Indian Journal of Pure & Applied Physics Vol. 45, March 2007, pp. 243-255

Dielectric and optical properties — Structure correlations in NiO doped Li2O-MgO-B2O3 glasses

R Balaji Rao, M Srinivasa Reddy, P Yadagiri Reddy+ & NVeeraiah*

Department of Physics, Acharya Nagarjuna University P.G. Centre, Nuzvid 521 201

*E-mail: nvr8@rediffmail.com +Department of Physics, OU College of Science, Osmania University, Hyderabad 500 007

Received 7 June 2006; revised 24 October 2006; accepted 8 January 2007

Glasses of the system, Li2O-MgO-B2O3 containing different concentrations of NiO (ranging from 0 to 1.0 mol %) were prepared. A number of studies viz., dielectric properties (constant ε, loss tanδ, ac conductivity σac over a range of frequency and temperature and dielectric breakdown strength) optical absorption, magnetic susceptibility and thermoluminescence on these glasses were carried out as a function of NiO concentration. An anomaly has been observed in all these properties when the concentration of NiO is about 0.6 mol % in the glass matrix. This has been explained in the light variation in the environment of Ni2+ ions in the glass network.

Keywords: Li2O-MgO-B2O3 glasses: Optical absorption, Dielectric properties, Thermoluminscence IPC Code: G01R 27/26

1 Introduction

Alkali/alkaline-earth oxyborate glasses are well known due to their variety of applications in phosphors, solar energy converters and in a number of electronic devices. These glasses are relatively moisture resistant possess high mechanical strength when compared with the pure borate glasses. Extensive studies on various properties like ESR, optical, elastic and some electrical properties of various alkali/alkaline-earth borate glasses doped with different transition and rare earth metal ions are available1-6. The divalent nickel ion is an interesting paramagnetic ion to probe in the glass systems. Nickel ions are reported to occupy both tetrahedral and octahedral positions in the glass matrices. A number of recent studies on a variety of inorganic glass systems containing Ni2+ ions are available in literature7-12 Attempts were also made to detect the lasing action of these ions in certain crystal and glass systems since these ions possess several strong absorption bands in the visible and NIR regions where the pumping sources are easily available13. The octahedrally positioned Ni2+ ions are expected to exhibit eye safe laser emission of wavelength 1.56 µm (3T2→

3A2) even at room temperature with low threshold energy, which is of great importance in telecommunications14. The concentration of the ions present in tetrahedral or octahedral positions depends

on the quantitative properties of modifiers and glass formers, size of the ions in the glass structure, their field strength, mobility of the modifier cation etc. Further, the investigation on the co-ordinate chemistry of Ni2+ ions in alkalioxyborate glass network is of interest in itself, because, these ions are expected to influence the physical properties of the glasses to a large extent. Hence, the structural aspects of Li2O-MgO-B2O3 glasses containing small concen-tration of nickel ions by studying some of their physical properties viz., dielectric properties (the dielectric constant, εl, the loss, tanδ, ac conductivity, (σ), in the frequency range 102-106 Hz and temperature range 30-300°C and the dielectric breakdown strength in air), spectroscopic properties (optical absorption, and IR spectra), magnetic properties (magnetic susceptibility), have been studied. Further, study on radiation induced defect centers in glasses has been an interesting subject of investigation in recent years, since such studies help in examining the suitability of the glasses for radiation dosimetry applications and the structural aspects of the glasses. Extensive studies on the activating or killing effect of luminescence due to some transition metal ions like Fe2+, Cu2+, Mn4+etc, in amorphous materials are available in literature15-19. In recent years, Pontuschka et al.

