Structural Investigations on SodiumâLead Borophosphate Glasses

Article pubs.acs.org/JPCA

Structural Investigations on Sodium−Lead Borophosphate Glasses Doped with Vanadyl Ions K. Srinivasulu,† I. Omkaram,‡ H. Obeid,§ A. Suresh Kumar,† and J. L. Rao*,‡ †

University Science Instrumentation Center and ‡Department of Physics, Sri Venkateswara University, Tirupati-517502, India § Laboratory of Materials, Catalysis, Environment and Analytical Methods, Department of Chemistry, Lebanese University, Beirut, Lebanon ABSTRACT: Electron paramagnetic resonance (EPR), optical absorption, and FTIR spectra of vanadyl ions in the sodium−lead borophosphate (Na2O−PbO−B2O3− P2O5) (SLBP) glass system have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of VO2+ ions. The spin Hamiltonian parameters g and A are found to be independent of the V2O5 content and temperature. The values of the spin Hamiltonian parameters indicate that the VO2+ ions in SLBP glasses are present in octahedral sites with tetragonal compression. The population difference between Zeeman levels (N) is calculated as a function of temperature for an SLBP glass sample containing 1.0 mol % VO2+ ions. From the EPR data, the paramagnetic susceptibility (χ) is calculated at different temperatures, and the Curie constant (C) is calculated from the 1/χ versus T graph. The optical absorption spectra of the glass samples show two absorption bands, and they are attributed to V3+ and V4+ ions. The optical band gap energy (Eopt) and Urbach energy (ΔE) are calculated from their ultraviolet absorption edges. It is observed that, as the vanadium ion concentration increases, Eopt decreases and ΔE increases. The study of the IR absorption spectrum depicts the presence of BO3, BO4, PO3, PO4, and VO5 structural units.

1. INTRODUCTION Phosphate glasses have been extensively studied since they have many technological applications, such as bone transplantation,1 fuel cell refractory ceramics sealing glass,2 solid-state lasers,3 electrolytes in electrochemical devices,4 fast ion conductors,5 anode material for Li ion batteries,6 nuclear waste vitrification,7 sealant,8 nonlinear optical devices, etc.9 The network structure of simple phosphate glass consists of P tetrahedra linked to neighboring tetrahedra through bridging oxygens (BOs). They result from the formation of sp3 hybrid orbitals by the P outer electrons (3s3p3). The fifth electron is promoted to a 3d orbital where strong π-bonding molecular orbitals are formed with oxygen 2p electrons. These tetrahedra link through covalent bridging oxygens to form various phosphate anions. The tetrahedra are classified using the Qn terminology, where n represents the number of bridging oxygens per tetrahedron.10 Addition of B3+, Al3+, Bi3+, etc. has been found to improve the chemical stability because of the formation and relative stability of the M3+−O−P bond.11 These ions modify various physical properties, including thermomechanical and optical behavior basically due to a change in the glass structural network through formation of cross-linked bonds. Phosphate glasses exhibit very important physical properties, such as low melting temperature, high thermal expansion coefficient, low glass transition temperature (280−380 °C), low softening temperature, high ultraviolet (UV) transmission, and high refractive index,12 and their applications can be found in optoelectronics, solid-state batteries, high-power lasers, etc. The © 2012 American Chemical Society

poor chemical durability and low thermal stability of phosphate-based glasses have been major constraints for the practical utility of these glasses. However, the addition of boron atoms13 (less than 15 mol %) into the phosphate glass network has been shown to increase the chemical durability or control the volume nucleation (precursor of glass ceramics).14 The combination of the two network formers B2O3 and P2O515 enables considerable modifications of the properties of the materials compared to simple borate and phosphate networks alone. The role played by P2O5 and B2O3 in the glass structure, and the interaction with other elements in the glass network, is an interesting subject under study. For that reason, the mixing of borate and phosphate groups16 doped with 3d transitionmetal ions is an interesting subject of glass science. Phosphate glasses doped with transition-metal ions are technologically important because of their semiconducting properties, optical absorption, memorization, and photoconducting properties. Vanadium ions are very suitable for use in glasses because they are characterized by a partially filled d shell which can exist in at least two valence states. A large number of papers reported that, in vanadium-doped phosphate glasses, the main redox state is V4+, but spectroscopic data show the appearance of vanadyl VO2+ ions. The vanadyl ion (VO2+) incorporated into glasses, as a spectroscopic probe, is useful in Received: October 29, 2011 Revised: March 12, 2012 Published: March 12, 2012 3547

