main three valence

    2018-11-02

    Thus, the Fe3+ ion in the main three-valence state possessing total spin 5/2 as revealed by ESR, is to have the 4s2(1)4p1(3) hybridization, whereas the neutral iron p2x receptors does not contain electrons in 4p-orbitals. So, the tetra-coordinated iron ion in the BSO crystal donates three valence electrons (one of them is a d-electron) into its surrounding to form three sigma-bonds with three ligands (oxygen ions). At that the fourth hybrid orbital of the iron ion is unoccupied and forms a donor-acceptor bond with the one of 2s2(1)2p4(3)-hybrid oxygen ions. So, at the stage of the synthesis of crystals during the formation of the sillenite crystalline lattice, the following situation is formed. The oxygen atom, when linking to the iron atom, “withdraws” one of six iron atom 3d-electrons from internal d-orbitals and inserts it into one of four 4s2(1)4p1(3)-hybrid orbitals of the Fe3+ ion forming the chemical bond. The same situation occurs in the three-valence iron ion in hemoglobin. The electron configuration of the outer shell of the iron ion enables the hybridization described the only possible when the conditions of Fe3+ ion four-coordination, three-valence and total spin of 5/2 are fulfilled simultaneously. It is reflected formally in the substitution of the superscript from 0 in 4p0(3)-orbital of the iron atom to 1 in 4s2(1)4p1(3)-hybridization. This initial state is illustrated in Fig. 5a. For the excited two-valence light-induced state of Fe2+ ion, another hybridization takes place such as 4s2(1)4p0(3), which corresponds to the return of the electron “withdrawn” by oxygen into one of five 3d-orbitals. One of the possibilities of this return is as follows. The light quantum breaks one of the sigma-bonds of the iron ion with oxygen and the excited electron is transferred into one of five 3d-orbitals of Fe3+ ion, jumping a potential barrier of a sigma-bond. This, firstly, reduces the iron ion valence from 3+ to 2+, and secondly, lowers the total magnetic moment from 5/2 for Fe3+ to 4/2 for Fe2+ ion (see Table 1) due to the coupling of the coming sixth electron with one of the five have been possessed yet. So, in the state excited by light, two valence electrons are available for the formation of two sigma-bonds of Fe2+ ion with two ligands (oxygen ions), while one of the hybrid orbitals becomes empty. In this state the third and the fourth hybrid orbitals being unoccupied form the donor-acceptor bonds with two 2s2(1)2p4(3)-hybrid orbitals of the oxygen ion (Fig. 5b). The above said signifies that the new donor-acceptor bond of the Fe2+ ion appears instead of sigma-bond. It follows that, as a result of lighting, the re-distribution of electron density also takes place in the oxygen ion which possessed the sigma-bond with the iron ion before illuminating. One of the lone pairs which was donated to one of donor-acceptor bonds of the Bi3+ ion before the lighting now is transferred to the Fe2+ ion, and the bismuth ion has one electron on this bond. This forces Bi3+ ion also to redistribute its electron density in such a way as to reestablish the disturbed bond with the oxygen ion. For this purpose the Bi3+ ion takes one electron from the three-coordinated oxygen ion (Fig. 1) which, in spite of being three-coordinated, was in the 2s2(1)2p4(3)-hybridization state. Therefore, this oxygen ion had the lone electron pair which has not taken part in the bond formation in the crystalline lattice. The electron density redistribution in the oxygen ion mentioned produces the free light-excited electron vacancy (a hole) in the valence band of a crystal. This vacancy is able to migrate through the crystal like it occurs in any semiconductor. It follows from the general physical considerations that the excitation of an electron vacancy is more probable for the three-coordinated oxygen ions than for the four-coordinated ones. It is due to the fact that the energy of the electron detachment from the three-coordinated ions has to be lower than that for the oxygen atom, all hybrid orbitals of which take part in the formation of chemical bonds in a crystalline lattice. Also it follows from the description given that the valence band of the sillenite crystal has to be formed from the orbitals of oxygen atoms which donate electrons easily, whereas the conduction band is more likely to be formed from the orbitals of atoms accepting electrons (Bi, Fe).