18 have carried out a commendable work on thermoluminescence

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mechanisms in borate glasses20-22. Borate glasses are very advantageous materials for the radiation dosimetry applications and their effective atomic number is very close to that of human tissue. Lithium oxyborate glasses mixed with different alkaline earth oxides are considered as good materials for dosimetry applications since they are relatively moisture resistant when compared with the pure borate glasses. Hence, the thermoluminescence studies on Li2O-MgO-B2O3: NiO glasses, have been carried out. 2 Experimental Details

The compositions of the glasses chosen for the present study 30Li2O-10MgO-(60-x)B2O3: xNiO (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol % and the samples are labeled as N0, N2, N4, N6, N8, and N10 respectively). Appropriate amounts of Analar grade reagents of Li2CO3, MgCO3, H3BO3 and NiO were thoroughly mixed in an agate mortar and melted in a platinum crucible at 950 ± 10°C for about 1h until a bubble free liquid was formed. The resultant melt was then cast in a brass mould and subsequently annealed at 300°C. The amorphous state of the glasses was checked by the X-ray diffraction method. The samples were then ground and optically polished. The final dimensions of the samples used for dielectric and optical studies were about 1 cm× 1 cm×0.2 cm. The density (d) of the glasses was determined to an accuracy of 0.001 by standard principle of Archimedes' using o-xylene (99.99% pure) as the buoyant liquid. A thin coating of silver paint was applied (to the larger area faces) on either side of the glasses to serve as electrodes for dielectric measurements. Infrared transmission spectra for these glasses were recorded using a Perkin-Elmer 1600. Infrared Spectrophotometer in the frequency range 400-4000 cm−1 by KBr pellet method. The optical absorption spectra of the present glasses were recorded at room temperature in the wavelength range 350-1400 nm up to a resolution of 0.1 nm using a

Shimadzu -UV-VIS-NIR Spectrophotometer Model 3101. Magnetic susceptibility measurements were made by Guoy's method using fine powder of these glasses. The other details of the experimental techniques like dielectric measurements, thermoluminescence and D S C etc., used in the present investigation are similar to those reported in our earlier papers 23,26. 3 Results and Discussion Various physical parameters such as total nickel ion concentration Ni, mean nickel ion separation Ri, are evaluated using the measured values of density d and the calculated average molecular weight M for the present glasses and are presented in Table 1. From the differential scanning calorimetric patterns of the glasses, the value of glass transition temperature Tg of pure Li2O-MgO-B2O3 glasses is determined as 480°C as shown in Table 1. The doping of NiO, with gradual increase in concentration, up to 0.6 mol %, caused a gradual increase of the glass transition temperature Tg of the glasses. Beyond this concentration, the value of Tg is found to decrease (Table 1). The optical absorption spectra of NiO doped Li2O-MgO-B2O3 glasses recorded at room temperature in the wavelength range 350-1400 nm are shown in Figs 1(a & b). The spectrum of the glass (N6) has exhibited, five clearly resolved intense absorption bands in the visible and NIR regions at 1270 (Oh1), 1040 (Td1), 780 (Oh2), 620 (Td2), 420 (Oh3) nm and a sixth weak band at 475 nm. As the concentration of NiO is varied, the following changes have been observed in the spectra: (i) from 0.2 to 0.6 mol % the positions of the bands Oh1, Oh2 and Oh3 are shifted towards slightly lower wavelength, with decreasing intensity whereas the meta centres of Td1 and Td2 absorption bands remain unaltered, however, a gradual increase in the intensity has been observed with the increase in the concentration of NiO. (ii) in

Table 1 — Summary of data on various physical parameters of Li2O-MgO-B2O3: NiO glasses

Glass Density d (g/cm3)

Avg. mol. wt. ( M )

Total nickel ion concentration

Ni (1021 ions/ cm3)

Inter-ionic distance of Ni ions Ri (Å )

Glass transition temperature

(TgoC)

N0 2.850 54.94 - - 480 N2 2.864 54. 96 6.3 5.42 490 N4 2.892 54.98 12.71 4.29 505 N6 2. 987 55. 0 17.6 3.71 520 N8 3.087 55.02 26.4 3.36 475 N10 3.186 55.04 35.3 3.05 460

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Fig. 1(a) — Optical absorption spectra of Li2O-MgO-B2O3: NiO glasses recorded between 300 and 900 nm

Fig. 1(b) — Optical absorption spectra of Li2O -MgO -B2O3: NiO glasses recorded between 900 and 1350 nm

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the range 0.8 to 1.0 mol %. The positions of the Oh1, Oh2 and Oh3 bands are shifted towards slightly higher wavelength with increasing intensity at the expense of Td1 and Td2 bands and no detectable changes are observed on the position and the intensity of the sixth band located at 475 nm. The summary of the data on the positions of various absorption bands is furnished in Table 2. Using Tanabe-Sugano diagrams for d8 ion, the spectra have been analyzed and the bands Oh1, Oh2 and Oh3 are assigned to the transitions 3A2 →

