FTIR, Raman, EPR and optical absorption spectral studies on V2O5-doped cadmium phosphate glasses

Physica B 406 (2011) 3142–3148

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FTIR, Raman, EPR and optical absorption spectral studies on V2O5-doped cadmium phosphate glasses N. Kerkouri a,n, M. Haddad b, M. Et-tabirou a, A. Chahine a, L. Laˆanab c a

Laboratoire de Physico-Chimie des Mate´riaux Vitreux et Cristallise´s (LPCMVC), Faculte´ des Sciences, Universite´ Ibn Tofail, BP 133, Ke´nitra 14000, Morocco Laboratoire de Spectrome´trie, des Mate´riaux et Arche´omate´riaux (LASMAR), Faculte´ des Sciences de Me´kne s, Morocco c Laboratoire Conception et Syste me Dept. Physique, Faculte´s des Sciences, Universite´ Mohamed V Agdal Rabat, Morocco b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2010 Received in revised form 24 February 2011 Accepted 26 April 2011 Available online 20 May 2011

Vanadium doped cadmium phosphate glasses xV2O5–(50  x)CdO–50P2O5 with 0 rx r20 mol% were prepared and FTIR, Raman, EPR and optical absorption measured. Infrared and Raman investigations show the formation of P–O–V and V–O–V bonds, which replace P–O–P ones. The simulations of EPR spectra of most glass samples 2.5 r x r 10 mol% reveal a superposition of two resonance signals. Spin Hamiltonian parameters indicate that the VO2 þ ions are present in octahedral sites with tetragonal 2 compression and belong to C4v symmetry. The molecular bonding coefficient b1 and e2p show an appreciable degree of covalency. & 2011 Elsevier B.V. All rights reserved.

Keywords: Phosphate glasses FTIR spectroscopy Raman spectroscopy EPR spectroscopy Optical absorption spectroscopy

1. Introduction Glasses doped with transition metals (TM) attract much attention because of their interesting optical [1], semi-conducting [2], memorizing and photo-conducting properties [3]. They find potential application in solid state lasers [4], luminescent solar energy concentrator and optical fibers for communication devices [5,6]. Phosphate glasses usually of low melting point, are believed to be highly acidic and hence can produce a reasonable concentration of the reduced state of the transition metal ions even on ordinary melting in air [7], are potentially good UV-transmitting materials [8], offer an important range of compositional possibilities with which it is possible to tailor physical and chemical properties of interest for specific technological applications. The vanadyl ion (VO2 þ ) incorporated in glasses as spectroscopic probe has been studied by several researchers in order to characterize local structure [9–11], which evolves many particular aspects such as the geometry of the structural units, the character of chemical bonds as well as the coordination polyhedra (local symmetry) of transition metal ions. By varying the chemical composition of a glass, the local environment of the transition metal ion incorporated into the network can be changed leading to in-homogeneities in the

n

Corresponding author. E-mail addresses: [email protected], [email protected] (N. Kerkouri). 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.04.057

crystal field around the TM ion. These structural modifications may be well reflected in the EPR and optical absorption spectra of the TM ions. Previous EPR and optical results of VO2 þ : ZnO P2O5 glasses [10] suggest tetragonally distorted octahedral site symmetry for VO2 þ . It is well known that ZnO  P2O5 composition is similar to that of CdO  P2O5 [12]. The present spectroscopic studies are undertaken to present a comprehensive structural view and correlation of EPR and ligand field parameters of VO2 þ in cadmium phosphate glass system.

