Photoluminescence Studies on Lithium-Calcium Borophosphate Glasses doped with Transition Metal Ions

The mixed alkali borophosphate 25Li2CO3:25Ca2O3:30B2O3:20P2O5 glasses doped with 1 mol% of different transition metal ions were prepared and studied using photoluminescence techniques. The transition metal ions used included chromium, manganese and iron. Chromium doped glass exhibits a violet emission band at  ̴ 316 nm and this band has been assigned to the transition of zD1/2 → aP5/2. While manganese doped glass show a violet emission band at  ̴ 408 nm which is assigned to zP4 → aD4 and a red emission band at  ̴ 632 nm which is assigned to the transition of zP3 → aG6. Lastly, for the iron doped glass shows many emission bands along the visible range. | Borophosphate glass| Lithium | Calcium | transition metal ions | photoluminescence | ® 2012 Ibnu Sina Institute. All rights reserved. http://dx.doi.org/10.11113/mjfas.v8n2.128


INTRODUCTION
Glass can be understood as an amorphous solid which is completely lack of long range and periodic atomic structure.Glasses are important optical materials usually made to be transparent in the visible spectrum [1].Phosphate glasses have a wide range of technical applications [2].However, it is well known that a pure phosphate network is very hygroscopic and therefore is not stable.It has been demonstrated that the addition of B 2 O 3 to a phosphate network improves the chemical durability as well as the thermal and mechanical stability of pure phosphate glass [3,4].Borophosphate glasses belong to interesting glassy materials, e.g.zinc-calcium borophosphate glasses were studied as candidates for applications as low-melting glass solders or glass seals [5].Adding of network modifier to the glasses such as Lithium, Sodium or Potassium alters fundamentally the glass properties.Besides adding modifier, doping of transition metal ions also has attracted a great deal of attention due to their potential applications in solid state laser luminescent solar energy concentrators and fiber optic communication devices [6].
Furthermore, the most interesting features of transition metal ions is that they exhibit variable valency(oxidation state) and the valency changes in unit of one.When the transition metal ions are coordinated with other glass ions, the energy levels of the 3d electrons are split by the electric field of the coordinating ions.The delectron orbitals are strongly directional, so the splitting is sensitive to the arrangement of the surrounding ions.Thus, transition metal ions are used to probe the glass structure and to study the coordination numbers of the central ions due to their outer d electron orbital functions have a broad radial distribution.In general, the presence of transition metal ions additives increases the refractive index and transmission range the expense of increasing the ultraviolet absorption [7].A great number of researches have been carried to understand the luminescence properties of various rare earth ions doped glasses.While on the other hand, the research on transition metal ions doped materials are still very few [8].In order to examine the importance of glass doped transition metal ions, we have recently investigated the luminescence properties on the lithiumcalcium borophosphate glasses doped with transition metal ions.

Materials, method and instruments
The glass samples were prepared from raw material of Li 2 CO 3 , CaO, B 2 O 3 and P 2 O 5 according to the compositional formula 25Li 2 CO 3 :25CaO:30B 2 O 3 :20P 2 O 5 doped with 1 mol % of different transition metal ions (Cr, Mn, Fe).The corresponding weight of the starting material were weighed by the analytical balance and mixed thoroughly in porcelain crucible.This followed by placing the samples in an electric furnace at 400ᵒC for 20 minutes to facilitate evaporation of water and CO 2 .The samples were then melted in an electric furnace at 1100ᵒC for 5 to 10 minutes depending on the types of dopant used.Finally, the melts were then poured onto a stainless steel plate and press quickly with another stainless steel plate.
| 90 | The glass samples were polished with sand paper until the surface of the samples was flat to obtain the data from photoluminescence spectroscopy.Then, the glass samples of approximately 1cm× 1cm were selected to paste it onto a black paper as a holder.The photoluminescence properties of the glass samples were measured using Perkin-Elmer LS55 Spectroscopy in room temperature.Different excitation energy was employed to obtain the best spectra. .

RESULTS & DISCUSSION
The sample was visually checked and found to be homogeneous, colored and physical stable (nonhygroscopic).Table 1 summarized the samples composition that has been prepared.The emission spectrum of 25Li 2 CO 3 -25CaO-20B 2 O 3 -30P 2 O 5 with (a) 1 mol % of Cr ions, (b)1 mol % of Mn ions and (c) 1 mol % of Fe ions are shown in Fig. 1.The spectrum of Cr ions doped sample is obtained when the excitation of 240 nm radiation is used.While for the spectrum of Mn ions doped sample, the excitation of 260 nm radiation is used.Lastly, the spectrum of Fe ions doped sample, 260 nm radiations is used.
From Fig. 1, it can be seen that the Cr ions spectrum exhibit fewer emission bands than the Mn ions and Fe ions spectrum.As going from Cr ions → Mn ions → Fe ions, the number of emission peaks becomes more.This might be due to the nature of hyperfine structure of the transition metal ions.The hyperfine structure of the ions, lead to small shifts and splitting in the energy levels of atoms, molecules and ions.Therefore, it is more peaks observed in the Fe ions spectrum.
The emission spectrum of 1 mol % Cr ions doped 25Li 2 CO 3 -25CaO-20B 2 O 3 -30P 2 O 5 excited at 240 nm is shown in Fig. 2. The 240 nm excitation gives rise to three groups of peaks mainly in the range of ~283 to 377 nm, ~410 to 787 nm and ~808 to 857 nm.The location of the emission lines and their assignments are indicated in the energy level diagram in Fig. 3 and are summarized in Table 2.
From the transition of energy level obtained, it can be seen that, the transition of Cr ions in the ultraviolet region centered at 325, 347, 373 and 384nm respectively.As is shown in Fig. 1, the emission spectrum exhibits a blue emission band (~ 499 nm) and green emission band (~ 512 nm).
The Fig. 4 shows the emission spectrum of 25Li 2 CO 3 -25CaO-20B 2 O 3 -30P 2 O 5 with 1 mol% of Mn ions in the excitation of 260 nm.From the figure, it is clear that there are three main bands region which are from 293 to 419 nm, 551 to 698 nm and 754 to 888 nm respectively.The emission lines and their transitions are indicated in Fig. 5 and are summarized in Table 3.
From the Table 3, it shows that the transition of Mn ions emitted at ultraviolet region which is at ~ 235, ~ 261, ~ 282, ~ 352 and ~ 367 nm.The emission spectrum of Mn ions shows yellow emission (~ 552 nm), orange emission (~ 626 nm) and red emission (~ 669 nm).The Fig. 5 shows the emission spectrum of 25Li 2 CO 3 -25CaO-20B 2 O 3 -30P 2 O 5 with 1 mol% of Fe ions in the excitation of 260 nm.From the figure, it is clear that there are three main bands region which are from 272 to 489 nm, 516 to 689 nm and 754 to 861 nm respectively.The location of the emission lines and their assignments are indicated in the energy level diagram in Fig. 5 and are summarized in Table 4.
As summarized in Table 4, there are emissions bands

CONCLUSION
In summary, it is concluded that we have developed transparent and colored Cr, Mn and Fe ions doped lithiumcalcium borophosphate glasses for studying their photoluminescence properties.Due to the doping of transition metal ions in the glasses, they have been found that glasses are more stable.

Table 1 Nominal
compositions (mol%) of lithium calcium borophosphate glasses doped with transition metal ions.