Spectroscopic studies of copper doped alkaline earth lead zinc phosphate glasses

Physica B 434 (2014) 159–164

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Spectroscopic studies of copper doped alkaline earth lead zinc phosphate glasses S. Sreehari Sastry a,n, B. Rupa Venkateswara Rao a,b a b

Department of Physics, Acharya Nagarjuna University, Nagarjunanagar 522510, India Department of Physics, V.R. Siddhartha Engineering College, Vijayawada 52007, India

art ic l e i nf o

a b s t r a c t

Article history: Received 4 November 2013 Accepted 9 November 2013 Available online 16 November 2013

In this paper spectroscopic investigation of Cu2 þ doped alkaline earth lead zinc phosphate glasses was done through the spectroscopic techniques like X-ray diffraction, Ultra Violet (UV) absorption Spectroscopy, Electron Paramagnetic Resonance (EPR – X band), Fourier Transform Infra Red (FTIR) and Raman Spectroscopy. Alkaline earth lead zinc phosphate glasses containing 0.1% copper oxide (CuO) were prepared by the melt quenching technique. Spectroscopic studies indicated that there is a greater possibility for the copper ions to exist in Cu2 þ state in these glasses. The optical absorption spectra indicated that the absorption peak of Cu2 þ is a function of composition. The maxima absorption peak was reported at 862 nm for strontium lead zinc phosphate glass. Bonding parameters were calculated for the optical and EPR data. All these spectral results indicated clearly that there are certain structural changes in the present glass system with different alkaline earth contents. The IR and Raman spectra noticed the breaking of the P–O–P bonds and creating more number of new P–O–Cu bonds. & 2013 Elsevier B.V. All rights reserved.

Keywords: Phosphate glasses Transition metal Electron paramagnetic resonance Infrared spectroscopy Raman spectroscopy

1. Introduction In view of the superior physical properties such as high thermal expansion coefficients, low melting, softening temperatures and high ultra-violet transmission, the phosphate glasses are more advantageous than conventional silicate and borate glasses [1,2]. However, due to the poor chemical durability, high hygroscopic and volatile nature, phosphate glasses are not preferred mostly to replace the conventional glasses which have a wide range of technological applications. The physical properties and chemical durability of phosphate glasses are found to get improved by introducing a number of heavy metal oxides into P2O5 glass network [3]. Such glass systems have been investigated extensively for the past two decades; but, still there is a great scope in developing a new glass according to the suitability of industry demands and technology. These systems have great potential applications in solid state laser hosts, novel glass polymer composite materials etc. [4]. Alkaline earth lead zinc phosphate glasses are well known for their bioactive properties and one can find a range of applications in low temperature modeling operations for optical elements and glass to metal seals [5]. In the present work the spectroscopic studies for transition metal ion doped zinc-phosphate glasses are carried out to assess the bond character for understanding the structural phenomenon and to get the

n

Corresponding author. Tel./fax: þ 91 866 2484582. E-mail address: [email protected] (S.S. Sastry).

0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.11.017

microscopic insight of the glass network through the techniques like X-ray diffraction, Ultra Violet (UV) absorption Spectroscopy, Electron Paramagnetic Resonance (EPR – X band), Fourier Transform Infra Red (FTIR) and Raman Spectroscopy. Transition metal ions are the simplest and the most well suited dopants for the glass systems since these are characterized by the presence of partially filled d shells. Glasses containing CuO possess better semi-conductor properties and hence, these are used for several applications [6,7]. It is a well known fact that Cu2 þ is a coloring agent in different glasses [8–11]. Using effective spectroscopic techniques, the local structure of transition metal ions in glasses is determined by the Electron Paramagnetic Resonance (EPR) and optical absorption spectral data [12]. The EPR and optical absorption studies suggested that transition metals (Cr3 þ and VO2 þ ) can distort from tetragonal to octahedral [13]. 2. Material and methods 2.1. Sample preparation Appropriate amounts of AnalaR grade reagents; phosphorus pentoxide (P2O5) (99.9% pure), lead oxide (Pb3O4), zinc oxide (ZnO), 0.1 mol% of copper oxide(CuO) and magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO) were added an intermediate compound to each glass composition. The composition was thoroughly grinded in an agate mortar, and the powder taken in a porcelain crucible was placed in a high