dx.doi.org/10.1021/jp210398k | J. Phys. Chem. A 2012, 116, 3547−3555

The Journal of Physical Chemistry A

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glass samples were recorded at room temperature on an EPR spectrometer (Bruker EMX Plus) operating at the X-band frequency (9.205 GHz) with a modulation frequency of 100 kHz. The magnetic field was scanned from 2480 to 4480 G with a scan speed of 250 G/min. A powdered glass specimen of 100 mg was taken in a quartz tube for EPR measurements. Since the shape of the spectra under investigation remained unchanged with a variation in the content of different additives, it was assumed that the intensity of the EPR absorption signal at the maximum is proportional to the concentration of paramagnetic centers. The spectrum of the CuSO4·5H2O powdered substance was also recorded as a reference to calculate the spin concentration. The EPR spectra of the 1.0 mol % V2O5-doped sodium−lead borophosphate glass sample were recorded at low temperatures using a variable temperature controller. Optical absorption spectra of all glass samples containing V2O5 were recorded at room temperature on a UV−vis−NIR JASCO-V570 spectrophotometer in the wavelength region of 400−1500 nm. Glass samples of 1 mm thickness were used for optical measurements. Powder X-ray diffraction (XRD) patterns were recorded with an X-ray diffractometer using Cu Kα radiation. The chemical compositions of the glasses were examined with the distribution spectra to determine the homogeneity, which were obtained with a ZEISS EVO MA 15 EDS model scanning electron microscope and semiquantitative elemental analysis system. The nominal composition of the glass was determined by EDS analysis by analyzing one sample at different positions. FT-IR transmission measurements were recorded over the range of 400−2000 cm−1 using a Perkin-Elmer Spectrum One FT-IR instrument.

characterizing the glass local structure. The ability to characterize the local structure of a paramagnetic center and sensitive detection of structural changes form the basis for the increasing number of applications of the electron paramagnetic resonance (EPR) technique to glasses. EPR has been successfully applied to obtain information about transition-metal ions in various glass systems.17−19 Studies on EPR and optical properties of transition-metal ions in glasses have made it possible not only to interpret the energy levels involved in the observed transitions but also to know the chemical and structural environments about the metal ion center. The object of the present work is to determine the main structural units forming the glass network in the fourcomponent Na2O−PbO−B2O3−P2O5 glass system. Hence, structural studies of the borophosphate network are motivated by the improvement of the glass properties. The vanadyl-doped borophosphate glasses are interesting due to their optical and magnetic properties. The spin Hamiltonian parameters and optical absorption bands of VO2+ ions in glasses are sensitive to the ligands present around them in the glass matrix. In view of the aforementioned properties, it is worthwhile to investigate the structural and corresponding spectroscopic data for an interpretation of the EPR and optical spectra in complex oxide glasses, the details of which are reported in this paper.

2. EXPERIMENTAL SECTION 2.1. Glass Preparation. Seven glasses of the quaternary sodium−vanadium borophosphate glass system (30 − x)(NaPO3)6 + 30PbO + 40B2O3 + xV2O5 were prepared by varying the V2O5 content in the range 0 ≤ x ≤ 7. At first, reagents were precisely weighed in an electronic balance, mixed thoroughly, and ground to a fine powder. The ground mixture was taken in porcelain crucibles and melted slowly in an electrical muffle furnace under ordinary atmospheric conditions. The temperature of the furnace was first increased to 673 K to remove water vapor, and then the temperature was increased to about 1173 K. The melt was kept at this temperature for 1 h in the furnace, during which the molten glass samples were taken out with certain intervals and stirred to ensure homogeneity. The homogenized melt was poured onto a preheated steel mold plate to avoid breaking of the samples due to thermal strains and pressed with another plate quickly to obtain circle-shaped samples. To prevent breaks and cracks, these glass samples were taken out of the mold, kept in a sintering furnace at 573 K for 30 min, and then cooled slowly to room temperature. The glasses thus obtained were greenyellow in color. The glass samples were kept in a liquid paraffin to prevent possible attack by moisture. As the obtained samples were made in porcelain crucibles at high temperatures, it is possible that porcelain crucibles can be attacked by corrosive melts. Moreover, the integrity of our crucibles after the melt was quenched was observed to be intact, and the set compositions are taken for granted. There are few reports in the literature wherein the glasses were prepared at these temperature ranges, and no chemical analysis has been carried out on them.20 However, to justify the given compositions, we have performed chemical analysis on one of our samples (1 mol %), and the results of the nominal and analyzed batch compositions were in good agreement with each other. 2.2. Spectral Measurements. EPR spectra of (30 − x)(NaPO3)6 + 30PbO + 40B2O3 + xV2O5 (x = 1, 2, 3, 4, 5, 6, and 7 mol %) (hereafter referred to as sodium−lead borophosphates (SLBPs) V1, V2, V3, V4, V5, V6, and V7)

