Optical and Physical Properties of Bismuth Borate Glasses Doped With Dy

This study reports on physical and optical properti es of Dy doped bismuth borate glass. The glasses containing Dy 3+ in (70-x)B2O3:30Bi2O3:xDy2O3 (where x = 0.0-2.5 mol%) have been prepared by melt-quenching method. In order to understand the r ol of Dy2O3 in these glasses, the density, molar volume and optical spectra were investigated. The r esults show that molar volume of the glasses increa se with the increasing of Dy 2O3 concentration and consequently generating more non -bridging oxygen (NBOs) into glass matrix. The absorption spectra of Dy doped in bismuth borate glass correspond with several bands, which are assigned from the ground s tate, H15/2 to F3/2(761 nm), F5/2(806 nm), F7/2(907 nm), (H7/2, F9/2)(1099 nm), ( F11/2, H9/2) (1283 nm) and H11/2(1695 nm). Moreover, the optical basicities were also theoretically determined.


INTRODUCTION
Glasses doped with Rare-Earth ions (RE n+ ) are proving to be luminescence materials as they have high emission efficiencies. These emissions correspond to 4f-4f and 4f-5d electronic transitions in the RE n+ . The 4f-4f transition gives an especially sharp fluorescence pattern from the Ultra Violet (UV) to the infrared region. This is due to shielding effects of the outer 5s and 5p orbitals on the 4f electrons. In recent years, glasses doped with rare-earth ions have drawn much attention due to their potential applications in solidstate lasers, optical amplifiers and three-dimensional displays (Malchukova et al., 2005;Vetrone et al., 2002;Biju et al., 2004;Lakshminarayana et al., 2009).
On the other hand, for higher valent oxides, such as Bi 2 O 3 when used as a modifier, the cation produces important structural effects due to its highest valence. In the literature, it is supposed that Bi 2 O 3 occupy both network-forming and network modifying positions. Therefore, the physical properties of such glasses exhibit discontinuous changes when the structural role of the cation switches over in this way (Baia et al., 2003). Also, glasses containing Bi 2 O 3 have attracted a considerable attention because of their wide applications in the field of glass-ceramics, thermal and mechanical sensors, reflecting windows, radiation shielding and because they may be used as layers for optical and opto-electronic devices (Venkataraman and Varma, 2006). These glasses have a Long Infrared (IR) cut-off, which makes them ideal candidates for optical transmission (Bale et al., 2008). Bismuthate glasses containing alkali oxide act as ionic conductors and possess high conductivity compared to other heavy metal glasses (Gahlot et al., 2005). However, the study of the physical properties of the ionic glasses has been paid little attention.
In this study we report on optical, physical and structural properties of Dy 3+ ion-doped bismuth borate glass in formula (70-x)B 2 O 3 :30Bi 2 O 3 :xDy 2 O 3 (where x = 0.0-2.5 mol%). All these glasses were prepared by using high purity grade of Bi 2 O 3 (Fluka), H 3 BO 3 (Fluka) and Dy 2 O 3 (Fluka) as raw materials. Each batch weighing about 30 g was mixed homogeneously and melted at 1100°C for 3 h in an alumina crucible, in air. The melts were poured onto a preheat stainless steel plates. These glasses are in rectangular designs with a good transparency. All the glasses were annealed below the glass transition temperature to remove thermal strains. Finally, the as-prepared glass samples were cut and then finely polished to a dimension of 1.0×2.0×0.3cm for properties investigation.

MATERIALS AND METHODS
Measurements: By applying Archimedes principle, the weight of the prepared glass samples was measured in air and in xylene using a 4-digit sensitive microbalance (Denver, Pb214). Then, the density, ρ, was determined using the relation: Where: W a = The weight in air, W b is the weight in xylene ρ b = The density of xylene (ρ b = 0.863 g cm −3 ) The corresponding molar volume (V M ) was calculated using the relation, V M = M T /ρ, where M T is the total molecular weight of the multi-component glass system given by: The optical absorption spectra were recorded at room temperature using a UV-visible-NIR spectrophotometer (Shimadzu, UV-3100), working in 190-2100 nm.

