Structural study on lithium-calcium borophosphate glasses doped with transition metal ions using infrared spectroscopy

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Wan Ming Hua, et al. / Structural study on lithium-calcium borophosphate…

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1. Introduction

Glass is an amorphous (non-crystalline) solid material. Glasses are typically brittle, and often optically transparent. B2O3 is one of the most common glass forming oxides and has been incorporated with various types of modifier metal oxide in order to attain the desired physical and chemical properties. Borate glasses have the advantageous property of good thermal shock resistance, which can be utilized for sealing to metals.

Recently, phosphate based glasses have been interest due to their several special properties such as large thermal expansion coefficients, low melting temperatures, solubility, etc.[1, 2]. Although phosphate glasses have a wide range of technical applications, it is well known that a pure phosphate network is very hygroscopic and therefore it is not very stable [3]. Therefore, it has been demonstrated that the addition of B2O3 to a phosphate network in order to improves the chemical durability as well as the thermal and mechanical stability of pure phosphate glass [4, 5]. The basic units of pure amorphous phosphate glasses are PO4

tetrahedra linked through covalent bridging oxygen, whereas the basic units of pure amorphous borate glasses are trigonal BO3 groups. Borophosphate glasses are among the multicomponent glasses studied for some interesting applications. Zinc-calcium borophosphate glasses were studied as candidates for applications as low melting glass solders or glass seals [6]. These glasses are advantages due to their properties can be effectively controlled by changing their composition.

In recent years, glasses doped with transition metal ions have attracted a great deal of attention because of their memory and photoconductivity properties. They find potential applications in solid state laser luminescent solar energy concentrators (LSCS) and fiber optic communications devices [7]. Much research has been done on the optical effects associated with transition elements in glass, primarily because these elements are useful coloring agents.

This paper is devoted to study the interaction with Lithium Calcium borophosphate was investigated in the compositional series of 25Li2O:25CaO:xB2O3:(50-x)P2O5 and sample doped with Cr²⁺, Mn²⁺ and Fe²⁺ ions. The aim of the present work is to investigate the vibrational mode about the local order around phosphorus tetrahedral structures and the boron coordination change from trigonal to tetrahedral structures due to the incorporation of mix alkali and alkali earth cation (Li, Ca) oxide using FT-IR spectroscopy.

2. Experimental Procedure

The investigated Li-Ca borophosphate glasses have the synthetic composition 25Li2O:25CaO:(x)B2O3:(50-x)P2O5 glasses where 0≤x≤50 mole % and 25Li2O:25CaO:

30B2O3: 20P2O5 doped with 1 mole % of Cr²⁺, Mn²⁺ and Fe²⁺ ions were prepared from Li2O, CaO, B2O3 and H3PO4 (85% liquid) by melt quenching technique. Well mixed powders containing the exact amounts of the raw materials were calcined at 200ôC for 30 minutes and 400ôC for another 30 minutes to release water and melted in a porcelain crucible at 1100ôC for another 15 minutes in an electric furnace. Finally, the melts were then poured onto a stainless steel plate and press quickly with another stainless steel plate.

The structural of the prepared samples were analyzed using analytical tools such as FT-IR spectroscopy. Measurements were carried out using KBr pellets method. The infrared spectra of the glasses had been recorded using a Perkin-Elmer Spectrum One FT-IR spectrometer over the range of wavenumber 400-4000 cm^-1 at room temperature using 100 scans at 4 cm^-1 resolution.

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21 3. Results and Discussion

Figure 1 shows that the FT-IR spectra of 25Li2O:25CaO:(x)B2O3:(50-x)P2O5 with x varies in the range 0≤x≤50 mole %. The infrared spectrum of the 25Li2O-25CaO-50P2O5

glass contains 1200 cm^-1, 970 cm^-1 and 940 cm^-1. The band at 940 cm^-1 can be ascribed to the asymmetric stretching vibration of the bridging oxygen atoms in the P-O-P bonds, Vas

(P-O-P). Several previous studies [8] have shown that the band corresponding to the stretching vibration of doubly bonded oxygen could be found in the frequency range 1200- 1390 cm^-1. Whereas the other broad band at 1200 cm^-1 belongs to the asymmetric vibrations, Vas (PO2), of the non-bridging oxygen atoms in the phosphate chains. The other band in the infrared spectra at 970 cm^-1 is ascribed to the symmetrical stretching vibration, Vs (PO2).

