Optical and luminescence properties of manganese doped sodium lead alumino borosilicate glass system

Glass system of composition 20Na2O-10PbO-(5-x)Al2O3-40B2O3-25SiO2: xMnO with ranging from 0.3 to 0.9 mol% has been prepared by melt quenching technique. Further, the samples have been characterized by X-ray diffraction technique (XRD). A number of studies have been carried out, viz., Optical absorption and photoluminescence techniques. Glass formation is confirmed by Xray diffraction spectra. The optical absorption spectra of these glasses has exhibited a predominant broad band peak at about 21,052 cm-1 (475 nm) is identified due to A1g(S)→T1g(G) octahedral transition of Mn2+ ion. From the optical absorption spectral data, optical band gap (Eopt) and Urbach energy (ΔE) are evaluated. The emission spectra of Mn2+: NPABS glasses have shown single and broad emission band at about 600 nm assigned to electronic transition T1g (G)→A1g(S) displaying red emission upon excitation at 413 nm. Octahedral coordination of Mn2+ ions has been estimated from the positions of emission in luminescence spectra. Various physical properties and optical basicity are also evaluated with respect of the concentration of Mn (II) ions.


INTRODUCTION
Glasses are amorphous material and are transparent in the visible region [1]. The structure of oxide glasses reveals their ionic conductivity and potential usage as solid electrolytes in a variety of electrochemical devices like solid state batteries, fuel cells, chemical sensors and smart windows [2]. Among all the classical network formers, B2O3 and SiO2 are one of the significant glass formers and flux material due to their high phonon energies [3]. The addition of Na2O reduces the melting temperature and facilitates the homogenization of the glass system, reducing defects and bubbles. Al2O3 is an important component of the glass systems. In tetrahedral coordination, it replaces silicon in the glass network. But at larger concentrations, Al2O3 can act as both network former and modifier and enhance the glass forming ability, chemical durability and thermal stability. So, Al2O3 plays an important role in borosilicate glass system. Lead glasses are widely used for decorative purposes because of their bright brilliance due to high refractive index [4]. Borosilicate glass is widely used in the manufacture of laboratory glassware, pharmaceutical containers, High-power electric bulbs etc. Because of their good heat resistant properties and thermal shock resistance they are used in chemical industry, domestic kitchen cooking utensils (microwave or oven ware). The main ingredients of borosilicate glass are silica (SiO2-70-80%) and boric acid (7-13%). Oxides of sodium, potassium and aluminium are added to borosilicate composition to get good chemical durability [5,6]. An addition of small amount of MnO to borosilicate glasses facilitates the enhancement in mechanical, optical, electrical and non-linear optical properties. Mn 2+ ions have strong bearing on the optical, magnetic and electrical properties of glasses. These ions can exist in different valence states with different co-ordinations in glass matrices, for example as Mn 3+ in borate glasses with octahedral coordination whereas in silicate and germinate glasses as Mn 2+ with both tetrahedral and octahedral environment [7]. Both Mn 3+ and Mn 2+ ions are well known paramagnetic ions. Mn 2+ ion has half-filled d orbital with d 5 configuration and 6 S as the ground state. For these reasons, the total orbital angular momentum for Mn 2+ ion is zero. Since the total spin is 5/2, this ion exhibits zero field splitting which is sensitive to the local environment [8]. Mn 3+ ion has a large magnetic anisotropy due to its strong spin-orbit International Journal of Luminescence and applications Vol6 (4) January, 2017, ISSN 2277 -6362 interaction of the 3d orbital while Mn 2+ ion has small anisotropy energy due to its zero orbital angular momentum. The manganese ions may be linked with antimony/borate groups, there by strengthening the glass structure and probably increasing the chemical resistance of the glasses [9]. The objectivity of the present investigation is to have a comprehensive understanding over the local environment of manganese ion in Na2O-PbO-Al2O3-B2O3-SiO2 glass system, by a systematic study of various physical parameters, coupled with spectroscopic (Optical and luminescence properties) investigations.