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temperature furnace at 1100–1150 1C for about 1 h until a bubble free liquid formed. The resultant molten liquids were quenched to room temperature and poured on a brass disc and subsequently annealed at 300 1C for about 5 h to relieve mechanical strains and cracking of the glass samples. The compositions of the glass samples employed in the study are given in Table 1. The X-ray diffraction patterns were recorded on powder samples at room temperature using Philips X-ray generator (Model PW1170) with CuKα radiation (λ ¼1.5418) in the 2θ ranges of 101–701 at a scanning rate of 21 per minute. The optical absorption spectra of these glasses were recorded to a resolution of 0.1 nm at room temperature in the spectral wavelength range of 200–900 nm using JASCO model V-670 UV–vis–NIR spectrophotometer. EPR spectra of the samples are recorded at room temperature through JES-FA series EPR spectrometer operating in the X-band frequency (9.16 GHz) with 100 kHz field modulation. The magnetic field was scanned from 0 to 800 mT and the microwave power used was 1 mW. The Infrared spectra of alkaline earth lead zinc phosphate glasses were recorded on SHIMADZU 8201 PC FT-IR Spectrophotometer in the range 4000–400 cm  1 using KBr pellets. The fine glass powder was examined using Fourier Transform Raman spectrometer (model BRUKER RFS 27: Standalone FT-Raman Spectrometer) equipped with Nd:YAG laser 1064 nm, which eliminates the problem of sample fluorescence and photodecomposition. The Raman spectra were recorded using a 0.7 W laser power, 64 scans and 2 cm  1 resolution. The spectra of the samples were measured in the range 3700–200 cm  1. The procedures and formulae applied here to determine the physical properties in the study were put to test in the earlier works of authors [14].

3. Results The data in Table 2 indicated that the average molecular weight increases from TG1A to TG1D glass which influences density, refractive index and other physical properties.

3.2. UV spectroscopy An amorphous material is the one wherein the optical band gap is analogous to energy gap between the valence band and conduction band of a semiconductor. The absorption edge study in the UV region is a useful method to understand optical transition and electronic band structure in glasses. The indirect band gap is calculated by the following equation:  EOpt ¼ hν 

αhν A

1=2 ð1Þ

where α is a function of v and A for a constant. This relation can be applied to any oxide glasses [15]. The Urbach energy plots of the samples studied are presented in Fig. 2. The intersection point that is formed on extrapolating the linear region of the curve to the X-axis, indicated the optical band gap αðνÞ and Urbach energy that is evaluated by the following equation:  αðνÞ ¼ constexp

hν ΔE

 ð2Þ

where ΔE indicating the width of the band tails of the localized states. The Urbach energy values are obtained from plots drawn using the parameters ln αðνÞ against hν. Urbach energy ΔE is determined by finding the slopes of the linear regions of the curves and by taking their reciprocals. The values of the optical band gap and Urbach energies of the glasses are presented in Table 3. The optical absorption spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses (TG1A, TG1B, TG1C and TG1D) are shown in Fig. 3. The maximum optical absorption peak is recorded at 862 nm for strontium lead zinc phosphate glass. 300

3.1. X-ray diffraction

TG1A

250

Fig. 1 shows the XRD patterns of Cu doped alkaline earth lead zinc phosphate glass systems. No significant crystalline peaks (Bragg peaks) recorded in the patterns, so the amorphous nature of glass samples was confirmed.

Table 1 Compositions of the glass samples (mole %). Samples TG1A TG1B TG1C TG1D

Composition (mole %) 59.9P2O5 – 10Pb3O4 – 10ZnO 59.9P2O5 – 10Pb3O4 – 10ZnO 59.9P2O5 – 10Pb3O4 – 10ZnO 59.9P2O5 – 10Pb3O4 – 10ZnO

Counts (arb.units)

2þ

200

TG1D

150

TG1C

100

– – – –

20MgO – 0.1CuO 20CaO – 0.1CuO 20SrO – 0.1CuO 20BaO – 0.1CuO

TG1B

50 20

30

40

50

60

70

2theta (Degree) Fig. 1. XRD spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

