ABSTRACT

Results of the direct current (DC) Electrical Conductivity, thermoelectric power and Electron Spin Resonance (ESR) of CdO substituted PbO-V₂O₅ glass system are reported. Conduction in these glasses is found to be electronic and the hoping of polaron seems to be the dominant process in the transport mechanism. There is a remarkable decrease in the activation energy for conduction in the annealed and devitrified samples when compared to their amorphous counter parts. It is observed that there is remarkable improvement in the conductivity of the crystalline samples when compared to their amorphous counter parts. The thermoelectric power measurements indicates that the amorphous samples are n-type at room temperature where as the crystalline samples are p-type at room temperature. In crystalline samples the hyperfine structure is nearly smeared out and a relatively broad line with an isotropic g value characterizes the spectra.

1. Introduction

The PbO-V₂O₅ glass system has been extensively studied and the crystal structure of PbV₂O₆ or PbV₂O₇ Phases are known [1,2]. This is an ideal system to study the effect of doping or substitution on the physical properties can be explained to the possible changes in the glass structure especially when doping or substitution is carried out at specific composition for which structural information is available. Recently Ramesh and Sastry carried out the work in the field of semi conducting glasses to understand the effect of different oxide groups in the place of PbO in PbO-V₂O₅ glass system chosen at the eutectic composition i.e. 1:1 molar ratio with the substitution of ZnO, CuO and TiO₂ in the place of PbO of PbO-V₂O₅ eutectic glass composition which can be represented as follows, by the general formula XMO:(50-x)PbO:50V₂O₅, where MO is the substituted metal oxide [3-6]. In all compositions x varied as 0, 5, 10 and 15 mole%. Recently Bhujanga Rao et al. [7,8] reported the studies to CdO substituted glass systems with similar compositional variations. In the present paper we reported the results of DC Electrical Conductivity, thermoelectric power and ESR studies of CdO substituted lead Vanadate glass systems in the crystalline form compared with glass form in the range of 0 to 15 mole%.

Glasses of the following compositions xCdO:(50-x) PbO:50V₂O₅ (where x = 0, 5, 10 and 15 mole%) have been prepared using high purity reagent grade materials.

2. Experimental Work

The chemicals were weighted in required molar ratio for each batch of composition using an electrical balance (Dhona Model 200D) of 0.0001 gm accuracy. Each batch of composition was taken in agate mortar and was mixed thoroughly for several hours for homogeneity. This mixture whose total weight was around 2 gm for each batch of composition was taken in a glazed silica crucible with lid and was melted in an electrical furnace kept at a temperature of 950˚C - 1000˚C range. The completely melted samples were periodically stirred for homogeneity and were kept at that state for nearly 15 minutes before quenching. The melts were quenched on a large stainless steel block maintained at room temperature (30˚C) and constituting of 9 mm cylindrical cavities to get samples of cylindrical shape of 2 to 3 mm width. The glass samples were annealed at 150˚C below the glass transition temperature for nearly 2 hours. The annealed samples were washed with very fine lapping papers. The samples were washed with acetone and dried.

For the samples used for conductivity measurements a gold coating was deposited on both the polished surfaces using a thin film coating unit. (Model JFC 1000 of JEOL). The T_g (Glass transition temperature), T_c (Crystalline Temperature) and T_m (Melting Point Temperature) for glass systems were studied by thermo gravimetric or differential scanning calorimetric techniques and these values has been reported [7]. The as prepared glass systems were annealed at temperatures corresponding to their of crystalline temperature for four hours for perfect crystalline form by confirming through X-Ray diffraction. Conductivity, Thermo power studies when carried out on such poly-crystalline materials which were obtained in the form of discs of 9 or 10 mm diameter and 3 or 4 mm thickness. In the present studies ESR spectra were recorded using JEOL (FE-3X) ESR Spectrometer operated at X-band (9.3 GHz) and employing a 100 KHz field modulation.

