As2O3 for the following redox equilibrium to

As2O3 is a strong network former with corner
sharing AsO3 pyramidal units; normal bond lengths of As-O lie in
between 1.72-1.81 Å and O-As-O and As-O-As bond angles lie in the range 90-103o
and 123-135o respectively 22-23. PbO in general is a glass
modifier and enters the glass network by breaking for the As-O-As bonds
(normally the oxygen’s of PbO break the local symmetry while Pb2+
ions engaged interstitial positions) and commencement coordinate defects known
as dangling bonds forward with non-bridging oxygen ions. However, PbO may also
participate in the glass network of PbO4 structural units when lead
ion is linked to four oxygen’s in a covalence bond configuration. V2O5
is an incipient glass network former and as such does not readily
participate in the network forming but does so in the presence of modifiers
like PbO with VO5trigonalbipyramids and VO4 tetrahedral
units 24.

Vanadium ions are expected to exist mainly on V5+
states in the present PbO-As2O3 glass network. However,
during the liquescent of the glasses at more advanced temperatures there is
every eventuality for the following redox equilibrium to take place:

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2 V5+ + O2- ? 2 V4+ + O2á;

Further, Chung and Mackenzie 25 suggested that that a small
number of V5+ ions may also reduce to V3+ state, as per
the following equations:

As2O3 + V2O5 ? As2O5
+ V2O3

The V5+ ions take part network forming positions of VO5
structural units where as the V4+ ions may distort the glass
structure. As mentioned earlier As2O3 participate in the
glass network of the three member rings of AsO3 pyramids. Addition
of VO4 tetrahedral into the PbO- As2O3 glass
network will twist or distort the interconnected chains of AsO3 units
and increase the randomness of the glass matrix.

The optical absorption
spectrum of V2 glass shows two broad absorption bands of 645 and 971
nm; there is a noticeable shifting of the Meta centers of these bands of higher
Wavelength side with a gradual hike   in
the intensity with increase in the concentration of V2O5 up
to 1.0 mol%. V4+ ion belongs to d1 configuration with a
ground state of 2D. The presence of pure octahedral crystal fielded
the 2D state splits between 2T2g and 2Eg,
while an octahedral field with tetragonal distortion further splits the 2T2g
level into 2Eg and 2B2g; and 2Eg
level splits into 2a1g
and 2B1g.. Among these, the 2B2g
will be the ground state. Thus the vanadyl ions we can expect 3 bands (on the
basis of energy level scheme for molecular orbital’s of VO2+ ion in
a ligand field of C4v symmetry provided by Bullhausen and Gray 26)
corresponding to the transitions 2B2g?2B1g
(?^), 2B2g ?
2Eg (???) and 2B2g
? 2A1g.


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