3. RESULTS 3.1. XRD Analysis. X- ray diffraction is a useful method to readily detect the presence of crystals in a glassy matrix if their dimensions are greater than typically 100 nm. The X-ray diffraction pattern of an amorphous material is distinct from that of crystalline material and consists of a few broad diffuse halos rather than sharp rings. The doped sample was tested, and the results showed the absence of crystalline characteristics. Figure 1 shows the typical X-ray diffraction pattern for this

Figure 1. XRD pattern of SLBP glasses doped with V2O5. 3548



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composition. As seen from the figure, the diffraction pattern of the doped sample does not exhibit any detectable peak. This pattern indicates that glass samples are amorphous in nature and have noncrystalline structures. The EDS (energy-dispersive spectrometry) investigations were performed on SLBP V1 glass to identify changes in the chemical composition. The EDS analysis (Figure 2) at several different spots on the glass sample

Figure 3. EPR spectra of SLBP glasses doped with different V2O5 concentrations.

Figure 2. EDS spectrum of 1 mol % V2O5 SLBP glass.

gives almost an identical spectrum, confirming the homogeneous character of the sample. The chemical composition calculated from the EDS spectrum is in excellent agreement with the nominal composition of the glass. From the EDS spectrum, the atomic percentages for O, Na, P, and Pb are found to be 68.55, 14.13, 16.44, and 0.89. It is understood that, with the measurements in this region, an almost homogeneous distribution was formed in the glass. The white spots are visible, and these spots are considered to be the result of the interaction of glasses with atmospheric humidity, which is a typical behavior of borate-containing glasses. With the testimony of the spots on the surface, it was concluded that glass containing 40% B2O3 interacts with air. 3.2. EPR Studies. No EPR signal is observed in the prepared glasses, confirming that the starting material used in the present work was free from transition-metal impurities or other paramagnetic centers (defects). When various concentrations (mol %) of V2O5 were added to sodium−lead borophosphate glasses, the EPR spectra of all the investigated samples exhibited resonance signals as shown in Figure 3. The spectra have structures which are characteristic of a hyperfine interaction arising from an unpaired electron with a 51V nucleus whose nuclear spin is 7/2 and which is present in 99.76% abundance. The EPR spectrum shows a well-resolved eight-line hyperfine pattern which is due to the dipole−dipole interaction (S = 1/2) between the magnetic moment of the 51V nucleus (I = 7/2) and the electronic moment of the paramagnetic V4+ ions. The well- known expression given by Weil et al.21 was used to calculate the spin concentration (N) participating in resonance (population difference between Zeeman levels) by comparing the area under the absorption curve with that of a standard (CuSO4·5H2O) of known concentration. N in this glass system was calculated for different concentrations of vanadyl ions (1 mol % ≤ x ≤ 7 mol %). Figure 4 shows a plot

Figure 4. Variation of the spin concentration with the concentration (mol %) of V2O5 in SLBP glasses.

between the concentration of spins and vanadyl content for this glass system. The number of spins increases with increasing of V2O5 content. This results in strong magnetic coupling among the V4+ ions, which are close enough to cause dipolar broadening. The formation of associated V4+ ions in the composition range (1 mol % ≤ x ≤ 7 mol %) is due to the dipole−dipole interaction between vanadium ions. The EPR spectra were recorded at different temperatures for the SLBP V1 glass sample and are shown in Figure 5. It is observed that, as the temperature is lowered, the spin concentration increases, obeying the Boltzmann law. 3.3. Calculation of Susceptibility from EPR Data. The EPR data can be used to calculate the paramagnetic susceptibility22 of the sample, and the paramagnetic susceptibilities were calculated for the SLBP V1 glass sample at various temperatures. A plot is drawn between the reciprocal of susceptibility (1/χ) as a function of absolute temperature (T). It is clear that, as the temperature is increased, the susceptibility of the sample decreases, obeying Curie’s law. 3549



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one broad and intense band is observed at 916 nm. In addition to this, a weak band is observed at 659 nm. Experimental data on optical absorption in glassy materials were reported by Mott and Davis.24 The optical absorption coefficient measurements have generally shown an exponential dependence on the photon energy.25 One important difference between amorphous and crystalline solids is that there is a sharp well-defined lattice absorption edge for crystalline solids given by the relation Eg = hc/λ, where Eg is the energy gap, h is Planck’s constant, and c is the velocity of light, whereas in amorphous materials the absorption edge has a finite slope. Hogarth and Hosseini25 showed that the shape and position of the absorption edge for a high-absorption region could be represented by an equation of the form α(ω) = A(ℏω − Eopt)n /ℏω

