Investigations on Spectroscopic Properties of Manganese Ions in Sodium Lead Alumino Borate Glasses

: Divalent manganese ions containing ranging from 0 to 1 mol%) NPAB glasses are prepared by using melt quenching technique and are characterized by several spectroscopic techniques. Various physical parameters are calculated from the measured density values. Structural properties of MnO 2 doped NPAB glasses are investigated by Powder XRD, EPR, UV-VIS absorption and FT-IR spectroscopic techniques. The amorphous nature of prepared glass samples are confirmed by X- ray diffraction spectra. EPR spectra of these glasses exhibited a well resolved six line hyperfine structure around g = 2.00 is the characteristic nature of Mn(II) ions in distorted octahedral site symmetry. Optical absorption spectra of these glasses revealed a single broad band at 21000 cm -1 which has been assigned to the transition 6 A 1g (S) → 4 T 1g (G) of Mn(II) ions. Optical band gap energies and Urbach energies are evaluated through the fundamental Ultra Violet absorption edges of the glasses. FT-IR spectra clearly exhibited different vibrational bands and assigned to borate groups and water molecules.


Introduction
Lead alumino borate glasses find several applications in battery ceiling and other microelectronic packaging because of their properties like toughness, good strength, transparency, excellent corrosion resistance, high mechanical strength and low thermal expansion coefficients [1]. B 2 O 3 acts as one of the most important glass formers and flux materials. Melts with compositions rich in B 2 O 3 exhibit rather high viscosity and tend to formation of glasses [2]. Thus, borate glasses are enormously studied by different groups across the world due to its rich structural variation [3]. By the addition of various alkali modifiers, drastic structural changes occur in the borate structural units with the evolution of four coordinated boron and non bridging oxygen (NBO) ions in the glass network. Alkali-borate glasses have high mechanical strength, high electrical conductivity and relatively moistureresistant compared with pure borate glasses. Due to high electrical conductivity and ease in preparation of alkali-borate glasses, these are suggested as potential fast ion conducting solid electrolytes [4]. Al 3+ ions are generally used as intermediate species [5]. When alkali-borate glasses consist of heaviest metal oxides such as Al 2 O 3 , are more promising for photonics and optoelectronics. Because of its attractive properties of high melting point, excellent resistivity, high wears resistance, high chemical durability, and low cost availability of raw materials, these glasses have been widely studied [6][7][8][9]. Addition of PbO to these glasses makes the materials more magnetically and electrically inert and also decrease the viscosity, increase the electrical resistivity and the ability to absorb X-rays [10]. PbO in contrast with the conventional alkali metals, has ability to form stable glasses due to its dual role-one as modifier and the other as glass network former in both ionic [11] and covalent bonding [12,13] with PbO 4/2 pyramidal units connected in puckered layers [14]. The study of the glasses doped with transition metal ions and its physical properties is of considerable interest in the application point of view. The physical properties of these glasses can be enhanced by the addition of PbO to alkali alumino borate glasses. In recent years the study of glasses doped with transition metal ions have a wide range of applications in electronic, tunable solid state lasers, luminescent solar energy concentrators (LSCs) and fibre optic communication systems due to their memory and photo conducting properties [15].
Among various transition metal ions, Mn(II) is a distinctive luminescent ion with good potential applications. Mn-doped semiconductors are commonly used in magneto-optical, magneto-transport, spintronics, lasers, and infrared devices. Manganese-based thin film oxides can be utilized as electrode materials [16,17], electrochemical capacitors [18,19], rechargeable batteries, catalysts, sensors [20], magneto electronic devices [21]. Manganese is next to iron element, one of the most common impurities in glasses and it is used as coloring and decolouring agent [22]. In glass matrices, manganese ions exist in different valence states viz. Mn 2+ and Mn 3+ , in borate and phosphate glasses with both tetrahedral and octahedral coordination [1]. The content of manganese oxide present in the glass matrix at a given temperature depends upon the quantitative properties of modifiers, glass formers, size of the ions in the glass structure, their field strength and mobility of the modifier cation, etc., [3].
Studies of the coordination, bonding characteristics and covalency state of transition metal ions in glasses are very useful to understand the structure of the glasses. Changes in the chemical composition of glass may change the local environment of the transition metal ion incorporated into the glass, leading to ligand field changes, which may be reflected in the optical absorption and EPR spectra. Structural properties of vitreous systems are frequently studied by means of EPR of Mn(II) ions used as paramagnetic probes [23]. To the best of our knowledge there is no previous literature reported on EPR

Experimental
For the present study, a particular composition 20Na 2 O-15PbO-(5-x)Al 2 O 3 -60B 2 O 3 : xMnO 2 with six values of 'x' ranging from 0 to 1 is taken. The detailed NPAB compositions of all the glass samples are listed in the Table 1.

Glass Preparation
The starting materials used in the present work are analar grade reagents sodium carbonate (Na 2 CO 3 ), lead oxide (PbO), aluminum oxide (Al 2 O 3 ), boric acid (H 3 BO 3 ) and manganese oxide (MnO 2 ) with 99.9% purity. All the chemicals are weighed accurately using an electrical balance which are grounded to fine powder and mixed thoroughly. The batches are taken in silica crucibles and melted in an automatic temperature controlled electrical furnace in air at 1100 0 C about 20 minutes. The resultant bubble free transparent liquid is then poured on a brass mould and subsequently annealed to relieve the structural stress, about 24 hours at 350 0 C.
The glasses thus obtained are transparent and brown. After polishing, good quality glasses obtained have been used for optical measurements.