Sample preparation
A particular glass composition 20Na2O-10PbO-(5x)Al2O3-40B2O3-25SiO2: x MnO with x ranging from 0.3 to 0.9 mol% is chosen for the present study. The details of the composition and their corresponding labelling are given below: Raw materials of sodium carbonate (Na2CO3), lead oxide (PbO), aluminium oxide (Al2O3), Boric acid (H3BO3), Silicon dioxide (SiO2) and Manganese oxide (MnO) were taken in appropriate ratios. All the materials of chemicals were of analytical-grade with purity of 99.9 %. All reagents were thoroughly mixed in an agate mortar and melted in a silica crucible in an electric furnace at temperature 1200 0 C for 20 min until a bubble free liquid is formed. At the end of the melting process in order to obtain homogeneous and the melts are poured on brass plate and annealed at a temperature 400 0 C for 3 h and cooled slowly to release the thermal stress associated with these glasses during the quenching process. The glass matrix is obtained transparent.

Characterization techniques
The optical absorption spectra were recorded on a JASCO UV-VIS-NIR spectrophotometer (Model V-670) at room temperature in the range 200-1400nm. The X-ray powder diffraction pattern of prepared glass samples were recorded using on XRD-6100 SHIMADZU X-Ray diffract meter in the scanning range of 10-80 0 (2θ) using Cu Kα radiation having a wavelength of 1.5406 Å at room temperature. The photoluminescence spectra (PL) were recorded at room temperature on the fluorescence spectrometer (SPEX Flouorolog-3) using with a 450 W Xe-lamp as the excitation source. By using Archimedes's principle, the density of the glasses was determined to an accuracy of ±0.001 by means of O-xylene (99.99% pure) as the buoyant liquid. The refractive index of the glasses was measured using Abbes Refractometer and mono-bromonaphthalene as the contact layer.

XRD Spectra
From the Fig. 1 the XRD pattern of all the glass samples shows no sharp Bragg's peak, but only a broad diffuse hump around lower angle region. This is indication of amorphous nature within the resolution limit of XRD instrument [10].

Physical parameters
The physical properties of prepared glasses are very interesting and provide useful information regarding the structure and transmission mechanism due to transport of ions. The density of glass is one of the most important properties in manufacturing glass production and it is required for calculating other properties such as refractive index, elastic properties and thermal conductivity. The measured values of density and physical parameters such as dopant ion concentration (Ni), mean separation (ri), refractive index and optical basicity of these prepared glasses have been evaluated in Table 1. The progressive introduction of MnO has caused enhance in the density of the samples, the degree of structural compactness, the modification of geometrical configuration of the glassy network [11].

Optical absorption spectra
UV-vis spectroscopy is one of the most widely used techniques for structural characterization of glass materials. The absorption spectra of transition metal ions are influenced by the nature of the host matrices into which those ions are accommodated owing to the excitation spectra of 3d electrons. The absorption spectra of transition metal ions are fairly broader and sensitive to the changes in coordination and symmetry. Due to the presence of various oxidation states, each of the states can give increase to different absorption spectra which can be explained by the application of ligand field theory.  Table 2. The fundamental optical band gap of these glasses has been computed based on their optical absorption spectra, for understanding their optically  Table. 3.

Photoluminescence spectra
Luminescence characteristics are very sensitive and complex property mainly depends on the spin and parity forbidden transitions of electronic configuration and also on the local structure of luminescent species which is affected by the surrounding matrix.     6. In the present case, spectral position of emission band peaking at 600 nm exhibiting red emission having six coordination number with strong ligand field strengths confirms octahedral site symmetry of Mn 2+ [13][14][15].

CONCLUSIONS
The conclusions are drawn from studying various properties of Na2O-PbO-Al2O3-B2O3-SiO2 glasses doped with manganese ions are as follows: Amorphous nature of the samples is confirmed by the broad diffused haloes in XRD pattern. The density and refractive index of the samples are found to increase with increasing concentration. Optical absorption spectra of these glasses exhibit a predominant broad band peak at about 475 nm is identified due to 6 A1g(S) → 4 T1g(G) octahedral transition of Mn 2+ ion. MnO doped NPABS glasses have displayed a broad red emission band at 600 nm assigned to a spin forbidden transition of 4 T1g(G)→ 6 A1g(S). The CIE chromaticity color coordinates calculated from emission spectra of NPABSMn glasses show that the glasses emit warm white light. The band position of manganese emission confirms Mn 2+ state in octahedral position having six coordination numbers with strong crystal field strength. From this observation we have to conclude that the Mn 2+ ions predominately occupy octahedral positions in this glass network.