3T2(F), 3A2 →

3T1(F), 3A2 →3T1(P) of octahedrally positioned

Ni2+ ions. The Ligand field parameters Dq (crystal field splitting energy) and B (Racah parameter) are evaluated using energies of these transitions and the values obtained are also presented in Table 2. The bands with peak positions at 1040 (Td1) and 620 (Td2) nm are attributed to the transitions 3T1(F) → 3A2(F) and 3T1(F) → 3T1(P) respectively of Ni2+ ions in tetrahedral sites 14, 27; the band at 475 nm represents a spin forbidden octahedral transition 3A2 → 1T2

14. Thus, the optical absorption spectral studies on Li2O-MgO-B2O3: NiO glasses suggest that Ni2+ ions exist both in octahedral and tetrahedral sites in the glass network. When NiO is in smaller (less than 0.6 mol %) concentrations, Ni2+ ions occupy both tetrahedral and octahedral positions whereas when it is in larger concentrations, these ions prefer mostly octahedral sites in the present glass network. The nephelauxetic ratio has also been evaluated using the formula β1= B (complex)/B (free ion), for all the glasses and presented in Table 2. The value of βl and the inter-electronic repulsion parameter B are observed to be the highest for glass N10 (Table 2) indicating the highest nephelauxetic effect for this glass. Such

Table 3 — Magnetic susceptibilities and effective magnetic moments of Li2O-MgO-B2O3: NiO glasses

Glass Conc. NiO Magnetic susceptibility µeff

(mol%) χ (10−5 emu) (µB)

N2 0.2 6.12 3.85 N4 0.4 12.3 3.92 N6 0.6 17.1 4.05 N8 0.8 25.6 3.40 N10 1.0 34.3 3.0

observations also suggest the highest ionic environment of nickel ion in the network of glass N10 and the highest covalent environment in the network28 of glass N6. Further, the d-d transitions of the tetrahedral complexes are electric dipole allowed whereas those of octahedral complexes are electric dipole forbidden and are mainly due to the static or dynamic distortions from the regular octahedral geometry of the glass network and they can also be magnetic dipole allowed. Because of these reasons, the oscillator strength of the octahedral complexes is expected to be several orders of magnitude lower than that of tetrahedral units14. Magnetic susceptibility (χ) of the Li2O-MgO-B2O3: NiO glasses measured at room temperature is observed to increase with increase in the concentration of NiO in the glass matrix (Table 3). From the values of magnetic susceptibilities, the effective magnetic moments (µeff) are evaluated. The value of µeff is found to be the highest for glass N6 (4.05 µB) and the lowest (3.0 µB ) for the glass N10

(Table 3). The magnetic properties of these glasses arise from the paramagnetic Ni2+ (both tetrahedral and octahedral) ions. Since, the ground state 3T1 (F) of

Table 2 — Summary of the data on the positions (in nm) of optical absorption transitions of Li2O-MgO-B2O3: NiO glasses

Ni2+ transitions Glass N2 N4 N6 N8 N10

Octahedral transitions 3A2(F)→ 3T2(F) 1278 1274 1270 1285 1294 3A2(F) → 3T1(F) 788 785 778 788 795 3A2(F) → 3T1(P) 422 420 418 426 430 3A2(F) → 1T2(D) 475 475 475 475 475

Tetrahedral transitions 3T1(F) → 3A2(F) 620 620 620 620 620 3T1(F) → 3T1(P) 1040 1040 1040 1040 1040