2. Experimental 2.1. Glass preparation The glass samples having the general formula xV2O5–(50x) CdO–50P2O5 with 0rxr20 mol% have been prepared by weighing suitable amount of (NH4)2HPO4, CdO and V2O5. The samples were mixed together by grinding the mixture repeatedly to obtain a fine powder. 16 g of this mixture was melted in alumina crucible in an electrically heated furnace under ordinary atmospheric conditions at a temperature of 500 1C for about 2 h in order to evaporate ammonia, carbonate and water from the batch and to minimize the tendency of subsequent phosphate volatilization. The temperature was then raised gradually to 1000 1C and held constant at this value for 30 min to homogenize the melt. The melt so obtained was quickly quenched between two stainless steel plates and random pieces of glasses were collected. The glasses were then stored under vacuum in a desiccator

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over silica-gel until use. All the glass samples were prepared under the same conditions in order to keep the same parameters for the oxidation and reduction reactions. 2.2. Characterization procedure X-ray measurements were recorded to check the non-crystallinity of the glass samples using a Philips (X’PERT-PRO) diffractometer. The IR spectra of the samples were recorded at room temperature using KBr pellet technique on Fourier transform spectrometer (BRUCKER Tensor 27) in the range 400–1400 cm  1. For this powdered glass samples were thoroughly mixed with dry KBr in a ratio of 1:25 and the pellets were formed under a pressure of 15 tons for few minutes. All spectra were run at 4 cm  1 resolution. Raman spectra were measured at room temperature with a Horiba Jobin Yvon Micro Raman spectrometer (LABRAM Dilor) coupled to an internal laser He–Ne (18 mW) using the 632.8 nm line and to an external Spectra-Physics laser Ar þ (200 mW) using the 514.5 nm line. The spectra were measured between 200 and 1400 cm  1. EPR spectra of glass samples were recorded at room temperature on an EPR spectrometer (Bruker ER 200D) operating in the X-band frequencies (9.5 GHz) with a modulation frequency of 100 kHz. A powdered glass specimen of 100 mg was taken in quartz tube for EPR measurements. Polycrystalline DPPH with g value of 2.0036 was used as a standard field marker. The optical absorption spectra of all glass samples containing V2O5 were recorded at room temperature using Perkin Elmer UV–vis–NIR spectrophotometer (lambda 900) in the wavelength range 400–1000 nm.

3. Results and discussion The XRD patterns (Fig. 1) show that the glasses did not reveal any discrete or sharp peaks, but a broad humps characteristic of the amorphous materials. 3.1. Infrared study The FTIR spectra of xV2O5–(50  x)CdO–50P2O5 glasses with 0 rxr20 mol% V2O5 are illustrated in Fig. 2. The infrared

Fig. 1. X-ray diffractograms of xV2O5–(50  x)CdO–50P2O5 glasses.

Fig. 2. FTIR spectra of xV2O5–(50  x)CdO–50P2O5 glasses.

spectrum of the base undoped cadmium phosphate glasses possess a sharp band with a peak at about 1273 cm  1, another sharp one at 1086 cm  1 showing a small kink at about 1160 cm  1 at the descending lobe, a doublet strong and broad band with two obvious peaks at about 893 and 1030 cm  1, a doublet medium band with two peaks, a secondary kink at about 720 cm  1 and a prominent one at about 775 cm  1 and finally a doublet band with two peaks, a prominent kink at about 473 cm  1 and a secondary kink at about 520 cm  1. The addition of V2O5 up to 10 mol% to the base glass matrix reveals no evident changes in the number of all the previously mentioned IR peaks. It is obvious that the increase of V2O5 until 10 mol% does not cause any major influence on the main characteristic phosphate groups, but only minor effects are observed in the intensities of some of the IR bands. The two absorption bands at about 1273 and 893 cm  1 progressively shift to lower and longer wavelengths reaching about 1256 and 915 cm  1, respectively, in the high V2O5 content (20 mol%) which is accompanied by the appearance of new band at about 1020 cm  1. The experimental IR bands observed in the undoped and V2O5-doped cadmium phosphate glasses has been discussed in agreement with the concept about the independent vibrations in the glasses, previously introduced by Tarte [13,14] and Condrate [15]. Dimitriev et al. [16–18] have also applied the same approach. It is supposed that vibrations of characteristic groups of atoms in the network are independent of vibrations of other groups. The infrared interpretation of cadmium phosphate glasses (50CdO–50P2O5) have been reported in Ref. [19]. The characteristic features are the PO2 asymmetric stretching vibration band at 1273 cm  1 (nas PO2), the symmetric stretching vibration band at 1160 cm  1(ns PO2), nas PO3 groups chain end at 1086 cm  1, ns PO3 groups at 1030 cm  1, nas POP groups at 893 cm  1, ns POP groups at 775 and 720 cm  1 and the bending vibration d(P–O) bands at 520 and 473 cm  1 [20–22]. The presence of bands at 1273 cm  1 (nas PO2), at 1160 cm  1 (ns PO2) and at 775 and 720 cm  1 (ns POP) indicates the existence of metaphosphate chain structure [23–25]. It is observed that the introduction of vanadium ions up to 20 mol% leads to the decrease of the intensity of PO2 asymmetric stretching vibration band and of POP bridges. These changes in

