Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 3(ISC-2013), 336-339 (2014) Res. J. Recent. Sci. International Science Congress Association 336 Effect of Alkaline-earth Metals on Physical and Optical Absorption Studies of Alkali Fluoroborate Glasses V. Rajashekar Reddy, R. Vijaya Kumar, V. Atchaiah Naidu and P. KistaiahDepartment of Physics, University College of Science, Osmania University, Hyderabad-500007, INDIA School of Physics, University of Hyderabad, Hyderabad-500046, INDIAAvailable online at: www.isca.in, www.isca.me Received 29th October 2013, revised 21st February 2014, accepted 3rd March 2014 AbstractAlkali fluoroborate glasses were prepared using melt quench technique. The prepared glass samples have been characterized using XRD, MDSC, density and molar volume measurements. Density and glass transition temperature increase with atomic mass of alkaline earth metals. The thermal stability values decrease with the increasing mass of alkaline-earth metal. The values of cut off wave length, optical energy band gap are determined from the optical absorption spectra. The cut-off wavelength decreases while the band gap energies increase with increasing the mass of the alkaline-earth metal. Urbach energy, electron polarizability of oxide ion and optical basicity values decreased with increasing mass of alkaline earth metal. Keywords: Fluoroborate glass, alkaline earth metal, XRD, MDSC, thermal stability, optical absorption. Introduction In recent years alkali fluoroborate glasses have attracted great attention because of their interesting optical properties. Addition of alkali fluorides make the glasses more moisture resistant when compared to alkali oxides. The borate glasses containing alkali fluorides are suitable for radiation dosimetry applications. Addition of fluorine lowers the dielectric constant and polarizability of the glass . The addition of alkali earth ions increases the chemical stability of the glass. Material and MethodsAnalar grade chemicals were used to prepare the glass samples according to the molecular formula 30NaF-10AF-60B(A=Ca, Sr and Ba). The batch compositions in mol% of glasses prepared in the present work are given in table-1. Table-1 Chemical composition of prepared glassesS.No. Glass composition Glass code 1 30NaF-10CaF 2 -60B 2 O 3 NCB 2 30NaF-10SrF 2 -60B 2 O 3 NSB 3 30NaF-10BaF 2 -60B 2 O 3 NBB Analar grade reagents NaF, CaF2, SrF, BaF2 and HBO were taken in appropriate proportions and ground together to constitute a 15 g batch. The ground mixture was melted in a porcelain crucible at temperature of 1050°C for 2 hours until bubble free liquid is formed. The melt was then poured into pre-heated steel mould and annealed at temperature 320C for 5 hours to remove residual internal strains. For an easy reference, these glasses have been labeled as NCB, NSB and NBB as presented in Table-1.The glass formation was confirmed by X-ray diffraction measurements. The XRD profiles were recorded on a Philips X-ray diffractometer PW/1710 with Cu-Kradiation in the Bragg’s angle region 20° 2 80° and are shown in Figure-1. Figure-1 XRD patterns of prepared glasses The glass transition temperature (T) was measured in all the glass samples using Modulated Differential Scanning Calorimeter (TA Instruments, DSC 2910). All the samples were heated at the rate of 10°C/min in the temperature range 350 -550 C. The MDSC thermograms are shown in Figure-2. Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 336-339 (2014) Res. J. Recent. Sci. International Science Congress Association 337 Figure-2 MDSC thermograms of prepared glasses The optical absorption spectra of the glass samples were recorded at room temperature in the UV-Vis region by using a UV-Vis Elmer Lambda 950 spectrophotometer. Results and Discussion The XRD profiles of the prepared glass samples show no discrete sharp peaks but exhibit broad hollow band in the Bragg angle region 20 2 300 indicating their amorphous nature. The increase in glass transition temperature is attributed to increase in molecular mass of alkaline earth metals. Glass transition temperature can be used to indicate the glass network rigidity. Yiannopoulos et al also observed similar increase in glass transition temperature (T) in binary glass systems xMO-(1-x) B (MO= MgO, CaO, SrO and BaO) and hence the observed increase in T from CaF to BaF can be attributed to increase in boron-oxygen connectivity from CaF to BaF Thermal stability: The temperature difference between the crystallization temperature (T) and glass transition temperature (T), i.e. T = Tc - T is a measure of the thermal stability of prepared glasses. The values of thermal stability of glasses NCB, NSB and NBB are 43C, 38C and 30C. Table- 2 shows that the thermal stability (T) decreases with the increasing mass of alkaline earth metals from calcium to barium. Density and molar volume: The density (D) of the glasses was determined by the standard Archimedes’s principle using xylene (99.99% pure with density 0.86 gm cm-3at 25C) as the buoyant liquid. Micro balance with ± 0.1 mg of variation is used for weighing of the glass samples. The glass sample was suspended on a very thin copper stand that was set in the xylene container. The weights of the sample were determined in the presence of both the liquid and air. The density of all glass samples was estimated from the formula 86.0 (1) where w and w2 are the weights of the glass sample in air and xylene respectively. The density of xylene is 0.86 gm/cm. Density values are precise to ±0.02g/cm. The molar volume of a given glass composition is calculated using the formula, D MViim  (2) where M denotes the molar mass of the glass. The molar volume relates directly to the spatial distribution of the oxygen in the glass network. The density