Dq (cm−1) 782 785 787 778 773

B (cm−1) 754 751 746 757 762

Nephelauxetic ratio (βl) 0.704 0.701 0.697 0.707 0.712

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tetrahedral Ni2+ ions has much inherent orbital angular momentum, the magnetic moment of perfect tetrahedral Ni2+ should be ~ 4.2 µB. Even a slight distortion reduces this value markedly because of the orbital degeneracy. The fairly regular tetrahedral complexes of these ions are expected to have the magnetic moment29 in the range 3.5 to 4.1 µB. The value of the effective magnetic moment (4.05 µB) obtained for the glass N6 indicates that Ni2+ ions occupy mostly tetrahedral positions in this glass network. Magnetically, the octahedral Ni2+ complexes have relatively simple behaviour and their magnetic moments29 are expected to lie in the range 2.9 to 3.4 µB depending on the magnitude of the orbital contribution. The gradual decrease of the effective magnetic moment with the concentration of NiO from 4.05 µB (for glass N6) to 3.0 µB (for glass N10) confirms the gradual transformation of the positions of Ni2+ ions from the tetrahedral sites to the octahedral sites in the glass network. Figure 2(a) shows the thermoluminescence (TL) glow curves of Li2O-MgO-B2O3: NiO glasses. The thermoluminescence emission of pure glass exhibits a broad glow peak at 573 K (Peak I). With the introduction of NiO into the glass matrix, an additional peak at 353 K (Peak II) has been observed; with the increase in the concentration of NiO, no detectable change in the II glow peak temperatures

(Tm) has been observed; the first glow peak (high temperature peak) has however been observed to shift towards higher temperature when the concentration of NiO is increased beyond 0.6 mol %. Nevertheless, the intensity of both the glow peaks has been observed to decrease with increase in the concentration of NiO up to 0.6 mol % and thereafter the intensity is found to increase (Fig. 2(b)). Using the glow peak temperatures, the average trap depth parameter Eδ corresponding to both the glow curves is evaluated using the equation30: (βEδ/ΚTm

2) = ν exp (-Eδ/KTm) …(1) In Eq (1), Κ is the Boltzmann constant, β is the rate of heating and ν is the frequency factor; this formula is based upon the assumption that the refilling of the traps corresponding to the glow peaks does not occur. Taking the value of ν as 2.5×1013 Hz from the infrared spectroscopy, the values of Eδ is computed as 0.996 eV respectively for glow peak II (low temperature peak). A summary of the data on thermoluminescence peaks and the corresponding trap depth parameters of the present glasses is presented in Table 4. Prior to TL measurements, the optical absorption spectra of all the glasses before and after X-ray irradiation have been recorded. X-ray irradiation on

Fig. 2(a) — TL glow curves of X-ray irradiated Li2O-MgO-B2O3: NiO glasses. (b) Variation of TL light output with the concentration of NiO

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these samples was carried out only for ½ h with an X-ray tube operated at 35 kV, 10 mA. Such irradiation does not produce any significant changes of the pattern and the intensity of the existing absorption bands indicating no considerable changes in the concentration of existing environment of nickel ions in these glasses due to X-ray irradiation. In Li2O-MgO-B2O3: NiO glasses, the Li+ ions have closed structure, and do not have energy levels within 10 eV of the ground state and hence, these ions do not participate directly in luminescence but may act as activator ions. The magnesium ions occur as Mg2+, ions. In the activation process, the substitution of Li+ by doubly charged alkaline earth ions (viz., Mg2+), the charge balance would be upset. Since the magnesium ion is doubly charged, the energy levels of the surrounding oxygen ions will be slightly lifted up in comparison with the normal ions and give rise to occupied energy levels close to the top of the valence band. These levels form the ground state of the luminescence. The action of X-ray irradiation on glasses is to produce secondary electrons from the sites where they are in a stable state and have an excess energy. Such electrons may traverse in the glass network depending upon their energy and the composition of the glass and are finally be trapped, thus forming colour centres. The trapping sites may be the metal cations that constitute the glass structure, ions of admixtures to the main composition and the structural defects due to impurities in the glass. Thus, this process leads to the formation of (1) boron electron centres,(2) non-bridging oxygen hole centres and (3) boron oxygen hole centres20-23. Thermoluminescence is a consequence of radiative recombination between the electrons (released by heating from electron centre) and an anti-bonding molecular orbital of the nearest of the oxygen hole centres. However, the TL emission due to such recombination process is generally possible only at low temperatures in borate glasses 20.