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the spectra of the glasses with V2O5 addition are due to the decrease of average phosphate chain length [26]. Absorption band of PO2 asymmetric stretching vibration mode shifts from 1273 cm  1 in 50CdO–50P2O5 glass to 1256 cm  1 in 20V2O5–30CdO–50P2O5 glasses. This is an expected result because the vanadium–oxygen bond is more covalent than the cadmium–oxygen bond, thus the phosphorus–oxygen bonds linked to vanadium ions P–O(–V) are more ionic than P–O(–Cd) bonds. The band of POP asymmetric stretching mode shifts to higher frequency from 893 cm  1 for x¼0 mol% to 915 cm  1 for x¼20 mol% V2O5 content. The band shift can be related to the increase of the covalent character of P–O–P bonds and indicates that these bonds are strengthened as CdO is substituted by V2O5. This band (915 cm  1) can also be attributed to the symmetric stretching vibration of VO2 groups of VO4 tetrahedra in metavanadates forming chains with V–O–V bridges [27,28]. The addition of V2O5 to 50CdO–50P2O5 leads to the disappearance of the frequency band at 775 and 720 cm  1 assigned to two bridges P–O–P of metaphosphate chains (P2 O2 6 ) [24] and the appearance at x¼20 mol% of only one broadened band centered around 765 cm  1. This result suggests the presence of pyrophosphate groups (P2O7)4  [24,25,29]. According to Bhargava and Condrate [30] this band can also be attributed to the vibration of V–O bond connected to phosphate groups P–O–V. For x ¼20 mol%, another absorption band emerges at 1020 cm  1 which can be ascribed to the vibrations of the isolated VQO vanadyl groups in VO5 trigonal bipyramids [31]. At low frequency, the intensity of the bands at 473 and 520 cm  1, assigned to the bending vibration of PO4 tetrahedra d(PO4), decreases and transforms into only one band centered at 500 cm  1 for x ¼20 mol% V2O5 content. This band can be ascribed to the bending vibration of d(P–O) bonds in (PO4)3  groups [32]. 3.2. Raman study The Raman spectra of xV2O5–(50 x)CdO–50P2O5 glasses with 0 rxr20 mol% V2O5 are represented in Fig. 3. The Raman spectrum of undoped cadmium phosphate glasses reveals two sharp peaks at about 696 and 1172 cm  1, three medium peaks at about 330, 500 and 1250 cm  1 and two kink at about 789 and 1065 cm  1. The Raman spectra of V2O5-doped cadmium metaphosphate glasses reveal numerous peaks and careful inspection

