Synthesis, physical, optical and structural studies of B2O3-CdO-Al2O3-PbF2 glasses modified with MoO3 ions

This manuscript deals with the preparation and detailed physical and structural study of B2O3-CdO-Al2O3-PbF2 glasses reinforced with MoO3 ions. With the addition of MoO3, the density and band gap values decreased while the refractive index increased, confirming the transformation of Mo6+ ions to Mo5+ ions and resulting in the generation of NBO's in these glasses. The FTIR spectra revealed the presence of different borate and aluminate units. The FTIR spectra also indicated the conversion of Mo6+ ions to Mo5+ ions. The effect of MoO3 ions on Raman spectra was clearly visible and indicated the conversion of BO4 to BO3 units, increasing NBO’s which helped to explain the density variation.


Introduction
The extreme importance of glasses has been increasing in day to day regular needs and in most of the areas, which in turn impelled researchers to magnify their view in various chemical compositions with borate, silicate, phosphate, etc. for its large area of applications (Wang et al. 2019;Venkateswara and Shashikala 2014). Because of its low melting point, transparency, thermal stability, and solubility, borate is being considered mostly as a glass forming material (Chandra et al. 2018;Mahesh et al. 2017). Physical, chemical, and optical characteristics all change significantly when Al 2 O 3 is incorporated to the borate matrix. Depending on the proportion of Al 2 O 3 , it can be detected in the glass network as AlO 4 or AlO 6 units. An increase in Al 2 O 3 concentration can greatly improve mechanical strength, chemical durability, and moisture resistance capacity. Alumino borate glasses

Experimental
The glass samples were prepared with unique composition 60B 2 O 3 -20CdO-5Al 2 O 3 -(15-x) PbF 2 -xMoO 3 (where x = 0, 0.5, 1, 1.5, and 2 mol%). The method employed in preparation of glass samples was melt quenching. All the required chemicals were annular grade, which are weighed with required mole percentages for five sets of glass compositions. The dissimilar mole percentages of chemicals being measured using an electronic monopan balance and then physically mixed in an agate mortar until the chemicals were uniformly blended. This mixer of chemicals was taken in porcelain crucible and kept in an electronic furnace, which is under normal atmospheric conditions, maintained at 1000 °C temperature for 1 h to melt congruently. Further melts are stirred to gain homogeneity. The obtained melt was quenched on steel plate, which is at 200 °C for couple of hours to release the internal stresses and finally the glass samples are ready for further characterization.

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On a Philips Xpert Pro X-ray diffractometer, X-ray diffraction (XRD) spectra being measured in the range of 10°-80° for 2θ to know the nature of the specimens. The Archimedes principle is then used to evaluate density where Xylene is being used as the dipping liquid. For density measurements, three experimental pieces from each glass were collected, and the aggregate density of the three sample pieces was used to get the final density. The accuracy of the computation is 0.005 g/cc. Fourier transform infrared (FTIR) spectra were recorded using a Bruker FTIR spectrometer with a resolution of 0.5 cm −1 and a precision of 0.01 cm −1 . On a J. Y. H. LABRAM-HR Raman spectrometer, Raman spectra of BCAPM glasses were scanned in the wavenumber range of 200-1800 cm −1 . On a Shimadzu UV-1800 Spectrometer in absorption mode, the UV-Visible spectra of BCAPM glasses were scanned across a wavelength range of 200 nm to 1000 nm. At room temperature, EPR spectra were scanned between 2500 and 4000G using a Bruker EPR spectrometer. Figure 1 shows the XRD patterns of the BCAPM glasses. The figure clearly displays wide humps between 2θ ≈ 20° to 30°. Furthermore, there are no sharp peaks, implying that BCAPM samples are amorphous (Table 1).