Alternatively, the TL emission in these glasses may be explained as follows. During the heating process, the electrons that were captured by metal ions are liberated and later trapped by holes in the recombination centers giving out TL light output. The first glow peak may be attributed due to such process. The thermoluminescence of X-ray irradiated Li2O-MgO-B2O3: NiO glass is found to reduce by the addition of nickel impurity to the glass (up to glass N6). The reasons for quenching of TL light output may be given as follows: The delocalised antibonding state of substitutional neighbouring Ni2+ ions in tetrahedral sites as described earlier may play an intermediate role in the recombination reaction either through by passing or by resonance energy conditions, so that the radiative transition is no longer possible in the site A1 as shown in Fig. 3. Alternatively, the quenching of TL light output may be due to the electron-hole recombination by means of the energy transfer of exciton recombination to the excited states of d band of nickel ions; this is followed by non-radiative relaxation of the excited ion with phonon emission. This is possible because the free electron (or the hole) does not have sufficient electron-phonon coupling necessary for the self-trapping to give rise to radiative recombination; on the other hand, the exciton has a strong coupling with the lattice so that predominant non-radiative recombination occurs with the phonon emission31. The decreasing presence of tetrahedral Ni2+ ions (in the glasses N6 to N10) probably reduces non radiative transitions there by increasing the TL light output as observed. The value of the trap depth parameter (Eδ = 0.996 eV) corresponding to the second glow peak (at 353 K) matches nearly with the energy associated with 3T2 → 3A2 transition which is a conventional lasing transition of Ni2+ ions. Hence, TL emission of

Table 4 — Data on various trap depth parameters of TL glow peak of Li2O–MgO-B2O3: NiO glasses

Glass Conc.NiO Tm(K) Eδ

(mol%) Peak I Peak II Peak I Peak II

N0 0 573 - 1.641 - N2 0.2 548 353 1.567 0.996 N4 0.4 503 353 1.435 0.996 N6 0.6 453 353 1.289 0.996 N8 0.8 583 353 1.670 0.996 N10 1.0 588 353 1.685 0.996

Fig. 3 — Mechanism of recombination of oxygen hole center and the electron from the neighbouring nickel ion impurity

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the second glow peak may be attributed due to this transition of Ni2+ ions. Further, the raise in the intensity of this peak with the increase in the concentration of NiO indicates an increase in the concentration of luminescence centres (octahedral Ni2+ ions) in the glass network. The dielectric constant ε′ and loss tanδ at room temperature (30°C) of pure Li2O-MgO-B2O3 glass at 10 kHz are measured to be 6.2 and 0.014, respectively. These values are found to increase with the decrease in frequency. With the introduction of NiO into the glass matrix (up to 0.6 mol %), the values of dielectric constant ε′ and loss tan δ are found to decrease at any frequency. For any particular concentration of NiO, the values of εl and tan δ are found to increase considerably with decrease in frequency. The temperature dependence of ε′ at different frequencies of glass T6 (glass containing 0.6 mol % of NiO) and for different concentrations of NiO at 1kHz are shown in Figs 4 and 5 respectively. The value of ε′ is found to exhibit a considerable increase at higher temperatures especially at lower frequencies.

The temperature dependence of tanδ of glass T10 (glass containing 1.0 mol % of NiO) at different frequencies is shown in Fig. 6 and for different concentrations of NiO at 10 kHz in Fig. 7. The loss curves have exhibited distinct maxima; with increasing frequency the temperature maximum shifts towards higher temperature and with increasing temperature the frequency maximum shifts towards higher frequency, indicating the relaxation character of dielectric losses in Li2O-MgO-B2O3: NiO glasses. The observations on dielectric loss variation with temperature for different concentrations of NiO further indicate a gradual increase in the broadness and (tanδ)max of relaxation curves beyond 0.6 mol % concentration of NiO. The summary of the data on the relaxation effects of tan δ with TiO2 concentration is presented in Table 5. Using the relation: f = fO exp( –Wd/KT), …(2)

the effective activation energy, Wd, for the dipoles is calculated for different concentrations of NiO and presented also in Table 5. The activation energy is found to be the highest for glass N6. The ac conductivity, σac is calculated at different temperatures using the equation:

Fig. 4 — Variation of dielectric constant with temperature at different frequencies of glass N6

Fig. 5 — A comparision plot of variation of dielectric constant with temperature at 1kHz for Li2O -MgO -B2O3 glasses doped with different concentrations of NiO

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Fig. 6 — Variation of dielectric loss with temperature at different frequencies of glass N10 (Solid lines) and for Pure glass (the dotted lines)