of these spectra indicate the combined presence of the peaks due to phosphate network and that due to vanadium ones. The addition of V2O5 to the base glass matrix reveals changes from x ¼2.5 mol% with the apparition of new bands at 974, 1027, 615 and 760 cm  1. The Raman spectrum of cadmium phosphate glasses (50CdO– 50P2O5) reported in Ref. [19] is typical of metaphosphate glass composition. The two main peaks observed at about 696 and 1172 cm  1 are referred to the symmetric stretching mode of P–O–P (ns POP) bridging bonds and to the symmetric stretching PO 2 bonds (ns PO2) of PO4 tetrahedra, respectively [33]. The two shoulders at about 1065 and 1250 cm  1 centered at 1172 cm  1 are related to the asymmetric stretching mode (PO3)2  chain end groups (nas PO3) and to the asymmetric stretching of PO 2 bonds (nas PO2), respectively [34]. A weak shoulder at 789 cm  1 could be due to the second symmetric stretching mode of P–O–P bridging bonds (ns POP) in short phosphate units [35]. The weak band at 500 cm  1 is assigned to a bending vibration of PO3 groups (dPO3). The low frequency band at 330 cm  1 is related to the bending motion of phosphate polyhedral [33]. The addition of V2O5 to cadmium metaphosphate glass causes progressive spectral changes. In particular, the intensities of the strong bands at 696 and 1172 cm  1 decreased and finally vanished for x¼ 20 mol%. These changes are accompanied by the appearance from x ¼2.5 mol% of new bands at 615, 760, 1027 and strong band at 974 cm  1. The disappearance of the bands at 1250 and 1172 cm  1 and the appearance of the weak broad band at 760 cm  1, which is assigned to the symmetric stretch of P–O–P bridge [21] indicate that no significant amount of phosphate chain remain in these glasses when V2O5 replaces CdO. The band at about 1027 cm  1 is attributed to the symmetric PO3 stretching modes of dimmer (P2O7)4  unit [36,37]. The band at 974 cm  1 which appears from x¼2.5 mol% V2O5 is referred to VQO vibration in isolated VO5 trigonal bipyramids [38]. With increasing V2O5 content, this band shifts from 974 cm  1 for x ¼2.5 mol% to 920 cm  1 for x¼20 mol%, this indicates change of the coordination of vanadium from 5 to 4, leading to the appearance of VO4 instead of VO5 groups [39]. The band at 615 cm  1 which appears from xZ5 mol% is assigned to the formation of PO4 tetrahedral units bridged through one oxygen to vanadium (V–O–PO3) [40]. A weak broad band at 760 cm  1 appearing for x ¼20 mol% is attributed to the P–O–P bridging vibration in pyrophosphate groups P2 O4 [41]. On this band can be superposed the V–O–V 7 bridging vibration, which arises by connecting neighboring VOx polyhedra through oxygen in the glass [40]. The appearance from x¼2.5 mol% of the band at 1027 cm  1 assigned to the symmetric PO3 stretching mode of dimmer (P2O7)4  units (pyrophosphate groups) indicates a depolymerization of the initial metaphosphate chains and therefore V2O5 acts as strong network modifier. Furthermore, for low concentration of V2O5 (xo10 mol%), Raman spectra are dominated by bands belonging to phosphate groups [42]. The appearance at x¼20 mol% of the band at 760 cm  1 assigned to V–O–V bridging vibration and the increase of the intensity of the two bands at 615 and 974 cm  1 attributed to P–O–V and VQO vibration in isolated VO5 trigonal bipyramids, respectively, underline the breaking of the phosphate chains and then V2O5 acts as network former since these two last bands prevail the spectra at x¼20 mol% [42]. 3.3. EPR study

Fig. 3. Raman spectra of xV2O5–(50  x)CdO–50P2O5 glasses.