3. Results and Discussion

The X-ray diffractograms showing perfect amorphous nature of the samples. The X-ray diffractograms for the samples annealed at 623 K which corresponds to the first crystallization temperature of the samples and evident from X-ray diffractograms which are perfectly crystalline after the heat treatment. X-ray diffractograms were recorded for all the samples are shown in the Figure 1. The DSC pattern for these samples has been reported [7]. It can be seen that there is a gradual increase in the glass transition temperature as CdO is substituted for PbO. The crystallization peak observed around 324˚C gradually broadens so does the crystallization peak observed around 435˚C. The crystallization peak observed around 324˚C moves to higher temperature as the CdO substitution increases from 5 mole% to 15 mole%. There is also a simultaneous decrease in enthalpy (heat released) from 66 J/gm to 40 J/gm and also the crystallization peak broadens as CdO substitution increases. This is probably an indication of the decrease in the glass forming ability of the compound as CdO substitution for PbO is increased from 5 mole% to 15 mole%.

In the present investigations on DC Electrical conductivity studies were carried out by using a two probe technique. The resistance of the sample was carried out by using a Keithlay (Model 614) digital electrometer. The conductivity of the sample was calculated from knowledge of the geometry of the sample using the expression

where R_x is the Resistance, t is the thickness and A is the area of the sample.

The logarithmic DC conductivity for amorphous samples as a function of reciprocal of temperature is shown in the Figure 2.

The activation energies obtained by fitting the samples to the Mott’s equation are given in the Table 1 [9-13].

where is the pre-experimental factor;

W is the activation energy;

K is the Boltzmann constant;

T is the temperature in Kelvin.

The DC conductivity obtained in the crystalline samples is shown in the Figure 3 the activation energies obtained are shown in the Table 1.

The chemical analysis data suggests that is not much change in V⁴⁺ concentration in crystalline samples when compared to the amorphous system. At a given temperature the conductivity increases as CdO substitution increases in these glass systems. At around 303 K, the conductivity of 5 mole% of CdO substituted sample is less than that of the pure sample. But as temperature increases the conductivity of all the samples increases and is greater than that of the pure sample. As per the reported literature [7] 5 mole% CdO substituted sample V⁴⁺ ion concentration is less than that of the pure sample. But the V⁴⁺ ion concentration increases as CdO concentration increases. The CdO substituted glasses posses higher activation energy for conductivity when compared to 50PbO:50V₂O₅ glass system.

Conductivity data at temperatures higher than room temperature can be fitted to Mott’s phonon assisted polaron hopping model. The conduction mechanism in all the glass systems studied can be shown to be due to highly localized small polarons hopping non-adiabaticcally between different sites.

It has been observed in literature that DC conductivity in vanadate glasses with conventional network formers like GeO₂ can be explained by adiabatic hopping process and Mott’s variable range hopping model can be fitted to conductivity data below 100 K [14-16]. In sharp contrast,

the conductivity data of lead vanadate glasses [xPbO: (100-x)V₂O₅, x varied between 10 to 50 mole%] can be explained by non-adiabatic hopping process and variable range hopping model is found to be applicable even at room temperature (about 300 K) [17]. The present studies in CdO substituted lead vanadate glasses are consistent with the latter observations.

In CdO glass systems the conductivity decreases to a value less than that observed in the un-substituted sample at 300 K for 5 mole% substitution. However conductivity increases as temperature increases and is characterized by higher activation energy when compared to the pure sample. The decrease in conductivity in these systems is consistent with the decrease in V⁴⁺ ion concentration as concentration of substitution is increased. The conductivity mechanism in these systems also can be explained by Mott’s phonon assisted non-adiabatic polaron hopping process.

In the reported studies of electrical properties of lead vanadate glasses [17], it has been observed that conductivity decreases (at a given temperature) as V₂O₅ concentration is decreased in the glass composition. In the present studies V₂O₅ concentration has been kept constant (at 50 mole%) and PbO concentration has been reduced by equivalent amounts of CdO substitution. It is interesting that such substitution in the place of network modifier has affected V⁴⁺ concentration and hence conductivity. It is also interesting that even though the network former (V₂O₅) concentration has been kept constant there has been a considerable increase in the activation energy when PbO has been replaced by CdO. Hence the increased conductivity may be due to increased hopping. The decrease in activation energies in crystalline samples is in accordance with this observation.

In amorphous samples indicates that the Seebeck coefficient is negative for all the samples at room temperature and changes to positive at temperatures higher than the room temperature. This indicates that the conduction is predominantly n-type at low temperature and p-type at higher temperatures.