Table 2 Various physical properties of Cu2 þ doped alkaline earth lead zinc phosphate glasses. Physical property

TG1A

TG1B

TG1C

TG1D

Refractive index (nd) at 589.3 nm Density, d (gm/cm3) Average molecular weight, M (gm) Cu þ 2 ion concentration N (  1022 ions/cm3) Mean atomic volume (gm/cm3/atom) Optical basicity Λth

1.6105 3.6972 169.861 0.1310 8.3609 0.4132

1.6200 3.7876 173.016 0.1318 8.3129 0.4442

1.6205 3.9137 182.524 0.1291 8.4872 0.4443

1.6300 4.0409 192.466 0.1264 8.6677 0.4525

S.S. Sastry, B.R.V. Rao / Physica B 434 (2014) 159–164

TG1A= 3.53eV TG1B=3.82eV TG1C = 3.51eV TG1D=3.76eV

161

TG1A TG1B TG1C TG1D

400 200

Intensity(arb.units)

(ahn)1/2 (eV1/2cm-1/2)

0 -200 -400 -600 -800 -1000 -1200 -1400 -1600

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

200

220

240

260

280

Energy(eV)

300

320

340

360

380

400

Field(mT)

Fig. 2. Urbach energy plots of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

Fig. 4. EPR spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

term (k) are calculated using the following equations [16,17]. Table 3 Optical band energies of Cu2 þ doped alkaline earth lead zinc phosphate glasses. Samples Band gap energy (eV)

Urbach energy (eV)

Transition from 2Eg(D)-2T2g(D) (cm  1)

TG1A TG1B TG1C TG1D

0.344 0.143 0.187 0.200

12239.9 12106.5 11890.6 12360.9

3.53 3.82 3.51 3.76

-TG1A -TG1B -TG1C -TG1D

Absorbance (arb units)

-0.18

-0.21

650

700

750

800

850

ð3Þ

k ¼ ðAO =PÞ þΔg O

ð4Þ

Here γCu is the magnetic moment of copper, βo for the Bohr magneton, βN for the nuclear magneton and r for the distance from the central nucleus to the electron. AO ¼ ðA J þ 2A ? Þ=3

-0.15

600

P ¼ 2γ Cu βo βN ðr  3 Þ ¼ 0:036 cm  1

900

950

Wavelength (nm) Fig. 3. Optical absorption spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

3.3. EPR spectra No resonance signal is detected in the EPR spectrum of undoped glasses due to the absence of paramagnetic elements. When the Cu2 þ ions are introduced into the present glasses, the EPR spectra of all glass samples exhibited characteristic resonance of Cu2 þ . The EPR spectra of samples studied at room temperature are plotted in Fig. 4. The spin-Hamiltonian parameters (SH) calculated from the EPR spectra of the ions reflect very sensitively even small variations in the coordination of the paramagnetic centers. The spinHamiltonian parameters, the dipolar term (P) and Fermi-contact

ð5Þ

where A J and A ? are the hyperfine coupling constants in the parallel and perpendicular direction to the fields. The g J and g ? are g-values of parallel and perpendicular to the field respectively and for Δg o ¼ g o  g e , where g o ¼ ðg J þ 2g ? Þ=3 and g e is the free ion and its value is 2.0023. The bonding coefficients of, α2, β21 and β1 described, the inplane s bonding, in-plane π bonding and out-of-plane π bonding respectively of the Cu2 þ complex in the glasses. The value α2 will lie in between 0 U5 and 1 which is the limits for pure covalent and ionic bonding. The values α2 and β1 were estimated using the following relations [16]: 3 AJ þðg J  g e Þ þ ðg ?  g e Þ þ 0:04 α2 ¼ 0:036 7

ð6Þ

g J ¼ 2:0023 8Q ½αβ1  ð1=2Þα0 ð1  β1 2 Þ1=2 TðnÞ

ð7Þ

where Q ¼ ðλo αβ 1 =ΔEÞTðnÞ ¼ 0:02 is a function involving metal ligand hybridization constant. ΔE is the transition energy obtained from the optical absorption spectra of Cu2 þ doped alkaline earth lead zinc phosphate glass and λo is the spin-orbit coupling constant ( 828 cm  1 ) [16]. The parameter α′ for normalization of d1 orbital was calculated from the normalization conditions on the ground state d1 orbital by the equation α′ ¼ ð1  α2 Þ1=2 þ αS