(1)

for indirect transitions, where n = 2 allowed indirect transitions, A is a constant, Eopt is the indirect optical band gap energy, α is the absorption coefficient for the indirect transition, and ω is the angular frequency. 3.5. FT-IR Analysis. FT-IR spectroscopy has been utilized to study the structural changes produced by the variation of the V2O5 content in (30 − x)(NaPO3)6 + 30PbO + 40B2O3 + xV2O5 (x = 1, 2, 3, 4, 5, 6, and 7 mol %) glasses. The infrared absorption spectra of the glasses shown in Figure 7 exhibit

Figure 5. EPR spectra of SLBP glass doped with 1.0 mol % V2O5 observed at different temperatures (93−333 K).

3.4. Optical Absorption Spectra. The optical absorption spectroscopy technique is very useful for the investigation of optically induced transitions and provides insight into the energy gap and the band structure of crystalline and noncrystalline materials. The absorption spectra of vanadiumcontaining glasses are generally of a very complex nature because of the simultaneous presence of three valence states of vanadium (V3+, V4+, and V5+), which usually exist in varying proportions depending mainly on the nature of the host glass, the conditions of melting (temperature and time), and the concentration of vanadium in the glass. In phosphate glasses, the vanadium ions have bands somewhat close to each other which can overlap and are assumed to exist mainly in lower valence states as V3+ (trivalent) and V4+ (tetravalent) ions.23 Figure 6 shows the optical absorption spectra of VO2+ ions doped in SLBP glass samples at room temperature observed between 400 and 1500 nm. From the figure, it is evident that

Figure 7. FTIR absorption spectra of SLBP glass doped with different concentrations of V2O5 observed at room temperature.

bands with maxima at 455, 529, 729, 870, 940, 1070, 1165, 1347, 1388, and 1651 cm−1. It seems that the addition of transition-metal ions in the doping level did not cause structural changes which could be detected by the IR spectra. The IR bands are correlated with their specific groups of vibrations.

4. DISCUSSION The vanadium ion (V4+) has the electronic configuration [Ar]3d1, which thereby leads to paramagnetism in VO2+. The oxidation−reduction (redox) equilibrium of transition-metal ions in glasses involves several concepts: redox, acid−base reaction (O2− exchange), and complexation with O2−. For instance, in vanadium-doped phosphate glass, V4+ is the main redox state, and spectroscopic data show the occurrence of the

Figure 6. Optical absorption spectra of SLBP glass doped with different concentrations of V2O5 observed at room temperature. 3550



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VO 2+ complex ion. A simple equation may then be considered:26

vanadyl ions in a square pyramidal site with C4v symmetry. The vanadyl oxygen is attached axially above the V4+ site along the z axis (VO bond), while the sixth oxygen forming the O− VO4−O unit lies axially below the V4+ site opposite the vanadyl oxygen. The predominant axial distortion of the VO2+ octahedral oxygen complex along the VO direction may be the reason for the nearly equal g and A values for all the glass samples.32 The g and A values obtained in the present work are close to those of vanadyl phosphate and borate glasses reported in the literature.23,33 No significant changes have been observed in the spin Hamiltonian parameters with a change in the composition of the glass or concentration of vanadium. Hecht and Johnston34 studied extensively the EPR and optical spectra of V4+ ions in oxide glasses and reported that there are two ways, either by 3-fold symmetry or 4-fold symmetry, to describe the crystal field of V4+ ions in glasses and concluded that the V4+ ions in sodium borate glasses exist in octahedral coordination with a tetragonal compression and have a C4v symmetry. The tetragonal compression was interpreted in terms of two of the V−O bonds being shorter than the other five; i.e., the ion in the glass was regarded as a vanadyl complex, VO2+. An octahedral site with tetragonal compression would give g|| < g⊥ < ge (ge = free electron value, 2.0023) and |A||| > |A⊥|.35 The contributions of an admixture of d electrons to the values of the hyperfine structure constants are evaluated by using the expressions developed by Kivelson and Lee:36