The EPR spectra of xV2O5–(50  x)CdO–50P2O5 glasses with 0.1rx r20 mol% V2O5 content recorded at room temperature are shown in Fig. 4. When various amount of V2O5 were added to the

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The EPR spectra of vanadyl ions could be best analyzed using a spin Hamiltonian [45] H ¼ b½gJ BZ SZ þg? ðBX SX þBY SY Þ þ AJ SZ IZ þA? ðSX IX þSY IY Þ

ð1Þ

where b is the Bohr magneton, gO and g? are the parallel and perpendicular principal components of the g tensor, respectively. AO and A? are the parallel and perpendicular principal components of the hyperfine coupling tensors, respectively, BX, BY and BZ are the components of the magnetic field and SX, SY and SZ, respectively and IX, IY and IZ are the components of the spin operators of the electron and nucleus, respectively. The quadrupole and nuclear Zeeman interaction terms have been ignored. The solutions [46] of the spin Hamiltonian are given in Eqs. (2) and (3) for the parallel and perpendicular orientations, respectively    2  63 A? BJ ðmÞ ¼ BJ ð0ÞmAJ  m2 ð2Þ 4 2BJ ð0Þ     2 63 AJ þA2? m2 B? ðmÞ ¼ B? ð0ÞmA?  4 4B? ð0Þ

ð3Þ

where m is the magnetic quantum number of the vanadium nucleus having values 77/2, 75/2, 73/2 and 71/2. BJ ð0Þ ¼

hn gJ b

B? ð0Þ ¼

hn g? b

where h is the Planck constant, n is the frequency of the spectrometer and b is the Bohr magneton. The computer simulation of the experimental spectra (Fig. 4a and b; dashed lines) was made possible by superposing two signals which will be called signal 1 and signal 2. The total simulated spectrum is constituted from the contribution of each of these signals depending on the vanadium content and according to their parameters given in Table 1. The values of spin Hamiltonian parameters g and A were used to calculate the dipolar hyperfine coupling parameter P and Fermi contact interaction term K using the expressions developed by Kivelson and Lee [46]       4 3 AJ ¼ P K þ DgJ  Dg? ð4Þ 7 7       2 11  Dg? A? ¼ P K 7 14 Fig. 4. (a) EPR spectrum of 0.1V2O5–49.9CdO–50P2O5 glass (—, experimental; y , simulated), (b) EPR spectra of xV2O5–(50 x)CdO–50P2O5 glasses with 2.5rx r20 mol% V2O5 (—, experimental; y , simulated).

cadmium metaphosphate glasses, the EPR spectra of all the investigated samples exhibited resonance signals. Numerical simulations of EPR spectra specially adapted for glasses have been implemented. In particular the concept of statistical distribution of parameters is introduced to take account of the disorder of the structure. Details on the simulation technique have been reported in Ref. [43]. For low V2O5 content (x ¼0.1 mol%), the spectrum shows a well resolved hyperfine structure (h f s) (Fig. 4a) which consists of sixteen, eight parallel and eight perpendicular, lines and is typical of the unpaired (3d1) electron of VO2 þ ion associated with 51V (I¼7/2) in an axially symmetric crystal field [44]. For high V2O5 content (2.5 rxr20 mol%) (Fig. 4b), the EPR spectra manifest a progressive disappearance of vanadyl hyperfine structure and finally the appearance of only one intense broad and isotropic line (Fig. 4b; x¼20 mol%).

ð5Þ

where DgO ¼gO–ge; Dg? ¼g?–ge and ge( ¼2.0023) is the g factor of free electron [47]. The calculated values of K and P are given in Table 2. Table 1 EPR parameters of signals 1 and 2 deduced from simulations of spectra. %V2O5

Signal

0.1

Signal Signal Signal Signal Signal Signal Signal Signal Signal Signal Signal Signal

2.5 5 7.5 10 20

1 2 1 2 1 2 1 2 1 2 1 2

%

gO

100 0 78 22 30 70 13 87 4 96 0 100

1.917

g?