The temperature depends of the Thermo Electric power above the room temperature for the amorphous samples are shown in the Figure 4. The thermo electric power in the crystalline samples is also measured as a function of temperature and is shown in the Figure 5.

In the crystalline samples the thermo electric power measurements indicate that the seebeck coefficient is positive even at room temperature as well as higher temperature as well as higher temperatures. This indicates that the crystalline materials are predominantly p-type even at room temperature. The thermo electric measurements were fitted to Heike’s formula [18] as below:

According to Heike’s,

where is change in entropy of the system due to the presence of a charge carrier. The values obtained in the amorphous and crystalline systems at room temperature are given in Table 2.

Austin and Mott [19] suggested that for large polaron and is for small polaron. Hence this observation we can assume that conduction in amorphous samples is predominantly due to formation of large polaron.

The presence of V⁴⁺ (electron spin S = 1/2 and nuclear spin I = 7/2 and electron ground state 3d¹) in vanadate glasses gives rise to good electron spin resonance (ESR) spectra which give information regarding V⁴⁺ local symmetry [20]. The V⁴⁺ generally exists in six coordinated distorted octahedral symmetry, the degree of distortion being a function of the composition, the nature of phases present and thermal treatment of the glass system [21]. The V⁴⁺ ion is assumed to be present in the form of VO²⁺ molecular ion or radical whose ground state has

been described by Ballhausen and Gray [22]. The radical VO²⁺ or V⁴⁺ has been studied in several vanadate or V₂O₅ containing glass systems [23]. H. El Makami et al. could identify two different V⁴⁺ ions in vanadium borate glasses [20]. No such phase separation has been reported in 50PbO:50V₂O₅ glass system [21]. In the present studies ESR spectra recorded in CdO substituted lead vanadate glass system have yielded interesting results which are discussed below. The ESR spectra of CdO substituted lead vanadate glass and crystalline systems are shown in Figures 6 and 7.

The g and A values observed in the present system are very similar to those reported for V⁴⁺ ion in vanadate glass systems [21] and given in the Tables 3 and 4. However a considerable variation is observed in the ESR spectra obtained in the devitrified samples. The CdO substituted lead vanadate sample even after devitrification there seems to be some residual hyperfine structure being present. It has been observed that the ESR spectra of crystalline semi-conducting glasses show a single ESR line due to the collapse of hyperfine interaction which results from increased hopping of electron from V⁴⁺ sites to V⁵⁺ sites. Thus the increased conductivity of the crystalline samples averages or reduces the hyperfine interaction. It can be seen from the conductivity studies that conductivity is reduced in CdO substituted samples. A similar result was observed by Ramesh in ZnO substituted lead vanadate glass system and it has been observed that there is some residual hyperfine interaction in ZnO substituted samples [24]. But in CdO substituted samples, the residual hyperfine interaction is more pronounced.

ESR spectra of CdO sample of 15 mole% exhibits higher V⁴⁺ ion concentration. Hence in line with these observations it can be seen that the ESR spectrum of 15 mole% of CdO containing sample is relatively less resolved probably due to broadening of the ESR lines due to the presence of higher concentration of V⁴⁺ ion.

4. Conclusions

It has been observed that there is a rem arkable improvement in the conductivity of the crystalline samples

when compared to their amorphous counter parts. There is also remarkable decrease in the activation energy for conduction in the annealed and devitrified samples when compared to amorphous samples. The thermo electric power measurements indicates that the amorphous samples are n-type at room temperature where as the crystalline samples are p-type at room temperature. The thermo electric power measurement also indicates the formation of large polar on in the crystalline samples.

ESR spectra of crystalline system shows a single ESR line due to the collapse of hyperfine interaction which results from increased hopping of electron from V⁴⁺ sites to V⁵⁺ sites. Thus the increased conductivity of the crystalline samples averages or reduces the hyperfine interaction. The increase in electrical conductivity in the crystalline samples can be attributed to the increased in electron hopping on V⁴⁺ or V⁵⁺ sites. The increase in the order

of the structural units in the crystalline samples causes the increased probability of electron hopping from V⁴⁺ to V⁵⁺ sites, and hence to increase in conductivity and decreased activation energies.

REFERENCES

NOTES

^*Corresponding author.