Fig. 7 — A comparision plot of variation of dielectric loss with temperature at 10 kHz for Li2O -MgO -B2O3 glasses doped with different concentrations of NiO

σac = ωε′εotanδ …(3)

(where εo is the vacuum dielectric constant) for different frequencies and the plot of log σac against 1/T for glass N8 is shown in Fig. 8 and for all the glasses at 100 kHz in Fig. 9. The conductivity is found to decrease with increase in the concentration of NiO at any given frequency and temperature up to 0.6 mol % of NiO and beyond that it is found to increase. From these plots, the activation energy for conduction in the high temperature region over which a near linear dependence of log σac with 1/T could be

Table 5 — Data on dielectric loss of Li2O-MgO-B2O3: NiO

glasses

Glass (tanδmax)avg εs ε∞ Activation energy for dipoles

αο

No 0.018 13.0 10.0 2.65 ≈0 N2 0.017 12.6 9.9 2.67 0.5 N4 0.015 12.1 9.6 2.70 0.5 N6 0.014 11 9.3 2.72 0.1 N8 0.087 15.7 9.9 2.30 6 N10 0.096 18.33 10.80 2.17 7.5

Fig. 8 — Variation of σ ac with 1/T of Li2O-MgO-B2O3 glasses doped with 0.8 mol % NiO (glass N8) at different frequencies

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observed, is evaluated and presented in Table 6; activation energy is found to be the maximum for glass N6 and minimum for glass N10 (Table 6). The dielectric breakdown strength for glass N0 (at room temperature in air medium) is determined to be 10.60 kV/cm. With the introduction of NiO, the value of the breakdown strength is observed to increase gradually up to 0.61 mol % of NiO and there after it is found to decrease (Table 6). Among various polarizations (viz., electronic, ionic, dipolar and space charge polarizations) that contribute to dielectric constant, the space charge contribution depends upon the purity and the perfection of the glasses. With the introduction of NiO beyond 0.6 mol %, the values ε′, tanδ and σac are found to increase at any frequency and temperature and the values of dielectric breakdown strength and activation energy for ac conduction are observed to decrease with respect to the values of pure glasses. Obviously, in this concentration (> 0.6 mol %) range, nickel ions mostly exist in octahedral positions, act as modifiers and create bonding defects. The defects thus, produced create easy path ways for the

migration of charges that would build up space charge polarization leading to the increase in the dielectric parameters as observed32-34. In general, the dielectric relaxation effects are observed only when the metal ions present in the glass network, are in the divalent state 35,36. Among the three constituents, viz Li2O, MgO and B2O3 of the pure glasses, the divalent ions of magnesium, nickel together with a pair of any cationic vacancies may form dipoles and such dipoles are responsible for the observed dielectric relaxation effects in the pure Li2O-MgO-B2O3 glasses. A typical Cole-Cole diagram corresponding to a temperature of 125°C in the relaxation region is shown in Fig.10 for glasses N10 and N0 (pure glass). The diagram shows that the dielectric relaxation in these glasses is a Debye-type relaxation with a certain set of relaxation times τ. The points of intersection of the circular arc with the X-axis in the Cole-Cole plot correspond to the low frequency and high frequency dielectric constants (εs and ε∞ respectively. Similar plots at different temperatures (within the relaxation region) have also been made for samples doped with different other concentrations of NiO and the obtained averaged values of εs and ε∞ for each concentration are presented in Table 5. The parameter α, characterizing the distribution function τ does not show any considerable change with temperature for a particular concentration; however, for different concentrations, the values are appreciably different (Table 5). When the glasses are doped with nickel ions, we have observed spreading of relaxation with a set of relaxation times τ which means the relaxation effects are due to several types of dipoles37,38. Since the spreading of relaxation is not observed in the pure glasses, the spreading in the NiO doped glasses is obviously due to the presence of Ni2+ ions that

Fig. 9 — Variation of σ ac with 1/T at 100 kHz of Li2O -MgO-B2O3 glasses doped with different concentrations of NiO

Table 6 — Summary of data on ac conductivity σac for Li2O-MgO-B2O3: NiO glasses

Glass N(EF) in Activation Energy Breakdown (1020, eV−1/cm3) for conduction strength Austin and Mott (eV) (kV/cm)

