SiO2 is a familiar glass former amongst all components of the glasses under study, with tetrahedral [SiO4/2]0 units. The following chemical equilibrium, de-polymerization, arises upon the addition of modifiers like Na2O, PbO :
[SiO4/2]0 + Na2O → [SiO3/2O]− + 2Na+
[SiO4/2]0 + Na2O → [SiO2/2O2]2− + 2Na+
[SiO4/2]0 + Na2O → [SiO1/2O3]3− + 2Na+
Further Bi2O3 exhibits the weak glass forming nature with BiO3 structural units when associated with SiO2 in addition to modifying action with BiO6 octahedral units . Molybdenum ions may occur in two valance states viz., Mo6+ and Mo5+ in the glass network. There is a possibility for the reduction of Mo6+ ions to Mo5+ state, which take the position at octahedral sites with distortions due to John-Teller effect .
3.1 Physical parameters
Evaluation of density ρ, molar volume Vm and average molecular weight M of the prepared samples guided to calculate various physical parameters such as Mo ion concentration Ni, Mo mean ion separation Ri and polaron radius Rp. All the computed parameters are furnished in Table 1. The gradual increment in the values of density from M0 (4.338 g/cm3) to M5 (4.633 g/cm3) is owing to the structural modification and the continuous substitution of relatively insubstantial silicon ions by the weighty molybdenum ions.
The XRD patterns of M0 and M5 glasses are shown in Fig. 1. The noisy broad Bragg peaks reported endorse the glassy arrangement of the investigated specimens.
3.3 Optical absorption spectra
Optical absorption spectra, of the specimens prepared in this study, recorded in the wavelength region 200 to 1000 nm are represented in Fig. 2. The cut off wavelength of Mo free sample is observed at 367 nm. Where the optical absorption edge of M1 sample with 1 mol% of MoO3 is significantly increased to 425 nm and it has red shifted with the amalgamation of MoO3 up to 5.0 mol%. This abrupt escalation of the cut-off wavelength is a sign of the depolymerization of the network. The spectra also exhibited a wide absorption band between 550 to 800 nm which is ascribed to the excited Mo5+ (4d1) ion . With the increase of MoO3 content from 1 to 5 mol % the intensity as well as half width of the spectra are noticed to increase with a red shift of peak position. This increase of intensity proposes the gradual conversion of Mo6+ ions into Mo5+ ions during melting and it suggests the existence of the highest concentration of MoO5+ ions in the glass network of the sample with 5 mol % MoO3. Such Mo5+ ions cause added bonding imperfections and NBOs by forming Mo5+O3− molecular orbitals leading to depolymerization of the glass network.
An optical band gap (Eg) of glasses under study are estimated from the extrapolated curves between (αhν)1/2 and hν as presented in Fig. 3 and values are furnish in Table 2. As an outcome the optical band gap is found to decrease from 3.38 eV to 3.03 eV with the rise of MoO3 Concentration. Further, Urbach energy (ΔE) explores the information about the breadth of the extensions of localized states in the band gap, values are assessed by plotting the Urbach’s relation .
ln(α) = (hυ/ΔE) + const (4)
Fig. 4 represents these Urbach curves between ln(α) and hν of the glasses taken for study. The Urbach energy values are also presented in Table 2. It is identified that the Urbach energy ΔE of M0 (0.14 eV) is minimum and maximum for M5 (0.35 eV) sample. This type of spectroscopic analysis indicates that growth in the density of Mo5+ (Oh) ions in the samples from lower to higher concentrations of MoO3. The development of donor centres and the ensuing overlap of excited states of localized electrons entrapped on Mo5+ sites with the vacant 4d Mo states on the nearest Mo6+ sites lead to more invade of polaron band into the main band gap. The red shift of absorption edge, significant fall in the band gap and enhancement in Urbach energy can be attributed to this polaronic development .
3.4. EPR spectra
Fig. 5 depicts the EPR signals of MoO3 contained Na2O−PbO−Bi2O3−SiO2 glasses traced at ambient temperature. A main central line, aroused due to molybdenum isotopes (I = 0), bounded by satellite lines, corresponding to the hyperfine structure of odd 95Mo and 97Mo (I = 5/2) isotopes , with wee intensity is discernible in the EPR spectra. From these spectra the peak height and half width of the central line is observed to amplify with the rise of MoO3 content in the glass. It noticeably indicates the enhancement of Mo5+ ions with the increasing content of MoO3 in the glass composition. The maximum intensity observed in the spectrum of M5 sample shows the existence of higher proportions of Mo5+O3- ions compared to other ones. It is well known that molybdenum take part in the glass network as MoO3+ complex. The high charge lingering on the central molybdenum in the interim the drop of the net charge on the neighbouring oxygen ions and accordingly reduced the ability to donate an electron (σ–bonds) stimulates the strong π-bond with two axial oxygens of the compressed octahedron. As a result, MoO6 transforms into complex Mo5+O3- molybdenum ion. In other words, in the axially distorted environment Mo(V) is strongly displaced in its octahedron so that it adopts a pyramidal MoO5 configuration with a very short Mo–O bond length matching to the molybdenyl ion. This endorses the coordination of Mo5+ ions with five oxygen ions by C4v symmetry through Mo=O in studied glasses.
The spin Hamiltonian function  for the illustration of Mo5+ spectrum is
H = βSgB + SAI (5)
Here the symbols have the standard meaning, i.e., the Bohr magneton β, electron spin S = ½, g-tensor g, the nuclear spin of molybdenum ‘I’, hyperfine structure tensor ‘A’ and the applied field B. The degree of axial distortion α of the coordination polyhedron is estimated by (ge−g||)/4(ge− g^). The values of g||, g^, A||, A^ and a evaluated from the spectra are presented in Table 3 and also observed g||< g^< ge (2.0023) together with A||> A^, demonstrating the octahedral site with tetragonal compression. The observed trends of these parameters with the rise of MoO3 content support the above viewpoints.
3.5. FTIR spectra
FTIR spectra of glasses considered for the present study are portrayed in Fig. 6. The bands noticed at about 941, 781 and 486 cm-1 are assigned to the asymmetric, symmetric and bending vibrations of Si−O−Si linkages of silicate structural units, respectively. The weak band reported at about 415 cm-1 is attributed to the vibrations of Bi−O bonds in BiO6 units. Another band observed at about 1112 cm-1 is assigned to O−Si−O asymmetric stretching mode of vibrations of SiO4 structural entities linked with NBOs . Another two bands at about 843 and 877 cm-1 are detected which are due to the vibrations of MoO42- and MoO6 structural units . The intensity of the vibrational band due to MoO42- tetrahedral units is found to reduce and shifted towards greater wave number side whereas the peak height of the band due to MoO6 structural units is found to increase with a red shift, with the increase of MoO3 content [14-16]. Thus it is confirmed that the conversion of molybdenum ions from MoO4 to MoO6 state with the rise of concentration of MoO3. As well the intensity of the bands pertinent to asymmetric stretching vibrational modes (Si-O-Si, O-Si-O-) is noticed to rise at the cost of intensity of symmetric stretching vibrations of Si-O-Si with the rise of MoO3 content. The above spectral analysis strongly evidences the modifier role of molybdenum ions at higher concentrations of MoO3 and consequent depolymerisation of the present glass network.