RESULTS
Based on the determined density, the various physical properties of the glass studied are present in Table 1. Therefore, the Figs. 1-2 illustrate density and molar volume, respectively, as function of Dy 2 O 3 concentration (%mol). Figure 3 shows the typical absorption spectrum of bismuth borate glasses undoped and doped with Dy 3+ 0.5-2.5 % mol.
Where: Λ 1 , Λ 2 and Λ 3 = Basicities of the oxide components x 1 , x 2 and x 3 = Their equivalent fractions (fraction of the total oxygen provided by the component oxide glass) The optical basicity of the glass samples are evaluated and listed out in Table 1. Figure 4 illustrate optical basicity as a function of Dy 2 O 3 concentration (%mol). Fig. 1, although the relative molecular mass of Dy 2 O 3 is higher than B 2 O 3 , density is found to not depend on Dy 2 O 3 concentration. Boron oxide is well known conventional network former. It consists of a random three-dimensional network of 6membered boroxol rings (Suzuki et al., 2002), when some modifier is added, coordination number of boron atoms changes from 3 to 4. As a result of this, Non-Bridging Oxygens (NBOs) would start to form. It is well reported that at low concentration of Dy 2 O 3 acts as network modifier in place of network former in bismuth borate glass system. So, the increase in molar volume (Fig. 2) may indicate that the volume of NBO sites produced by the modifier Dy 2 O 3 .

Physical properties: From
Optical absorption: The optical absorption edges are not sharply defined in glass samples under study, in accordance with their amorphous nature (Chimalawong et al., 2010). It is observed that the absorption intensity of the observed bands increase with the increase of Dy 2 O 3 concentration. From the Fig. 3, the spectra consist of various absorption levels corresponding to the transitions between the ground state and higher energy states. The bands are assigned from the ground state, 6 H 15/2 . The transitions from the next excited state 6 H 13/2 may be ruled out due to thermalization as the energy gap between 6 H 15/2 and 6 H 13/2 is around 3000 cm −1 . From this spectra, the levels of 4 I 13/2 , 4 F 7/2 , 4 G 11/2 , 4 I 15/2 are not observed. The absorption peaks at 6 F 3/2 (762 nm), 6 F 5/2 (805 nm), 6 F 7/2 (905 nm), ( 6 H 7/2 , 6 F 9/2 ) (1100 nm), ( 6 F 11/2 , 6 H 9/2 )(1280 nm) and 6 H 11/2 (1695 nm) are observed and well resolved. The position and intensity of certain transitions of rare-earth ions are found to be very sensitive to the environment around the ion. Such transitions are termed as hypersensitive transitions (Jorgensen and Judd, 1964). All known hypersensitive transitions obey the selection rule |∆S| = 0, |∆L| ≤2, |∆J|≤2 (Jorgensen and Judd, 1964). In the case of Dy 3+ ( 4 f 9 ) ion, the hypersensitive transition ( 6 F 11/2 → 6 H 9/2 ) is found to be more intense than the other transitions.
Optical basicity: Theoretical optical basicity serves in the first approximation as a measure of the ability of oxygen to donate a negative charge in the glasses. The theoretical optical basicity can be used to classify the covalent/ionic ratios of the glasses since an increasing Λ th indicates decreasing covalency (Sindhu et al., 2007). In context of modification, therefore, we may note the following: modifier oxides should be more basic than the glass forming oxides. When modifier oxides are added to glass-forming oxides, the resulting modification reaction is like and acid-base reaction in which the sites in the acidic (glass forming) oxide are approached by the oxide ion (of the modifier) in the order of decreasing acidities. It is clearly observed from Fig. 4 that the optical basicity increases when B 2 O 3 is replaced by one of the trivalent metal dysprosium oxide. The increase of optical basicity in this work means the higher ability of oxide ions to transfer electrons to the surrounding cations.

CONCLUSION
The Dy 3+ -doped bismuth borate glasses were prepared at various doping concentration of Dy 2 O 3 and characterized for their physical and optical properties. The density and molar volume increase with increasing concentration of Dy 2 O 3 . The increase of molar volume with Dy 2 O 3 content indicates that the extension of glass network due to the increase in the number of NBOs. The optical spectra were characterized using UV-VIS-NIR spectroscopy and show the six absorption bands in the range 190-2100 nm. The rise of optical basicity in the present glasses indicates the higher ability of oxide ions to transfer electrons to the surrounding cations.