ð8Þ

In above equation S is the overlap integral between the dx2  y2 orbital and the normalized ligand orbital, and α′ indicates the extent of overlap between the dx2  y2 orbital of the central metal ion and the normalized ligand orbital. The parameter β1 2 which is a direct measure of the covalency of the in-plane π-bonding between the copper and its ligands, is also calculated by using α′. The value of oxygen S (from Eq. (8)) used in the investigation is 0.076. The parameters α′ and β1 are calculated and the values given in Table 5. The α2 values indicated the intermediate ionic bond of the Cu2 þ –O  – in-plane s – bonding.

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Table 4 Spin-Hamiltonian parameters of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

Table 6 IR absorption peaks (cm  1) of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

Samples

gJ

g?

A J (10  4 cm  1)

A ? (10  4 cm  1)

TG1A

TG1B

TG1C

TG1D

Vibrations

TG1A TG1B TG1C TG1D

2.464 2.449 2.447 2.448

2.095 2.093 2.092 2.097

100.5 108.2 110.0 116.0

25.2 24.0 22.0 25.0

1071 986 915 874 711 678 610 558 512

1074 986 959 874 711 – 607 516 512

1073 986 915 877 714 – 616 558 512

1070 989 967 894 774 – – 539 520

ν3 ν1 P–O–P P–O–P P–O–P P–O–P P–O–P P–O–P ν4

Table 5 Bonding parameters of Cu2 þ doped alkaline earth lead zinc phosphate glasses. Samples

K

α

α′

β1

Γs (%)

Γπ (%)

TG1A TG1B TG1C TG1D

0.355 0.354 0.375 0.365

0.906 0.909 0.910 0.921

0.491 0.486 0.483 0.459

0.967 0.978 0.964 0.981

39 38 37 33

13 08 14 07

18000

748

907

1029

asymmetrical stretch asymmetrical stretch symmetrical stretch symmetrical stretch bend bend

1118

462

16000

TG1A TG1B TG1C TG1D

Intensity (a.u.)

14000

TG1A--TG1B--TG1C--TG1D---

1.2

1155

12000 690

10000 238

286

941

8000

564

(P-O-P)s

3-

700

800

900

1000

600

800

1000

1200

1400

Infrared spectrum of Cu2 þ doped alkaline earth lead zinc phosphate glasses is shown in Fig. 5. The observed band positions and the corresponding assigned vibrational modes are listed in Table 6.

0.0 600

400

1121

Fig. 6. Raman spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

(PO)4

500

1026

Raman shift (cm-1)

(P=O) strech

0.4

200

(P-O-P)as

907 747

4000

(P-O-P) bend

Absorption (a.u)

1012

431

6000 0.8

904

1100

1200

1300

1400

3.5. Raman spectra

Wavenumber (cm-1) Fig. 5. Infrared spectra of Cu2 þ doped alkaline earth lead zinc phosphate glasses.

The normalized covalency for Cu2 þ –O in-plane bonding for the s or π symmetry is expressed by [18]. Γ s ¼ 200ð1  SÞð1  α2 Þ=ð1  2SÞ; %

ð9Þ

and Γ π ¼ 200ð1  β21 Þ; %

ð10Þ 2þ



Here, the normalized covalency of (Γπ) Cu –O bonding of π symmetry indicated the basicity of the oxide ion. In general, covalency of in-plane π bonding (Γπ) increases while covalency of the in-plane s bonding (Γs) decreases.

It is known that the transition metal oxides (like CuO and V2O5) have usually a network modifier effect which consists of a depolymerization of the long phosphate chains. And in a local reorganization of the structural units appeared only the short range phosphate units or ring structures. A typical Raman spectrum of Cu2 þ doped alkaline earth lead zinc phosphate glasses is shown in Fig. 6. The most important bands observed in these spectra are:  336 cm  1 assigned to the bending vibration of the phosphate polyhedra [19],  747 cm  1 assigned to the symmetric stretching vibration of P–O–P bonds [19–22],  941 cm  1 band assigned to the vibration in the PO2  and PO3  groups [19,20,22], and 1155 cm  1 due to the symmetric stretching vibration in the PO2 groups [19,20].

