Optical properties of borotellurite glasses containing metal oxides

Glass samples of the system: 5MxOy-20B2O3-75TeO2 : MxOy = WO3, Nb2O5, PbO, Nd2O3, Y2O3, Eu2O3 were prepared by melt quenching and characterized by X-ray diffraction, density, Differential Scanning Calorimetry, UVvisible and FTIR spectroscopy. XRD patterns confirmed the amorphous structure of all samples. Glass transition temperature was maximum in borotellurite glass containing Y2O3. Refractive index, atomic polarizability and basicity increased in the following order of ions: Y< Eu < Pb < Nd < Nb < W. FTIR studies showed that PbO is outstanding in enhancing the concentration of tetrahedral borons in the borotellurite network.


INTRODUCTION
Tellurite glasses have a wide range of applications.One of the commercial applications of tellurite glasses is in the field of optical communication due to their outstanding properties like high refractive index, high optical non-linearity and good infrared transmittance [1][2][3][4][5].Tellurite glasses have been reported to exhibit 30 times higher Raman gain coefficients than silica glass and find application in Raman amplifiers and non-linear optical waveguides [6,7].Many studies are reported on the technological importance of tellurite glasses containing transitional metal and/or rare earth ions in memory switching devices and as cathode materials for batteries [8].
In this work, we report the study of short-range structure and optical and thermal properties of borotellurite glasses doped with W 6+ , Nb 3+ , Pb 2+ , Nd 3+ , Y 3+ and Eu 3+ .Glasses were characterized by density, X-ray diffraction (XRD), Differential Scanning Calorimetery (DSC) and UV-visible and FTIR spectroscopy.The chemicals in the above mentioned compositions were weighed and sintered at 300 o C for 24 h and then melted in a temperature range of 800 o C to 900 o C in a platinum crucible.Glass samples were prepared by normal quenching technique in which a small amount of melt was quenched on a brass block and a button shaped sample was obtained and immediately transferred to a furnace kept at a temperature of about 80 o C lower than the glass transition temperature.Samples were annealed for 30 min and then slowly cooled to room temperature.

RESULTS AND DISCUSSION
X-ray diffraction measurements were performed on Bruker D8 Focus X-ray diffractometer with Cu K α radiation (λ =1.54056 Å) in the 2θ range of 10 o -70 o .Figure 1 shows the XRD patterns of borotellurite Density of glasses were measured by Archimedes principle using an electronic balance with an accuracy of 10 -4 g.Turpentine oil was used as an immersion liquid.Density and molar volume values are given in Table 1.Density increases on adding metal oxides in the glass network and is maximum for glass containing PbO [Figure 2].Thermal studies were performed on SETARAM SETSYS Evolution-1750 system in the temperature range of 200-850 o C at a heating rate of 10 o C/min, in air flow rate of 20 ml/min in Pt pans.From DSC scans, glass transition temperature, T g was determined [Table 1].T g is maximum for the sample having 5-mol% of Y 2 O 3 and minimum for glass containing Pb 2+ ion.All metal ions except Pb 2+ strengthened the binary borotellurite glass network due to the incorporation of metal oxide bonds with higher bond enthalpies.
Optical absorption spectra of polished disk shaped borotellurite glasses were measured at room temperature on Shimadzu 1601 double beam UVvisible spectrophotometer in wavelength range of 200-1100 nm.The optical absorption coefficient α(λ) was calculated by dividing the absorbance A, with sample thickness and is plotted in figure 3.  From α vs λ plots, absorption edge, λ o was determined as the wavelength at which α = 9 cm -1 [Table 2].The absorption edge shifted towards longer wavelengths on adding metal oxides.Optical band gap, E g of glasses was determined from λ o .E g was used to calculate the optical electronegativity χ, refractive index n,   The FTIR absorption spectra show three bands in the wavenumber ranges of 500-800 cm -1 , 800-1150 cm -1 and 1150-1550 cm -1 [Figure 4].The first band in the wavenumber range of 500-800 cm -1 is due to Te-O vibrations in different Te-O units.The bands in the wavenumber ranges of 820-1140 cm -1 and 1150-1550 cm -1 are due to B-O stretching vibrations in BO 4 and BO 3 units respectively [4].Areas under the second and third bands i.e.A 4 and A 3 are calculated and the ratios of these areas A 4 /(A 4 +A 3 ) is considered proportional to the fraction of tetrahedral borons (N 4 ) in the glass network.It is clear from the data presented in table 2 that N 4 is nearly the same for the undoped borotellurite sample: 20BTe and the glasses doped with Nd 3+ and Nb 3+ .Maximum N 4 is in the sample containing Pb 2+ and the minimum is observed in the glass with W 6+ .

CONCLUSIONS
Borotellurite glasses were prepared with 5-mol% of W 6+ , Nb 3+ , Pb 2+ , Nd 3+ , Eu 3+ , Y 3+ and their properties were compared with undoped borotellurite glass.Glass transition temperature is maximum for 5Y-20BTe sample.Refractive index, atomic polarizability and basicity increases in the following order of dopant ions: Y 3+ < 20BTe < Eu 3+ < Pb 2+ < Nd 3+ < Nb 3+ < W 6+ .Conversely optical energy gap is lowest for glass containing WO 3 and is highest for the glass with Y 2 O 3 .Also, PbO produces a maximum increase in N 4 while WO 3 suppresses it.

FIGURE 2 .
FIGURE 2. Density and molar volume variation in borotellurite glasses containing metal oxides.

FIGURE 4 .
FIGURE 4. FTIR absorption spectra of borotellurite glasses doped with metal ions.

TABLE 1 .
Molecular weight, density, molar volume and glass transition temperature of borotellurite glasses doped with metal oxides.
Optical energy gap of glasses containing metal ions was lower than undoped borotellurite glass (i.e.sample 20BTe) except for the sample containing Y 3+

TABLE 2 .
Optical energy gap and N 4 in borotellurite glasses containing metal ions.