2V 5 + + O2 − → 2V 4 + + 1/2 O2 ↑

It is known that the undoped phosphate glass has diamagnetic properties and the vanadyl-doped phosphate is a paramagnetic material. The V4+ ion, being paramagnetic in nature, produces a characteristic eight-line ESR spectrum. For a lower content of V2O5 (x = 1.0 mol %) these spectra show a well-resolved hyperfine structure (hfs) typical of vanadium ions. The 16-line feature with 8 parallel and 8 perpendicular lines is typical of the unpaired (3d1) electron of the VO2+ ion associated with 51V (I = 7/2) in an axially symmetric crystal field.27 The spectra of these glasses show patterns very similar to those found in various phosphate glasses containing vanadium.28,29 The spectroscopic splitting factor (g) and hyperfine (A) tensors with axial symmetry have been assumed in the analysis of the well-resolved hyperfine structure of phosphate glass EPR spectra. An axial spin Hamiltonian, which includes the hyperfine interaction, has been used to describe the EPR spectra of V4+ ions:30 / = β0g Bz Sz + β0g⊥(Bx Sx + By Sy) + A SzIz + A⊥(SxIx + SyIy)

(2)

where β0 is the Bohr magneton, g|| and g⊥ are the parallel and perpendicular principal components of the g tensor, A|| and A⊥ are the parallel and perpendicular principal components of the hyperfine coupling tensor, Bx, By, and Bz are the components of the magnetic field, and Sx, Sy, and Sz and Ix, Iy, and Iz are the components of the spin operators of the electron and nucleus, respectively. The quadruple and nuclear Zeeman interaction terms have been ignored. The values of the magnetic field for the hfs peaks from the parallel and perpendicular absorption bands are given by the following equations:31

A = −P[β*2 (4/7 + K ) + Δg + (3/7)Δg⊥]

A⊥ = P[β*2 (2/7 − K ) − (11/14)Δg⊥] (6) 2 where β* is the measure of the degree of π-bonding and is unity for a purely ionic bond. Δg|| = ge − g||, Δg⊥ = ge − g⊥, and ge (=2.0023) is the g factor for a free electron. The calculated values of P (126 × 10−4 cm−1) are nearer to the free ion value P = 128 × 10−4 cm−1 37 for a positive ratio of A|| to A⊥. A|| and A⊥ are found to be negative by the method proposed by Muncaster and Parke.31 The Fermi contact term K is the dimensionless core polarization parameter, which represents the amount of unpaired electron density at the vanadium nucleus. The term −PK in eqs 5 and 6 is due to the s-character of the magnetic spin of the vanadium. Basically, this s-character results from the partial impairing or polarization of the inner electrons as a result of an interaction with the unpaired d electrons.38 From molecular orbital theory, 39 it can be shown that the components A|| and A⊥ consist of the contributions A||1 and A⊥1 of the 3dxy electron to the hyperfine structure and the PK term arises due to the anomalous contribution of the s electrons. Equations 5 and 6 can be rewritten in the following way:

B (m) = B (0) − A m − {(63/4) − m2}A⊥2 /2B (0) (3)

B⊥(m) = B⊥(0) − A⊥m − {(63/4) − m2}(A 2 + A⊥2) /4B⊥(0)

(5)

(4)

where m is the magnetic nuclear quantum number for the vanadium nucleus having the values ±7/2, ±5/2, ±3/2, and ±1/2 and B||(0) = hν/g||β0 and B⊥(0) = hν/g⊥β0, where ν is the microwave frequency. Measurements for the B|| position were taken which correspond to a maximum in the first-derivative curve of the parallel hyperfine structure component for a given m value, whereas the B⊥ position is enclosed between the firstderivative perpendicular peak and its “zero”. The spin Hamiltonian parameters of the VO2+ ion determined from eqs 3 and 4 for the present glass samples are g|| =1.943, g⊥ = 1.995, A|| = 183 × 10−4 cm−1, and A⊥ = 68 × 10−4 cm−1. The g and A values depend upon several molecular parameters and crystal field splittings. The changes in the g and A values depend upon the ligand coordination around the VO2+ ion and the properties of the ligands, such as their electronegativity, πbonding ability, ligand field strength, etc. The data obtained show that g|| < g⊥ < ge. The g values are generally lower than the free electron value, ge = 2.0023. This lowering can be related to the spin−orbit interaction of the ground-state dxy level, with low-lying excited states; the A|| > A⊥ relation corresponds to

A = A 1 − Pβ*2 K

(7)

A⊥ = A⊥1 − Pβ*2 K (8) 1 1 The values of A|| and A⊥ were calculated and are found to be around 80 × 10−4 and 35 × 10−4 cm−1. The degree of distortion is estimated from the Fermi contact interaction parameter K representing the amount of unpaired electron density at the vanadium nucleus. The isotropic term K is given by the following equation: K = A0/(P − Δg0), where Δg0 = ge − g0 and g0 is the isotropic g factor obtained from the relation g0 = (g|| + 2g⊥)/3. From the above equations, it is clear that K is an 3551