3.4. IR spectra 4. Discussion Infrared spectroscopy is a very sensitive and one of the most used spectroscopic methods applied in the investigation of local order characterizing vitreous materials like glass. The phosphate units (PO43  ) for phosphate glasses exist in the range 1400– 400 cm  1. The phosphate ions in the p state exist in tetrahedral symmetry and exhibit four fundamental bands viz., 1082 cm  1 ðν3 Þ, 980 cm  1 ðν1 Þ, 515 cm  1 ðν4 Þ and 363 cm  1 ðν2 Þ. Here ν1 is non-degenerate, ν2 doubly degenerate and ν3 and ν4 triply degenerate. In this case, ν3 and ν4 are the infrared active. A typical

The glass samples prepared with different chemical compositions which are shown in Table 1, have presented the changes in the structural properties with respect to the different spectroscopic techniques. The increase in refractive index is attributed to the formation of non-bridging oxygen (NBO) due to the incorporation of Cu–O in the glass network. The increase in density is due to higher molecular weight of doped Pb3O4 compared to that of P2O5. The increase in density for the glass system reveals the change in

S.S. Sastry, B.R.V. Rao / Physica B 434 (2014) 159–164

the structure of the glass with addition of alkaline earth content, since the density of a glass is found to be very sensitive to the ionic size and atomic weight [23]. It is evident that the mean atomic volume, and optical basicity of the glasses increased with the replacement of alkaline earth oxide (MgO/CaO/SrO/BaO). The increase in most of the physical properties is due to increase in bond length or interatomic spacing. The theoretical value of optical basicity (Λth ) reflects the ability of the glass to donate negative charge to the probe ion [24]. The basicity parameter from Duffy and Ingram [25] slightly increased from glass TG1A to TG1D. High optical basicity means high electron donor ability of the oxide ions to the cations [26]. Variations in physical properties are due to replacement of alkaline earth oxides in different glasses around Cu2 þ ions. The optical absorption spectra for all glass compositions are shown in Fig. 3 and exhibited a single broad peak in the range 800–850 nm, which can be attributed to the presence of Cu2 þ ion in the glass as octahedrally coordinated by six oxygen atoms and the octahedron being tetragonally distorted [11]. This absorption can be assigned to the 2Eg(D)-2T2g(D) transition of Cu2 þ ion. The cubic symmetry of Cu2 þ ions is disturbed by electronic hole in the degenerate orbital which has caused the tetragonal distortion. According to Jahn-teller theorem, any non-linear system with degenerate ground state should distort in order to eliminate the degeneracy. So, two structural changes may be possible, one is elongated and other is compressed in structure. The variation in peak position of TG1A, TG1B, TG1C and TG1D indicated the fluctuation in ligand field around the Cu2 þ probe ion, which is related to change in polarizability of oxygen ion surrounding the Cu2 þ and its dependence on field strength of network [27]. The maximum optical absorption peak in the present system can be explained on the basis of structural change in the glasses with variation of alkaline earth content. The optical band gap values shown in Table 3 vary with change of alkaline earth content from 3.51 to 3.82 eV. Urbach energy values lie between 0.143 and 0.344 eV. Smaller values of Urbach energy indicated that Cu2 þ doped alkaline earth lead zinc phosphate glasses are homogeneous and stable [28]. From Table 4 it is observed that the values of g J , g ? and g e are in the order of g J 4g ? 4 g e that suggested that Cu2 þ ion is located in tetragonally distorted octahedral sites. The variation of g J and A J for different MgO, CaO, SrO and BaO compositions is non linear. This is due to the change in the tetragonal distortion. Variation in g and A values is perhaps associated with the change in the environment of Cu2 þ , i.e. in the ligand field strength at the site of Cu2 þ . This is due to the structural changes in the glass [29]. Therefore, incorporation of alkaline earth content in the glass influenced the field at the site of Cu2 þ , which, in turn, reflected in the non-linear variation of the spin Hamiltonian parameters. The Fermi contact term k is a measure of the polarization produced by the uneven distribution of d-electron density on the inner core selectron. From Table 5 the calculated values of k are in good tune with the general order of k [30]. The results of the both EPR and optical absorption studies with various band gap energies revealed the possibility of formation of Zn–O–P, Pb–O–P, Mg–O–P, Ca–O–P, Sr–O–P, Ba–O–P and Cu–O–P bonds [31]. The calculated values of bonding parameters as shown in Table 5 indicated the covalency nature of Cu2 þ environment for different alkaline earth contents since the intermediate varies with the bond nature, i.e., Ca2 þ and Ba2 þ are participating the network linkage. From the results of IR spectral studies, the observed band at 1070 cm  1 are assigned to PO43  fundamental vibrational mode. The observed strong band in the region 870 900 cm  1 in the spectra of all the investigated glasses which is the characteristic of linear phosphates suggests the presence of linear chains in them [32]. It has been reported [33] that compounds containing P-O-P links produce characteristic absorptions near 900