Physical and optical properties
The density is indeed a significant physical characteristic which shares insights about solids; such evidence will be more beneficial if the structure is properly understood (Singh et al. 2014;Saddeek and Gaafar 2009). The volume of structural units contained in the glass may be determined using the density data of the samples. The density values in the current BCAPM glass system were dropped nonlinearly from 4.072 g/cc (BCAPM0) to 4.009 g/cc (BCAPM4). The molar volume (V m ) was calculated using the measured density and molecular weight (M) data (Table 2). In Fig. 2, the fluctuation of ⍴ and V m with MoO 3 is displayed.
The density of the BCAPM glasses drops non-linearly as the mole percentage of MoO 3 grows, as seen by the respective crystal densities of MoO 3 (4.70 gm/cc) and PbF 2 (8.44 gm/cc). Fractional substitution of PbF 2 (245.2 g/mol) by MoO 3 (143.94 g/ mol) might potentially explain the reduction in density. Absorption spectra of BCAPM glasses are presented in Fig. 3. The analysis of the valence and conduction band transitions using the primitive absorption edge in the UV-Visible zone is a fascinating method. The prime aspect of the absorption edge in non-crystalline solids is the absorption coefficient α(ν) with smaller values as a function of the photon energy hν (Urbach and Phys. 1324).
where ΔE is Urbach energy and C is a constant. The α (ν) can be obtained from the absorbance A by using the equation below; (1) where 'd' is the thickness. Davis et al. (1970) and Tauc et al. (1972) suggested relation: Figure 4 shows the Tauc plot of BCAPM glasses. The optical band gap (E g ) values (see Table 2) are acquired by extrapolating the linear region of the curve. With the enhancement of MoO 3 from x = 0 to 2 mol% in the current BCAPM glasses, E g declines from 3.063 eV (BCAPM0) to 2.951 eV (BCAPM4). With the insertion of MoO 3 , the bandgap drops, indicating that localised states for electrons near the conduction band are established. The bandgap can indeed be reduced by associating these localised states with the conduction band.
(2) The inverse slope of the linear section of the Urbach plot, as illustrated in Fig. 5, is used to calculate Urbach energy. The magnitude of disorder present in the glass network can be accessed by the Urbach energy. These values were found to be much lower for the BCAPM glass system, indicating that the glass structure had less disorder (Thakur et al. 2015).

Fig. 2 Variation of density and molar volume with MoO 3 in BCAPM glasses
The other parameters such as refractive index (n), molar electronic polarizability (α m ), and molar refraction (R m ) are evaluated using the following relations (Dimitrov and Sakka 1996;Dimitrov and Komatshu 2002;Duffy 1986). With an excess of MoO 3 in BCAPM glasses, the n values were elevated from 2.379 (BCAPM0) to 2.410 (BCAPM4). As both R m and α m are related to molar volume, they show a raising trend with composition.

FTIR spectroscopy
FTIR spectroscopy is the measurement for acquiring infrared absorption or transmission spectra in order to detect the presence of various functional groups in materials. The influence of MoO 3 on the structural characteristics of BCAPM glasses containing lead fluoride may be studied using FTIR spectra. The infrared absorption spectra of BCAPM glasses are shown in Fig. 6 in the wave number range of 400 to 1600 cm −1 . The various IR bands observed from FTIR spectra are 423,461,517,579,673,724,901,995,1061,1116,1168,1279,1376,1466, and 1549 cm −1 and their assignments are listed in Table 3. In general, the IR examination reveals two distinct frequency zones. The stretching vibrations BO 3 and BO 4 borate units are attributed to the ranges from 1200 to 1600 cm −1 and 800 to 1200 cm −1 , respectively (Saddeek 2004;Balachander et al. 2013;Chandra et al. 2021d). Significant differences in the infrared spectra during analysis revealed the emergence of three primary broad bands at around 670, 1050, and 1380 cm −1 . The widths of the bands become wider as the MoO 3 concentration increased, although the spectra indicated no change in the centre of the bands. Relying on their valence states, molybdenum ions can join the glass network as network modifiers or network formers, with Mo 5+ ions claiming the modifying sites and Mo 6+ ions claiming the former sites. By converting the hexavalent molybdenum ions to the pentavalent ions, the addition of MoO 3 at the expense of PbF 2 in BCAPM glasses increases Mo 5+ ions (which function as modifiers). The bands between 423 and 517 cm −1 in BCAPM glass system are attributed to Pb 2+ and Cd 2+ vibrations and vibrations of AlO 6 octahedral (Ahmed et al. 2022;Ashok et al. 2021;Vedavyas and Chandra 2021). The peak near 579 cm −1 might be attributed to the Mo-O bond vibrations in distorted MoO 4 (Iordanova 1994, Saddeek 2007Abouhaswa et al. 2021). IR band positions observed 673 cm −1 in BCAPM glasses indicates the elongations of B-O-B bonds in BO 3 units groups (Yadav et al. 2013).