The band at 750 cm^-1is assigned to the symmetric stretching vibrations, Vs (P-O-P), of the bridging oxygen atoms. With increasing B2O3 content and decreasing P2O5 content, a series of changes in the spectra can be observed. The vibrations Vas (PO2) at 1200 cm^-1 become broader and weaker. The decrease in the strength of the vibrations of the non-bridging PO2

groups seems to indicate a progressive increase in the connectivity of the glass with increasing B2O3 content. It is likely that this connectivity is due to the formation of P-O-B links, which replace the P-O-P bonds. The vibrations of the bridging P-O-P groups, Vas (P- O-P), at 940 cm^-1 decrease and are almost undetectable in the sample with the highest B2O3

content. The formation of P-O-B links as B2O3 is added is suggested by the broadening of the bands in the region of 940 cm^-1(which is due exclusively to the B-O stretching of the BO4 units) [9]. As the B2O3 content increases, new bands appear at around 1100, 1270 and 1400 cm^-1. At around 1100 cm^-1 is assigned to the vibration of some boron atoms attached to NBO in the form of BO4 vibration. Whereas at the band 1270 cm^-1 is representing to the B- O stretching vibration of BO3 unit in metaborate and orthoborate groups. The band at 1400 cm^-1 is assigned to the vibration of BO4 groups.

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22

Fig. 1: FT-IR spectra of 25Li2O:25CaO:(x)B2O3:(50-x)P2O5 with x varies in the range 0≤x≤50 mole %.

Fig. 2 shows that FT-IR spectrum of 25Li2O:25CaO:30B2O3:20P2O5: (y) with y for Cr²⁺, Mn²⁺ and Fe²⁺ and 25Li2O-25CaO-30B2O3-20P2O5 undoped glass. From Fig. 2, the doped and undoped samples are almost the same. There are not many changes for the doped and undoped samples. There is a band at around 510 cm^-1 for the doped and undoped glass which is assigned to the B-O-B bond bending vibrations, covalent Ca-O bond vibrations.

Another band is presence at about 600 cm^-1 for the doped and undoped glass was attributed to B-O-B bending vibration. The addition of doped Cr²⁺, Mn²⁺ and Fe²⁺ ions to the 25Li2O- 25CaO-30B2O3-20P2O5 glass matrix shows that the band is a slightly shift to the right side compare to the undoped glass. For the undoped glass, the band at 770 cm^-1 shift to the lower band (710cm^-1) after doped with Cr²⁺, Mn²⁺ and Fe²⁺. This is assigned to the symmetric stretching vibrations, Vs (P-O-P) of the bridging oxygen atoms. The other band in the infrared spectra at 1050 cm^-1 is ascribed to the symmetrical stretching vibration, Vs (PO2). It shifted around 50 cm^-1 compared to the undoped glass. Besides that, the band for the glass at 1150 cm^-1 doped with Cr²⁺, Mn²⁺ and Fe²⁺ ions becomes narrow. The band at 1450 cm^-1 for the doped and undoped glass is assigned to antisymmetrical stretching vibration with three (NBOs) of B-O-B groups. The glass doped with Cr²⁺, Mn²⁺ and Fe²⁺ ions shows that the intensity of the bands described above increase and the position of the main bands shifts to the lower frequencies.

Intensity (a.u.)

x=50 x=40

x=30

x=20 x=10

x=0

1270 1400

1270

1270 1270 1400

1400 1400

1400

1230 1230

1230

110 1100 1100 1100 1100 1100

970940 680

680 680 680 680

490 490 490 490 480 800

800

750

810

800

Wavenumber (cm^-1)

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300

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Fig.

x)P2O dope glass their chan P-O- the c abov tetrah broad P2O5

2: FT-Infrar

4. Conc Homoge O5 for x=0 ed materials

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U D

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1800

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Doped Cr²⁺

0 1700 1600 1

f 25Li₂O:25C Li₂O:25CaO

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500 1400 1300

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1100 10 10 10

0 1200 1100

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of the gla rresponding an increasi umber (cm^-1)

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1000 900 800

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710 710 710

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600 600 600

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n become decreasing

00

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24

Acknowledgments

The authors would like to thank Phosphor Research Group, Physics Department, UTM for the preparation of equipment, The Ministry of Higher Education (MOHE), The Ministry of Science, Technology and Innovation (MOSTI) and Universiti Teknologi Malaysia, Research University Grant Project Number Q.J130000.2626.04J29 for their financial support.

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