and molar volume values are given in Table-2. Table-2 Physical and thermal parameters of prepared glassesGlass code Density (g/cc ) Molar volume (cc/mole) Tg C) C)T C) NCB 2.39 39.57 470 513 43 NSB 2.58 38.50 474 512 38 NBB 2.71 38.49 476 506 30 Table-3 Glass code (nm) Optical band gap energy (eV) E (eV) o2- (Å 3 ) (Eopt indirect direct NCB 323 2.86 3.26 0.67 4.46 1.29 NSB 317 3.12 3.48 0.43 3.81 1.23 NBB 300 3.36 3.80 0.35 2.75 1.06 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 336-339 (2014) Res. J. Recent. Sci. International Science Congress Association 338 The density of present glasses increased with the increase in atomic mass of alkaline earth metals. A reverse trend is observed in the case of molar volume. The increasing values of density reflect the tightness of the glass structure and increase of network rigidity5,6. Decrease in the molar volume is easily explained by the difference between the atomic masses of Ca, Sr and Ba, as well as by the decrease in the total number of atoms in the volume of the glass. The increase in density with decrease of molar volume is another indication of more compactness of the glass structure.This result is in support of the DSC findings in the present study.Figure-3 showsthe variation of density and glass transition temperature with atomic mass of alkaline-earth metals. Figure-3 Variation of density and glass transition temperature with atomic mass of alkaline-earth metals Optical absorption studies: The optical absorption spectra of all glass samples are shown in Figure-4. A distinct cut off wave length () was observed for each glass sample and the corresponding values are presented in Table-3. Table-3 From the table, it is found that the observed values of cut-off wave length shifts towards lower wavelength with the increasing atomic mass of alkaline earth metals. The study of optical absorption is useful for the investigation of the band structure and energy gap in both crystalline and non-crystalline materials. The absorption coefficient ) can be determined near the edge using the relation. )=(1/d) ln(I/I) = 2.303(A/d) (3) where the factor ln(I/I) is the absorbance ‘A’ at a frequency and d is the thickness of the sample. For amorphous materials the optical absorption at higher value of ) ( 10cm-1) above the exponential tail follows the power law given by Davis and Mott , ) = B(h - Eoptn / h (4) Figure-4 Optical absorption spectra of prepared glasses where B is an energy independent constant and n takes values of 1/2, 2, 1/3, 3 for direct allowed, indirect allowed direct forbidden and indirect forbidden transitions respectively. By plotting (1/2 and ( as a function of photon energy (h) (figure-5 and figure-6), the respective values of E(opt) are obtained by extrapolating to (1/2 = 0 for indirect transitions and ( = 0 for direct transitions. The corresponding values are given in table-3. Figure-5 versus photon energy (h) Figure-6 versus photon energy (h) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 336-339 (2014) Res. J. Recent. Sci. International Science Congress Association 339 The non-linear variation of ln() with photon energy is found near UV absorption edge. This exponential behavior of absorption coefficient near the edge can be represented by ) = B exp (hE) (5) where E is the Urbach energy and is found as the inverse slope of ln () vs. h plot as shown in the figure-7. Figure-7 Urbach energy plot Urbach energy, which corresponds to the width of localized states, is used to characterize the degree of disorder in amorphous and crystalline systems. The E values of the present glass systems are in the range 0.67-0.35 eV (table-3). It can be observed that the Urbach energy deceases with the increasing atomic mass of alkaline-earth metals. Electronicpolarizability and optical basicity: Polarizability is related to several physical and chemical properties such as optical basicity, chemical stability and optical non linearity10. The electronic polarizability is useful in finding the non-linear optical (NLO) response of a material. According to Dimitrov and Sakka11, the electronic oxide ion polarizability (o2-) of a simple oxide is given by () ()2052.2optopt (6)= molar cation polarizability, N2-= number of oxide ions in formula = Molar volume, E (opt) = Optical band gap energy The calculated values of oxide ion polarizability (o2-) for the studied glasses vary between 4.46 to 2.75 Å (table 3). These values are comparable with those reported for other fluoroborate glasses. According to Duffy12,13 the optical basicity (), an ability to donate negative charges to the probe ion, is given by 67.1opt (7) Duffy and Ingram reported that the optical basicity, an ability to donate negative charges to the probe ion, can be predicted from the composition of the glass and the basicity moderating parameters of the various cations present in the glass. The values of optical basicity for the studied glasses are found to vary in the range 1.29 to 1.06 (table 3). Conclusion The absence of sharp peaks in the X-ray diffraction pattern indicates the amorphous nature of prepared glasses. Density and glass transition temperature values are increased with the mass of alkaline earth metals while the molar volume decreases. This indicates the increase of network rigidity with increasing the mass of the alkaline-earth metal. The direct band gap energy and indirect band gap energies are increased while the cut off wave length and Urbach energies decreased with the mass of the substituted alkaline earth metals. Alkali fluoro borate glasses are highly polarizable due to their large ionic radii and high electro negativity. Acknowledgments UGC-DAE Consortium, Indore centre for providing experimental facilities. 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