SPECTROSCOPIC PROPERTIES OF LITHIUM SODIUM BORATE GLASS BY THE ADDITION OF MoO3

M. Sugathri 1 and P. Madhusudana Rao 2 . 1. Department of Physics Rayalaseema University,Kurnool. 2. Deparment of Physics, JNTUH, Hyderabad. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

The spectroscopic properties of Li 2 O -Na 2 O 3 -B 2 O 3 glass containing different concentrations of MoO 3 (ranging from 0 to 1 mol %) were prepared by melt quenching are studied. Spectroscopic (XRD, infrared, optical absorption spectra and EPR) properties of these glasses have been carried out. As the concentration of MoO 3 is increased especially beyond 0.4 mol %, the colour of the glasses increased, which is indicating the reduction of molybdenum ions from the Mo 6+ state to the Mo 5+ state. The Optical absorption Spectrum contains one broad absorption band around 695 nm is attributed to the excitation of Mo 5+ ( 4 d 1 ) ion. EPR studies reveal Molybdenum ions are expected to exist mainly in the Mo 6+ state in the glass network. EPR, FTIR and Optical absorption spectra of paramagnetic ion in the above glasses reveal that the site symmetry of the transition metal ion is elongated octahedral.

…………………………………………………………………………………………………….... Introduction:-
The glasses containing transition metal ions came into prominence because of their notable spectroscopic properties and their suitability for fibre optic communications, luminescent solar energy concentrators (LSC) [1].Among the many glasses, borate glasses have been known to be excellent host materials for transition metal oxides because of their glass forming nature compared to other conventional glass systems. Borate glasses are used as wave-guides, electro-optic switches, electro-optic modulators, magneto-optic materials, solid-state laser materials and non-linear optical parametric converters [2][3][4]. In addition, they are often used as dielectric and insulating materials and it is known that borate glass constitute a good shield against IR radiation [5]. In general B 2 O 3 glasses have a property of showing minimum expansion due to the large amount of boric oxide which is of great importance in obtaining thermal durability. It also decreases the tendency for glass to devitrify or crystallize. B 2 O 3 is a basic glass former because of its higher bond strength, lower cation size and smaller heat of fusion. The ions of molybdenum introduce high activity and selectivity in a series of oxidation reactions of practical importance in the glass matrices [6,7]. 1978 stable valence states viz., Mo (V) and Mo (VI) in the glass network. Earlier EPR studies on the glasses containing molybdenum ions have identified the presence of octahedrally coordinated Mo (V) ions along with octrahedral distortion approaching tetragons [14,15]. These ions act both as network formers as well as network modifiers depending upon their concentration and nature of the host network. The Mo 6+ ions are expected to participate in the glass network with tetrahedral MoO 4 2structural units and may alternate with BO 4 tetrahedral units. Most of the studies available on MoO 3 containing glasses are on the understanding of their structure by spectroscopic investigations [16,17] and ionic conductivity studies [18].
In the present work the influence of MoO 3 in varying concentrations, on resonance and absorption properties of lithium-sodium-borate glasses are studied. Optical band gaps for both direct and indirect transitions and Urbach energies were calculated and those glasses were also characterized by XRD and FTIR studies.

Experimental:-
The glasses of the system 20Li 2 O -10 Na 2 O -(70-X) B 2 O 3 (X=MoO 3 ) have been prepared by melt quenching technique varying the concentration of molybdenum ranging from 0.2-1.0(mol%) . The raw materials used were analytical pure (99.5%). Appropriate amounts of weighed chemicals Na 2 O, Li 2 O, B 2 O 3 , MoO 3 were grounded thoroughly in a mortar to produce homogeneous mixture. This homogenized mixture was then placed in porcelain crucibles and melted in an electrical furnace at 870℃-920℃ for half an hour until a bubble free liquid was formed . The melt was then quenched to room temperature by pouring it on a plane brass plate and pressing it with another brass plate. Finally the vitreous samples were annealed for 3 hrs at 423K to relieve residual internal stress and slowly cooled to room temperature. The glasses so obtained were brown in colour with good optical quality and transparency. Details of experimental procedure and calculations are similar to the earlier work [14].
Results and Discussion:-X-ray diffraction:-X-ray diffraction technique has been widely used to characterize inorganic glasses as it readily detects crystals in glassy matrix.    Fig. 3. The absorption edge is observed to shift towards higher wavelength side.
1980  Table II. The values of direct and indirect band gap energies, decreases from M0.2 to M1.0.   The EPR spectra obtained for the molybdenum ions in the titled glasses are similar to those obtained for various other glass systems [20][21][22]. The spectra shown in Fig. 4 can best be analyzed using a spin-Hamiltonian of the form: H = βSgB + SAI, where the symbols have their usual meaning. It contains the electronic Zeeman term (β is the bohr magneton, S = 1/2 the electron spin, g is the g tensor and B is the applied field), perturbed by the hyperfine coupling term between the unpaired electron and the nuclear spin I of molybdenum ion, A being the hyperfine structure tensor. The central line arises from even molybdenum isotopes (I = 0), while the smaller lines correspond to the hyperfine structure from odd 95 Mo and 97 Mo isotopes (I =5/2). The nuclear magnetic moments of 95 Mo and 97 Mo being close, so the isotope splitting is not resolved in the EPR spectra. It can be assumed that Mo 5+ ions in the glasses studied has an octahedral coordination with a weak axial distortion. An octahedral site with a tetragonal compression usually shows of and g| | < g< ge (=2.0023) (=2.0023) [23].It is found that the values obtained for the glasses under study, are more close to those of the molybdenyl ions. The values of g ║ and g⊥ from these spectra have been evaluated and presented in Table III. Further, the g values obtained for these glasses are found to be consistent with the reported values for many other glass systems containing molybdenum ions. A|| and A⊥ are the parallel and perpendicular components of the hyperfine tensor A. It is observed that the intensity of the resonance signal increases with increase in the concentration of MoO3. The EPR transition can be explained as because of decrease of the effective charge of the surrounding oxygen ligands, there is decrease in their electron donating ability (σ-bonds), hence the charge of the central molybdenum remains high. Therefore, this induces the promotion of strong π-bond with the two axial oxygens of the compressed octahedron hence axially distorted environment is developed. By this the Mo(V) is strongly displaced in its octahedron so that it adopts a pyramidal configuration MoO 5 with a very short Mo-O distance, which would correspond to the molybdenyl ion [24].