N0 5.54 0.71 10.60 N2 5.42 0.79 10.91 N4 4.66 0.88 11.22 N6 3.95 0.92 11.68 N8 9.53 0.68 10.54 N10 12.1 0.63 10.23

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participate in relaxation effects in addition to the Mg2+

ions. If it is assumed that the electrical field in this glasses is a Lorenz field, the connection between, the number N of the dipoles per unit volume, the dipole moment µ, the low and high frequency dielectric constants εs and ε∞ can be written according to Classius–Mosetti Debye relation later modified by Guggenheim37 as:

2s

s

1 1 4

2 2 27

N

KT

∞

∞

ε − ε − π µ− =

ε + ε +. …(4)

In the present glasses, ions and dipoles can be approximately regarded as mere points and the concentration of dipoles is not abnormally high, one need not doubt the applicability of the Eq. (4) for these glasses. After rearranging the terms given in Eq. (4), it will be modified to:

2s

s

4

( 2)( 2) 9

NT

K

∞

∞

ε − ε π µ=

ε + ε +. …(5)

The quantity Nµ2, in the right hand side of the Eq. (5) is conventionally known as the strength of dipoles. Substituting the values of εs and ε∞, the quantity 4πNµ2/9K is calculated at 398 K for different concentrations of NiO and its dependence on the

concentration of NiO is shown in the Fig. 11 for these glasses. The curve is observed to rise at a faster rate beyond 0.6 % of NiO. Such behaviour obviously indicates an increase in the degree of spreading of relaxation times with several types of dipoles above this concentration. This observation further suggests the contribution to the dipolar relaxation effects from the octahedral Ni2+ ion is more dominant than tetrahedrally positioned ions. The lowest value of (tanδ)max and the highest value of the effective activation energy associated with the dipoles in the network of glass T6 obviously suggests the highest rigidity of glass network that prevents orientation of the dipoles in the field direction. The conduction phenomenon in these glasses in the high temperature region can be explained on the basis of mixed conduction mechanism39. In fact, the interrelation and reduction processes could occur only if the material exhibits simultaneously a mixed conductivity, i.e., both ionic and electronic39. The variation of conductivity with the concentration of NiO at higher temperature (Fig. 12) shows a decreasing trend (zone I) up to 0.6 mol % of NiO and beyond this concentration, the conductivity is found to increase (zone II). The conduction in the zone I (where σac decreases with NiO) is identified as electronic conductive zone whereas the zone II is identified due to mobility of ions40. It may be stated here that the highest conductivity and the lowest activation energy observed for the glass N10 is

Fig. 10 — A Cole-Cole diagram drawn at 125°C for a) pure and b) N10 glasses

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Fig. 11 — Variation of 4πΝµ2/9K with the concentration of NiO

Fig. 12 — Isotherms of logarithm of conductivity at three temperatures for Li2O -MgO-B2O3 glasses as a function of concentration of NiO measured at a frequency of 10 kHz obviously due to the highest concentration of charge carriers, which may find easy paths for migration in the high disordered network. When a plot is made between logσ(ω) versus activation energy for conduction, a near linear relationship is observed (Fig. 13); the near linearity between the conductivity and the activation energy suggests the conductivity enhancement is directly related to the increasing

mobility of the charge carriers. Since the alkaline earth ions are much less mobile than the alkali ions in the mixed alkali and alkaline earth glasses (like the present Li2O-MgO-B2O3: NiO glasses), the alkaline earth ions (Mg2+ ions) can be regarded as virtually immobile within the time window of hopping processes of the alkali (Li+) ions41. Therefore, the contribution to the conduction for the present glasses in the high temperature region (zone II) can be mainly considered due to monovalent lithium ions. As it appears, due to increasing modifying action of Ni2+ ions, easy paths for the movement of the charge carriers are created and hence, increase in the conductivity is resulted with increase in the concentration of NiO from 0.6 to 1.0%; however, when NiO concentration is varied from 0 to 0.6 mol % in the glass matrix, the conductivity is observed to decrease gradually with increasing activation energy. At low concentrations of NiO, there is higher concentration of tetragonally positioned Ni2+ ions in the present glasses; and hence, we may assume that the least rigidly bound Li+ ions are replaced by Ni2+ ions, the remaining Li+ ions are more tightly bound so that (if the Ni2+ ions are not able to move) the conductivity falls (zone I), as observed. The low temperature part of the conductivity (a near temperature independent part, as in the case of present glasses up to nearly 100°C) can be explained on the basis of quantum mechanical tunneling model42 similar to many other glass systems reported recently