Measurement
By Archimedes principle, the density (d) of the glass samples is measured at room

Physical parameters
Based on the measured glass density (d) and calculated molecular weight (M), various physical parameters such as manganese ion concentration (N i ), mean manganese ion separation (r i ), Polaron radius (r p ), and optical basicity (Λ th ) are evaluated using conventional formulae [24] and the values obtained are presented in Table 2.  Table 3.

Optical absorption studies
The  Table 4.
With an increase in the concentration of MnO 2 , the intensity of octahedral band is observed to grow gradually with minor red shift. The position of the bands is shifted slightly towards the lower frequency side with increasing concentration of MnO 2 .
The study of optical band gap in the amorphous materials is the subject of considerable research. This is related to the energy gap between valence and conduction bands [25]. In glasses, the study of optical absorption in the ultraviolet region is a useful method to understand the band structure. The conduction band is influenced by the glass forming anions, the cations also play an indirect but important role [25]. In general, there are two kinds of optical transitions at the fundamental edge of crystalline and non-crystalline semiconductors; direct and indirect transitions. In the direct optical transition from the valence band to the conduction band, it is essential that the wave vector for the electron be unchanged [26]. In case of the indirect transitions, interactions with lattice vibrations (phonons) takes place, thus the wave vector of the electron can change in the optical transition and the momentum change will be taken or given up by phonons. Plotting (αhν) 1/2 as a function of hν suggested by Tauc [27] and discussed totally by Davis and Mott [28].
The absorption coefficient α(ν) can be related to optical band gaps of direct and indirect transitions following Davis and Mott [28].
where B is the energy-independent constant [30], E opt is the energy of the optical band gap and hν is the incident photon radiation. For indirect transitions n = Urbach energies obtained in the present work are listed in Table 5.  Table 6.

Powder X-ray diffraction (PXRD) study
X-ray diffraction is a useful technique to detect the structural changes in any type of materials. The dimensions of glassy materials are greater than typically 100 nm [34]. The PXRD pattern exhibited a broad diffuse scattering at angle, which indicated a long-range structural disorder characteristic of vitreous solids. In glasses, the atoms are not regularly spaced and nonuniform as in the case of crystals. Due to the variations in inter atomic distances of glasses; the peaks get broader. Thus, the undoped and MnO 2 doped NPAB glass samples are found to be completely amorphous in nature.

Physical parameters
The modification of the geometrical configuration, co-ordination number, cross-link density and dimensions of interstitial space may have influenced the density of the glass matrix.
Therefore, density is a powerful physical parameter which reveals the degree of change in structure with change in composition of glasses. From Table 2, it is observed that density values are increased gradually with the content of MnO 2 [35,36]. Polaron radius (r p ) values are found consistently less than that of inter ionic distance (r i ) in all glasses. Further, the values of r p and r i gradually decreased with increasing concentration of MnO 2 .a

Electron Paramagnetic Resonance (EPR)
The g values obtained for Mn(II) ions in the present glass system is comparable with that of other glass systems, such as lead alumino borate glasses [1], soda lime silicate glasses [37] and alkali sulphate glasses [38]. For Mn(II) ion, if we neglect the nuclear Zeeman term, the spin Hamiltonian term can be written as where µ B is the Bohr magneton, g is the electronic g factor, B is the Zeeman field, S is the Mn 2+ electronic spin=5/2, A is the hyperfine interaction tensor, I is the Mn(II) nuclear spin=5/2 and D and E are the non-axial zero field splitting parameters. indicate that the Mn(II) ions are present in octahedral symmetry [40]. According to Van Wieringen, there is a regular variation of hyperfine coupling constant(A) with covalence [41].
From the large hyperfine coupling constant values the ionic nature of bonding is high between Mn(II) ions and its ligands. The nature of bonding is successfully explained based on the hyperfine splitting g-value. If g-value shows a negative shift (g < g e ) with respect to the free electron value (g e =2.0023), then the bonding is ionic and conversely, if the shift is positive (g > g e ), then the bonding is more covalent in nature. In the present work, the observed 'g' value

Optical absorption studies
The optical basicity of an optical glass serves in the first approximation as a measure [42] of the ability of oxygen to donate a negative charge to the probe ion. Duffy and Ingram [43] reported that the ideal values of optical basicity can be calculated from the composition of the glass and the basicity moderating parameters of the various cations present.
Theoretical optical basicity values of the glass can be estimated using the following formula. (6) where n be the total number of cations present, z i is the oxidation number of the i th cation, r i is the ratio of number of i th cations to the oxides present and γ i is the basicity moderating parameter of the i th cation. The basicity moderating parameter γ i can be calculated from the following equation [42].
A plot is drawn between optical band gap energy and Urbach energy as a function of MnO 2 content and is shown in Fig. 8. From this figure it is noticed that the band gap energy and Urbach energy for these NPAB glasses are existed in opposing manner.

FT-IR study
FT-IR spectroscopy is recognized to provide insights into the interaction between alkali metal ions and borate glass network. Boron has smallest mass as compared to other network forming elements, thus the major vibrational modes associated with the glass network resolve well above 500 cm -1 [49]. From the FT-IR spectra, one can clearly identify three broad and is due to B-O-B linkages. Boron is a network forming cation it may occupy the centers of oxygen triangles or tetragonal sites [51]. Besides the borate fundamental groups, this spectra also reveal a group of prominent absorption bands in the near infrared range from 2200-3200 cm -1 due to hydroxyl or water groups, these bands are persistent in all compositions [52]. However with increase in the content of MnO 2 it is observed that the position of the bands due to BO 3 , BO 4 and B-O-B units are shifted slightly towards the higher frequencies. The shifting of band positions in the IR spectra is observed as the concentration of manganese ions increased.

Conclusions
Density values are increased and r p , r i values are decreased gradually with the content of