AO (10  4 cm  1)

A? (10  4 cm  1)

1.992

192

79

– 1.914 1.988 1.960 1.913 1.988 1.960 1.914 1.990 1.960 1.912 1.988 1.960 – – 1.960

– 192

80 –

194

80 –

194

79 –

194

79 – –

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Table 2 P, K and bonding coefficients of VO2 þ ions occupying site 1. %V2O5

K

P (10  4 cm  1)

PK/g

b21

e2p

0.1 2.5 5 7.5 10

0.93 0.93 0.93 0.92 0.92

120 121 121 122 122

0.0076 0.0077 0.0077 0.0076 0.0076

– 0.836 0.846 0.843 –

0.344 0.483 0.483 0.415 0.484

also [54] a strong VQO interaction, which makes the bond length in the vanadyl group (V4 þ –Oa) shorter than the other V–O bonds of the complex. The geometry of the VO2 þ -complex is distorted from Oh toward C4v. We evaluated PK/g using the value of gyromagnetic ratio g ( ¼1.468) [51] for 51V nucleus and obtained the value as 0.007 for vanadyl ions-doped in cadmium metaphosphate glasses which is in good agreement with the theoretical value expected for VO2 þ ions. By increasing the vanadium content (Fig. 4b), the hyperfine structure of the spectra vanishes progressively and a broad line at g¼1.96 characteristic for the dipole–dipole coupled ions appears. Thus the spectra recorded from samples with 2.5rx r10 mol% are considered as a superposition of two EPR signals, one with resolved hyperfine structure typical of isolated VO2 þ ions and another one consisting of a broad line without hyperfine structure typical of associated V4 þ –V4 þ ions [55,56]. For x¼ 20 mol%, only the broad line at g ¼1.96 remains which indicates first the complete disappearance of isolated VO2 þ ions and second an increase of the number of clustered ions with V2O5 content.

3.4. Optical absorption study The optical absorption spectra (Fig. 6) of the samples were recorded at room temperature in the wavelength region 400–1000 nm. V4 þ ions belong to d1 configuration and will have a ground state 2D. In the presence of pure octahedral crystal field, the 2D state splits into 2T2g and 2Eg states, while an octahedral field with tetragonal distortion further splits the 2T2g level into 2 Eg and 2B2g and 2Eg level splits into 2A1g and 2B1g states. Among these, 2B2g will be the ground state. Thus for vanadyl ion; we can expect three bands corresponding to the transitions 2B2g-2Eg, 2 B2g-2B1g and 2B2g-2A1g. The absorption spectra of vanadiumcontaining glasses are generally of a very complex nature because Fig. 5. Variation of linewidth of signal 2 with V2O5 content in xV2O5–(50  x)CdO– 50P2O5 glasses.

From the EPR spectra, signal 1 (hfs, Fig. 4) implies that the interactions between the V4 þ ions located in the corresponding site are week or non-existent, because at low vanadium content the V4 þ ions in this site are far from each other. Signal 2 (broad line; Fig. 4b) is characterized by an important peak-to-peak linewidth decreasing with the increasing of vanadium content (Fig. 5) due to the presence of superexchange interactions in V4 þ –O2  –V4 þ chains [48] indicating that in the corresponding site the V4 þ ions are close to each other. At high concentration (x¼20 mol%), signal 1 disappears completely. The best-fit parameters of the Hamiltonian (Eq. (1)) that allowed the simulation of the signals 1 and 2 indicate that the values obtained in the studied glasses are in good agreement with other results reported in the literature [42,44,49,50]. An octahedral site with a tetragonal compression would give the values gO og? oge and AO 4A? [45,51]. From these observations it is concluded that the paramagnetic V4 þ ions in CdO  P2O5 glasses exists as vanadyl ion, VO2 þ , in an octahedral coordination with tetragonal compression and belong to C4v symmetry where the vanadyl oxygen forms the apex V–O bond [52]. The vanadyl oxygen is attached axially above the V4 þ site along the z-axis (VQO bond), while the sixth oxygen forming the O?VO4?O unit lies axially below the V4 þ site opposite to the vanadyl oxygen. The predominant axial distortion of the VO2 þ octahedral oxygen complex along VQO direction may be the reason for nearly equal g and A values for all the glass samples [53]. The large value of K (E0.92) indicates a large contribution to the hyperfine constant by the unpaired s electron. This suggests

Fig. 6. Optical absorption spectra of xV2O5–(50  x)CdO–50P2O5 glasses.