163

and 700 cm  1. The presence of absorption bands near 875 920 cm  1 is due to the asymmetric stretching mode of the P–O–P linkage, while two modes appeared at around 689  785 cm  1, are attributed to the symmetric stretching of P– O–P groups [34,35]. A weak band appearing at around 500 cm  1 in all the spectra may be attributed to harmonic of the P–O–P bending vibrations [35]. With the basic assignments here in the spectra, identified three fundamental frequencies that prove the role of vitreous network former of phosphorous. The band assignments in between 400 and 550 cm  1 are difficult because of the superposition of intermediates like MgO, CaO, SrO and BaO in the present glass system. Here some modes did not appear in the spectra for Ba based glass. The vibrational mode acts as partial modifiers of the glass network due to the oxygen atoms around the substituent ions that probably constitute the network structure. Addition of intermediates to the phosphate network has gradually broken it into short phosphate groups such as P4O136  , P3O105  , and P2O74  [36,37]. The vibrational modes observed at 511, 517, 522 and 511 cm  1 for all the glasses are due to the bending mode of the phosphate polyhedra and/or Pb–O and Zn–O vibrations. The formation of these bonds replaces easily the hydrolysable P–O–P bonds [36]. It appears that BaO has formed the network linkages with the phosphate tetrahedra and with some portion of O–Pb4 þ –O linkages. Absence of P¼ O in BaO based glass and formation of P–O–Cu band are due to increase of the cross link density in the glass network. The Raman spectra of the glasses exhibited considerable differences. In Raman spectra, the network modifiers ( MgO, CaO, SrO, BaO ) are to be incorporated with transition metal oxides. The bands occurred in the spectra is at  690 cm  1 band which is due to the vibration of P–O–P in-chain [22], at 747 cm  1 band which is assigned to P–O–P asymmetric stretching vibration, a shoulder at  904 cm  1 and a band at  1026 cm  1 which is due to the P–O stretching modes [19–22]. The intensity in the band at 1155 cm–1 is due to decrease in intensity of the PO2 groups as a result it becomes broader with the addition of BaO with transition metal oxide content [20]. There are several bands showing commonality to both IR and Raman spectra of the glasses. But it is difficult to identify and compare quantitatively the various bands due to their large sized half-widths. The FTIR and Raman spectra of the glasses studied are influenced by the network modifiers as well as transition metal oxide.

5. Conclusions In Cu2 þ doped alkaline earth lead zinc phosphate glass systems, g J 4g ? , indicates tetragonally elongated octahedral (D4h) site for the Cu2 þ ion in the glasses. By correlating EPR and optical results the in-plane Π-bonding (β1 2 ) was evaluated, that has supported the existence of covalency with partial ionic nature. The FTIR spectra of these glasses were analyzed to identify the spectral contribution of each component on the structure and to examine the role of alkaline earth as a modifier of the glass network. Both FTIR and Raman spectra showed that the concentration of copper ions depolymerized the phosphate glass network, which were caused by breaking of the P–O–P bonds and by creating new and more P–O–Cu bonds. Similarly the FTIR and Raman results supported both the EPR and optical absorption results that inferred the major role of barium in the formation of cross linking bands which are due to their high ionic radius.

Acknowledgments The authors gratefully acknowledge UGC DRS LEVEL III Program no. F.530/1/DRS/2009 (SAP-I), dated 09-02-2009 and DST FIST

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