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independent property of the central ion and indicates the interaction between the electronic and nuclear spins. For transition-metal ions, K is found to be positive.40 It is found that the K values are independent of the temperature and concentration of vanadyl ions present in these glasses. The large values of K indicate a large contribution to the hyperfine constant by the unpaired s electron. Pβ*2K is a measure of the s-character of the spin of the vanadium and also the tetragonal distortion, where K is the Fermi contact interaction term and P is the dipolar hyperfine coupling parameter. The obtained values of K and β*2 are found to be 0.8 and 1, respectively. Δg||/Δg⊥ could be used to determine the degree of tetragonal distortion of the vanadyl complex. The quantity Δg||/Δg⊥ reflects changes in the tetragonal distortion. The distortion takes place along the VO bond direction. The Δg||/Δg⊥ values decrease (from 9.253 to 7.361) with an increase in the concentration of V2O5 in these glasses. This implies that, as the concentration increases, the deviation from octahedral symmetry decreases and the VO2+ ions are less tetragonally distorted. For the 51V nucleus, the value of the gyromagnetic ratio γ is 1.468.38 Using this value, the authors evaluated PK/γ and obtained a value of 0.007 for the vanadyl ions doped in SLBP glasses, which is in good agreement with the theoretical value expected for VO2+ ions.41 Figure 8 shows the temperature dependence of the spin concentration of SLBP V1 glass. The linear relationship

Figure 9. 1/χ versus T of SLBP glass doped with 1.0 mol % V2O5.

accordance with Curie’s law. The Curie law is observed in the temperature range of vanadium ions. This suggests that the majority of the vanadium ions are magnetically isolated and that no magnetic order is present. This behavior agrees with the conclusions from the EPR study. The intercept on the x axis of this graph gives the Curie temperature, which is related to the strength and sign of the dominant interactions and is found to be Θp = 83 K. The nature of the magnetic interactions is indicated by the value of the Curie temperature. A low value (negative) of the Curie temperature is attributed to high antiferromagnetic interactions, whereas if the Curie temperature is positive, then the ions are ferromagnetically coupled. In the present work, a positive value of Θp indicates that the VO2+ ions are ferromagnetically coupled and the Curie constant (C) has been evaluated from the slope of the graph and is found to be 52.7 × 10−3 emu mol−1, which is on the same order as that reported for VO2+ complexes.42 4.1. Optical Absorption. The optical absorption spectra of these SLBP glasses (V1−V7) in the UV−vis−NIR range are shown in Figure 6. One broad and intense band is observed at 916 nm. In addition, a weak band is observed at 659 nm. When we dope V2O5 in the glass samples, initially the vanadium ions are in the V5+ state. During the melting process of preparing the glass samples, there may be a conversion of part of the vanadium ions in the V5+ oxidation state to V4+ and V3+ states. The broad band observed at 916 nm has been attributed to the V4+ ion, and this band is assigned to the 2B2g → 2Eg transition. There are a few reports in the literature26,41 on the observation of a band for the V4+ ion in this wavelength region, and this band position indicates that V4+ ions are in tetragonal symmetry (C4v). At higher concentrations the weak band observed at 659 nm shows a slight blue shift in the lower wavelength region (604 nm), and this band has been attributed to the V3+ ion and is assigned to the 3T1g(F) → 3T2g(F) transition. A similar blue shift is reported by Ramesh Babu et al.26 for VO2+ in lithium−strontium borate glasses. From Figure 6, it is also observed that the samples have a distinct cutoff wavelength. The values of the cutoff wavelength (λc) are different for the samples and are shown in Table 1. The experimental value of the absorption coefficient α(ω) near the edge is calculated by using the relation43 α(ω) = 2.303A/d deduced from the Lambert−Beer law. Theoretically,

Figure 8. log N as a function of 1/T of SLBP glass doped with 1.0 mol % V2O5.

between the logarithm of the spin concentration (log N; N is measured in spins per kilogram) and the inverse of temperature (1000/T; T is measured in kelvin) with a slope satisfying the Arrhenius relation. It is observed that the Boltzmann relationship is obeyed. The activation energy (E) is determined from the log N versus 1/T graph by using the least-squares fitting of the experimental data with the relation log N = 21.243 + 59.17/T. The activation energy (ΔE) thus calculated is found to be 0.013 eV (2.15 × 10−21 J), and it is on the same order as that expected for VO2+ complexes.41 The temperature dependence of the reciprocal of the magnetic susceptibility of the investigated samples is shown in Figure 9. It is observed that the paramagnetic susceptibility continues to increase with decreasing temperature, which is in 3552