Fig. 13 — Variation of σac (10kHz) with the activation energy in the high temperature region for Li2O-MgO-B2O3 glasses doped

with different concentrations of NiO

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from our laboratory43,44. The value of N (EF) i.e. the density of the defect energy states (between which the conduction takes place) near the Fermi level, is evaluated using the equation42 σω =(π/3)e2

KT [N(EF)]2 α−5ω[ln(νph/ω)]4 …(6)

with the usual meaning of the symbols reported in earlier papers43-45 and presented in Table 6 . The value of N(EF) is found to increase with increasing concentration of NiO up to 0.6 mol % (indicating a decreasing disorder in the glass network in this concentration range of NiO) and thereafter, it is observed to increase. When the dielectric is placed in the electric field, the heat of dielectric loss is liberated. If the applied field is an alternating field, the specific dielectric loss i.e., the loss per unit volume of the dielectric is given by46

ρl= E2 ω εl ε tanδ W/m3 …(7) Eq. (7) indicates that the higher the value of εl tanδ of the glass at a given frequency, higher is the values of ρl. In a dielectric across which the voltage is applied, heat is liberated, the temperature of the dielectric then raises and the loss increases still more. The dielectric breakdown strength is in fact inversely proportional to the specific dielectric loss presented by Eq. (7). Our observations on dielectric properties of Li2O-MgO-B2O3: NiO glasses, indicate that the rate of increase of εl tanδ with temperature is the highest for glass N10. Though, the breakdown strengths are actually determined at room temperature, the heat liberated during the breakdown, raises the temperature of the glass and hence, raises the εl tanδ value. Since the rate of increase of εltanδ with temperature is the highest for glass N10; the breakdown strength (which is inversely proportional to ρl) is the lowest when compared with that of other glasses. Thus, the experiments on the dielectric breakdown strength of Li2O-MgO-B2O3: NiO glasses reveal that there is a maximum internal distortion in the glass N10 that contains the highest concentration of (octahedral Ni2+

ions). 4 Conclusion The summary of conclusions drawn from the study of various properties of Li2O-MgO-B2O3 glasses doped with NiO is as follows:

(i) The optical absorption spectral studies indicate the presence of nickel ions predominantly in octahedral positions which act as modifiers if NiO is present in higher concentrations (> 0.6 mol %); when the concentration of NiO is in lower quantities (below 0.6 mol %), these ions participate also in the network forming with tetrahedral positions; (ii) The magnetic moment of Ni2+ ions evaluated from magnetic susceptibility measurements of the glasses shows the highest value (4.05 µB) for glass N6 and for the glasses N8 to N10 it is found to drop to ∼ 3.0 µB (Fig. 14). Such changes have been identified due to the transformation of nickel ions from tetrahedral positions to octahedral positions; (iii) The dielectric parameters viz., ε′, tanδ and σac are found to decrease while the dielectric breakdown strength and the activation energy for ac conduction are found to increase with the increase in the concentration NiO up to 0.6 mol %. This has been attributed to an increase in the concentration of tetragonally positioned nickel ions that take network forming positions. When NiO is present in higher concentrations (> 0.6 mol %), we observe that the values of the dielectric parameters to increase; such changes may be understood due to decrease in the concentration of nickel ions that take network modifying positions. The relaxation effects exhibited by dielectric loss of these glasses have been analyzed using Cole-Cole diagrams and the spreading of relaxation times has been established. The temperature independent part of the conductivity could be explained on the basis of quantum mechanical model whereas high temperature conductivity has been analyzed on the basis of mixed conduction mechanism.

Fig. 14 — Variation of log σ with log ω for glass N8 at 343 K; glass drawn for other glasses have also exhibited the similar

behaviour

RAO et al.: STRUCTURE CORRELATIONS IN NiO DOPED Li2O-MgO-B2O3 GLASSES

255

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