N. Kerkouri et al. / Physica B 406 (2011) 3142–3148

of the presence of three valence states of vanadium (V3 þ , V4 þ and V5 þ ) altogether which are usually existing in varying proportions depending mainly on the nature of the host glass, the condition of melting (temperature and time) and on the concentration of vanadium in the glass [57,58]. Also from the asymmetric curves of the absorption spectra observed by many workers of Ref. [59], it can be assumed that vanadium ions exhibit a relatively strongly distorted symmetry. Moreover, the vanadium ions have somewhat closer bands which can overlap each other. In phosphate glasses, the vanadium is assumed to exist mainly in lower valence states as V3 þ (trivalent) and V4 þ (tetravalent) ions [59–61]. For the optical absorption spectra of the most glasses, three absorption bands in the frequency ranges 11,494–11,521, 12,886–12,987 and 17,421–19,194 cm  1 are observed. Accordingly, the bands may be assigned to 2B2g-2Eg, 2B2g-2B1g and 2 B2g-2A1g transitions, respectively. Fig. 6 illustrates the representative optical spectra in the studied glasses. By correlating EPR and optical parameters, the molecular 2 bonding coefficient b1 and e2p are evaluated using the following formulae [46]:

b21 ¼

e2p ¼

ðge gJ ÞDJ 2

8lb2 ðge g? ÞD? 2

2lb2

ð6Þ

ð7Þ

2

where b1 and e2p are the degrees of the in-plane V–O s-bonds and of p-bonding with the vanadyl oxygen. DO and D? are the energies of the electronic transitions 2B2g-2B1g and 2B2g-2Eg, respectively. The free ion value of spin–orbit coupling constant for VO2 þ 2 is l ¼170 cm  1 and b2 is assumed to be equal to 1 for the in-plane p-bonding with the equilateral ligands, according to the results 2 reported by McGarvey [62]. Using Eqs. (6) and (7) the values of b1 and e2p were calculated and are given in Table 2. From the optical absorption spectra (Fig. 6), a small change in band maxima due to small structural change in the glass matrix with increase in V2O5 content is observed. The bands in the optical absorption spectra are characteristic of VO2 þ ion in 2 tetragonally distorted octahedral site. The value of the b1 ( E0.84) coefficient shows an appreciable degree of covalency of the in plane V–O s-bonds in agreement with a C4v local symmetry of isolated vanadium ions.

4. Conclusion Various investigations such as infrared, Raman, EPR and optical absorption spectroscopies carried out on xV2O5–(50  x) CdO–50P2O5 show that homogenous glasses can be obtained for 0 rxr20 mol% V2O5. The infrared spectra of the existing structural phosphate groups and their arrangement are observed to be slightly affected by the change of glass composition in the range of composition studied, the intensities show minor variations, while the Raman spectra show the appearance of new bands from x Z2.5 mole% V2O5. FTIR and Raman spectra reveal the breakage of the PQO bond and the formation of P–O–V and V–O–V bonds at the expense of the P–O–P ones. The EPR measurements of the studied glasses evidenced the presence of the vanadium ion as VO2 þ vanadyl ions. The shape of the spectra is modified as V2O5 is added to the base glass matrix. It is found that the vanadyl ions appear as isolated species for 0.1 rxr10 mol% and occupied tetragonally compressed octahedral sites and belong to C4v symmetry. In addition to the isolated species in the samples with xr10 mol% the presence of clustered ions is also evidenced. The large value of K indicates a large contribution of the vanadium 4s orbital to the vanadyl bond.

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