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gradual decrease in Eopt is due to an increase of the nonbridging oxygen ion concentration. A similar result was reported by Mirzayi and Hekmatshoar.45 The decrease in the value of the optical band gap Eopt with increasing V2O5 content can be understood in terms of the structural changes that are taking place in the glasses. Eopt decreases from 3.00 to 2.16 eV as the concentration of V2O5 increases from 0 to 7 mol % in SLBP glass systems. The introduction of V2O5 as a network modifier brings about the formation of nonbridging oxygens simply bound by O2− ions. In metal oxides, the valence band maximum (VBM) mainly consists of an O(2p) orbital and the conduction band minimum (CBM) mainly consists of an M(nS) orbital. The NBO ions contribute to the VBM. When a metal−oxygen bond is broken, the bond energy is released. The nonbridging orbitals have higher energies than bonding orbitals. Therefore, an increase in concentration of the NBO ions results in a shift of the VBM and a reduction of the band gap energy. The absorption coefficient α(ω) of the optical absorption near the band edge shows an exponential dependence on the photon energy ℏω and obeys the empirical relation due to Urbach:46 α(ω) = α0 exp(ℏω/ΔE), where α0 is a constant and ΔE is the width of the band tails of localized states. Tauc47 suggested that the exponential dependence of the absorption coefficient on the photon energy ℏω arises from electron transitions between localized states and is exponentially dependent on the energy. The Urbach energy, which corresponds to the width of localized states, is used to characterize the degree of disorder in amorphous systems. Materials with a larger Urbach energy show a greater tendency to convert weak bonds into defects. From the variation of ln α with the photon energy ℏω, the values of ΔE were calculated from the slope of the graph and are listed in Table1. It can be seen that the values of ΔE are concentration dependent and vary from 0.41 to 0.74 eV. The exponential dependence of the absorption coefficient α(ω) on the photon energy ℏω suggests that these glasses obey the Urbach rule. 4.2. FT-IR Studies. The FT-IR spectrum can provide information about molecular vibration or rotation associated with a covalent bond and is usually used to survey the variation of the glass structure with change in composition. As shown in Figure 7, IR measurements have been carried out to investigate the structure of the SLBP glasses and the effect of V2O5 on their physical properties. Infrared absorption spectra indicate the presence of phosphate and borate groups as the dominant species exhibiting characteristic vibrational bands. The existing structural groups and their arrangement are observed to be slightly changed with a change in the concentration of V2O5 in

Table 1. Cutoff Wavelength, Indirect Band Gap, and Urbach Energy for Vanadyl Ions in Sodium−Lead Borophosphate Glass Samples with an Increase of the V2O5 Content glass sample

V2O5 concn (mol %)

cutoff wavelength (nm)

indirect band gap Eopt (eV)

Urbach energy ΔE (eV)

V1 V2 V3 V4 V5 V6 V7

1 2 3 4 5 6 7

469 472 484 492 496 498 494

3.00 2.88 2.74 2.47 2.35 2.26 2.16

0.41 0.48 0.52 0.56 0.66 0.70 0.74

the optical band gap energy for indirectly allowed optical transitions can be calculated by using eq 1. The quantity (αℏω)1/2 is plotted as a function of the photon energy (ℏω) for all glass samples as shown in Figure 10. The straight portion of

Figure 10. (αℏω)1/2 versus ℏω for SLBP glass doped with V2O5.

the graph is then extrapolated to intersect with the x axis. The value of ℏω at the point where (αℏω)1/2 becomes zero yields an indirect measure of the optical band gap energy. The variation of the optical band gap energy Eopt with the concentration of V2O5 (Table 1) shows that Eopt decreases as the V2O5 concentration increases in these ternary phosphate glasses, and the same trend is reported by Altaf et al.44 The

Table 2. Assignment of Absorption Bands in the Infrared Spectra (with a Probable Error of 0.1 cm−1) for Vanadyl Ions in Sodium−Lead Borophosphate Glass Samples with an Increase of the V2O5 Content undoped