Raman spectroscopy
Vibrational spectroscopy is amongst the best means for understanding the structure of glasses. Glasses comprise structural units that are equivalent to crystalline examples and may be positioned freely in a 3D network while being amorphous. Among the most powerful vibrational spectroscopy approaches for exploring glass structure is Raman spectroscopy. Raman spectra of BCAPM glasses is shown in Fig. 7 and deconvolution spectra for BCAPM1 sample is depicted in Fig. 8 to locate exact Raman band positions. The various Raman bands observed from Raman spectra are 344 cm −1 , 498 cm −1 , 674 cm −1 , 789 cm −1 , (983 cm −1 shift 925 cm −1 ), 1079 cm −1 , 1229 cm −1 , (1347 cm −1 shift 1337 cm −1 ), 1437 cm −1 , and 1738 cm −1 and their band assignments are given in Table 4. The Raman band rising sharply at 344 cm −1 is ascribed to corner-shared MoO 6 octahedra (Kaur et al. 2016;Sekiya et al. 1995). Another band existing at 498 cm −1 is due to isolated diborate groups (Alemi et al. 2006). Anti-symmetric elongations of O-Mo-O linkages were observed in BCAPM glasses due to the presence of the Raman band at 674 cm −1 (Seguin et al. 1995). Another Raman band present at 789 cm −1 is attributed to stretching vibrations of Mo-O-Mo linkages and the formation of AlO4 tetrahedra (Ahmed et al. 2022;Kaur et al. 2016). An important band increasing sharply in BCAPM glasses is observed at ~ 950 cm −1 and is ascribed to the vibrations of Mo-O and Mo = O bonds in single and paired MoO 6 units (Kaur et al. 2016;Sokolov et al. 2009). The two Raman peaks, one at   Meera et al. 1990). The bands near 1437 cm −1 in this glass system are ascribed to B-O − vibrations of the BO 3 units and the band at 1738 cm −1 is due to chain and ring type metaborate units (Meera et al. 1990;Dwivedi and Khanna 1995;Padmaja and Kistaiah 2009).

Conclusions
The glasses with the formula 60B 2 O 3 -20CdO-5Al 2 O 3 -(15-x)PbF 2 -xMoO 3 (where x = 0, 0.5, 1, 1.5, and 2 mol%) is been prepared using conventional melt quenching method. These samples were analyzed using physical, optical, FTIR and Raman spectroscopic techniques to investigate the impact of MoO 3 on the structural changes occur in the samples. Drop in the density of the samples with MoO 3 is attributed to the molecular weights of PbF 2 /MoO 3 and the transformation of BO 4 to BO 3 units. Decline in the optical band gap values was observed, which were evaluated from UV-Visible spectra. This decline is due to transformation of Mo 6+ to Mo 5+ ions as MoO 3 content is increased at the expense of PbF 2 . FTIR and Raman spectra confirmed the presence of different borate and aluminate groups. These spectra also supports the transformation of Mo 6+ to Mo 5+ ions and confirmed that the MoO 3 act as modifier in these glasses.
Funding The authors have not disclosed any funding.

Conflict of interest
The authors whose names are listed immediately below the title of the manuscript certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.