V1

V3

V5

V7

assignment

1651 1388 1347 1165 1070 940 870 729 529

1654 1387 1347 1164 1068 942 872 729 530 455

1652 1385 1340 1166 1070 940 871 729 529 456

1650 1386

1652 1386

1165 1069 945 873 729 530 454

1165 1070 943 870

ascribed to hydroxyl or water groups due to asymmetric B−O stretching vibration due to asymmetric B−O stretching vibration due to vibrations of PO2 of metaphosphate groups nonbridging VO of the [VO5] groups asymmetric stretching vibration of P−O−P bonds stretching vibrations of B−O bonds O3B−O−BO3 bond bending vibrations asymmetric bending vibration of P−O bonds from PO43‑ groups coupled V−O and P−O bending modes for both VO4and PO4− groups

530

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the glass. The intensities of the characteristic groups of vibrations show minor variations with the V2O5 content. The FT-IR spectrum of the glass matrix (x = 0) exhibits three major broad bands and also some shoulders in the wavenumber ranges of 450−650, 650−1150, and 1150−1500 cm−1. These bands are characteristic of bond vibrations of the phosphate−oxygen and boron−oxygen networks (Table 2). In the wavenumber range of 450−650 cm−1, one absorption band is located at 455 cm−1, which is due to the coupling of P−O and V−O bending modes for both PO4 and VO4 groups.48 The bands in teh 500−800 cm−1 range are related to B−O stretching of tetrahedral [BO4] units and bond bending motion of B−O−B groups, respectively. The asymmetric bending vibration of P−O bands from PO43− groups are characterized by a broad band at 529 cm−1.49 The next band centered at 720 cm−1 is assigned to the O 3 B−O−BO3 bond bending vibrations.50 These two bands have similar immediate variations in intensity with the addition of vanadium oxide, but no further noticeable changes occur over the composition range. The small band centered at 870 cm−1, which is characteristic of stretching vibrations of B−O bonds in BO4 tetrahedra, is from diborate groups.51 The appearance of the absorption band at 1070 cm−1 is characteristic of the vibrations of the nonbridging VO of the [VO5] groups; the band at 940 cm−1 is attributed to asymmetric stretching of PO3 groups in PO4 units.52 The band at 1070 cm−1 increases in intensity with an increase of transition-metal oxide content because of an increase of the number of ionic (PO2) groups and VO bonds. A band at 1165 cm−1 is identified due to vibrations of PO2 of metaphosphate groups. The intensity of this band decreases and becomes broader with an increase of the transition-metal oxide content because of the depolymerization of the phosphate matrix and an increase of the disorder degree in these glasses.53 Hence, the phosphate network is depolymerized owing to the role of PbO as a network former; the PO2 units interact with PbO, forming P−O−Pb linkages/bridges, which connect the chains together. The absorption bands at 1347 and 1388 cm−1 are due to asymmetric B−O stretching vibrations of [BO3, BO2Φ] trigonal units; where Φ indicates an oxygen atom bridging two boron atoms.54 In general, the absorption band observed in the 1600−3500 cm−1 region is ascribed to hydroxyl or water groups present in the glass.

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5. The optical absorption spectra exhibit two bands, and they are assigned to V3+ and V4+ ions. 6. From EPR and optical studies, it is observed that vanadium ions in the glass system exist as isolated VO2+ ions in a square pyramidal site with a tetragonal compression having C4v symmetry. 7. From the ultraviolet absorption edges, it is interesting to observe that the optical band gap energies of these glasses slightly decrease and the Urbach energy increases with an increase of the vanadium concentration; this is due to an increase of nonbridging oxygens. 8. The FTIR spectra of these glasses have been analyzed to identify the spectral contribution of each component on the structure and to point out the role of the vanadyl ions. The spectra show the existence of BO3, BO4, several modes of PO4 units, and VO5 structural units in borophosphate glasses.

AUTHOR INFORMATION

Corresponding Author

*Phone: 91-877-2249666 ext 272, 91-877-2287258. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. J. L. Rao thanks the University Grants Commission, New Delhi (India) for the award of an Emeritus Fellowship. REFERENCES

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5. CONCLUSIONS The following conclusions were made: 1. The EPR spectra of the V2O5-doped SLBP glasses exhibit characteristic spectra of hyperfine lines arising from an unpaired electron with the 51V nucleus. It is found that the V4+ ions can be formed by redox reaction of V5+ ions and exist as the VO2+ complex in octahedral coordination with a tetragonal compression with a C4v symmetry in SLBP glasses. 2. It is observed that the spin Hamiltonian parameters (g and A) do not vary with the change in vanadium concentration and temperature. 3. The spin concentration (N) participating in resonance and magnetic measurements revealed dipolar interactions involving vanadyl ions. 4. The variation of Δg||/Δg⊥ values with an increase in concentration indicates that, as the concentration increases, VO2+ ions are less tetragonally distorted. 3554



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