Fe concentration dependent transport properties of LiI–AgI–B2O3 glass system

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Abstract

LiI–AgI–B2O3 glasses mixed with different concentrations of Fe2O3 (ranging from 0 to 2.0 mol%) were prepared. DC conductivity and dielectric properties over a range of temperature and also optical absorption and ESR spectral studies have been investigated. The optical absorption and ESR studies have indicated that iron ions do exist in both Fe2+ and Fe3+ states and the reduction ratio is the highest in the samples containing 0.9 mol% of Fe2O3. The analysis of the results of DC conductivity has indicated that T > θD/2, the small polaron hopping model seems to be fit and the conduction is adiabatic in nature. These results further indicated that there is a mixed conduction (both ionic and electronic) and the ionic conduction seems to prevail over polaron hopping in the glasses containing Fe2O3 more than 0.9 mol% of Fe2O3. The variations of dielectric constant and loss with temperature have been analyzed on the basis of space charge polarization. The low temperature part of AC conductivity is explained based on the quantum mechanical tunneling model.

Research highlights

LiI–AgI–B2O3 glasses mixed with different concentrations of Fe2O3 (ranging from 0 to 2.0 mol%) were prepared. DC conductivity and dielectric properties have been investigated. The analysis of these results (with the aid of the data on optical absorption and ESR studies) has indicated that there exists mixed conduction (both ionic and electronic) in these glasses; the investigation further indicated ionic conduction prevails over polaron hopping in the glasses containing more than 0.9 mol% of Fe2O3. The temperature independent part of AC conductivity is explained on the basis of quantum mechanical tunneling model.

Introduction

Study of electrical properties of solid electrolytes has received wide attention due to their potential applications in solid state ionic devices such as fuel cells, gas sensors, electrochemical capacitors, electrochromic displays, analog devices, cathodes in electro chemical cells, smart windows, etc. [1], [2], [3], [4] The conductivity of LiI–AgI mixed glasses especially, has been the subject of extensive investigation in recent years as a quest for new solid electrolytes with super ionic properties [5], [6]. The silver/lithium ions surrounded by iodide ions diffuse very rapidly and are the main contributors of the conductivity in the glasses; on the other hand, the silver ions interlocked with the oxide glass network are almost immobile and contribute poorly to the conductivity. Further, when these glasses are doped with multivalent transition metal ions like iron, mixed electronic and ionic, pure electronic or pure ionic conduction is expected depending upon the composition of the glass constituents. Electronic conduction in this type of materials is predicted due to polaron hopping between different valent states of transition metal ions, where as the ionic conduction is expected due to the diffusion of Li and Ag ions.

Among various transition metal ions, the iron ions are considered as effective and useful dopant ions owing to the fact that they exist in different valence states with different coordinations simultaneously in the glass network; for example as Fe3+ with both tetrahedral and octahedral and as Fe2+ with octahedral environment [7]. The content of iron in different environments and in different valence states existing in the glass however 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. Hence, the connection between the state and the position of the iron ion and the electrical properties of the host glass containing highly mobile ions like Ag+ and Li+ is expected to be highly interesting. Both Fe3+ and Fe2+ ions are well-known paramagnetic ions. Fe2+ ion exhibits a large magnetic anisotropy due to its strong spin–orbit interaction of the 3D orbital whereas such anisotropy energy of Fe3+ ions is small because its orbital angular momentum is zero. Several recent works on the valence states and the influence of iron ions on the physical properties including electrical properties of a number of glasses are available in the literature [8], [9], [10], [11]. Mogus Milancovic and her group have carried out extensive investigations that include Mossabauer spectra and DC conductivity studies recently on the glasses containing iron ions [12], [13].

In spite of the fact that a considerable number of studies are available on LiI–AgI mixed glasses including lithium silver borate glasses [14], the conductivity studies as such on LiI–AgI–B2O3 glasses containing transition metal ions like iron are very rare and hence still there is a lot of scope to investigate the role played by iron ions on the conduction mechanism in these glasses. Further, dielectric measurements on ionic materials also give useful information about dynamical processes involving ionic motion and polaron transfer. It is known that the conductivity of glassy materials is frequency dependent, so that the diffusivity of the mobile ions is not entirely characterized by the single steady state parameter σDC quantifying DC conductivity.

The main objective of this paper is to explore the changes in conduction mechanism that take place with the varied oxidation states of iron ions in the glass network and the role of silver and lithium ions in this process by a systematic study on DC conductivity and dielectric properties (viz., dielectric constant, loss and AC conductivity over a wide range of frequency and temperature) of LiI–AgI–B2O3 glasses mixed with varied concentrations of Fe2O3 from 0 to 2.0 mol%. Auxiliary experimental data viz., optical absorption and ESR that help to have some pre-assessment over the valence states of iron ions and their environment in the glasswork have also been reported.

Section snippets

Experimental

For the present study, a particular compositions (39−x) LiI–1.0 AgI–60 B2O3:xFe2O3 with seven values of x ranging from 0 to 2.0 is chosen. The detailed compositions are as follows:

  • F0: 39 LiI–1.0 AgI–60 B2O3

  • F3: 38.7 LiI–1.0 AgI–60 B2O3: 0.3 Fe2O3

  • F6: 38.4 LiI–1.0 AgI–60 B2O3: 0.6 Fe2O3

  • F9: 38.1 LiI–1.0 AgI–60 B2O3: 0.9 Fe2O3

  • F12: 37.8 LiI–1.0 AgI–60 B2O3: 1.2 Fe2O3

  • F15: 37.5 LiI–1.0 AgI–60 B2O3: 1.5 Fe2O3

  • F20: 37.0 LiI–1.0 AgI–60 B2O3: 2.0 Fe2O3

Analytical grade reagents of H3BO3, LiI, AgI and Fe2O3

Results

From the measured values of density d and calculated average molecular weight M¯, various physical parameters such as iron ion concentration Ni and mean iron ion separation ri of these glasses are evaluated using the conventional formulae and are presented in Table 1.

Fig. 1 represents differential scanning calorimetric (DSC) scan for one of the glasses (F3) of LiI–AgI–B2O3:Fe2O3 series; DSC trace indicate a typical glass transitions with the inflection point between 320 and 335 °C followed by a

Discussion

B2O3 is a well-known network former, participates in the network forming with BO3 and BO4 structural units. AgI and LiI do act as modifiers like any conventional modifiers and create bonding defects. In some of the recent investigations it has also been reported that Ag+ and Li+ ions in oxysalt glass matrices experience mixed oxygen–iodine coordination and do not induce any defects in the glass network [18], [19], [20]. According to this model AgI and LiI mainly act to expand the glass network,

Conclusions

LiI–AgI–B2O3 glasses mixed with different concentrations of Fe2O3 (ranging from 0 to 2.0 mol%) were prepared. DSC, optical absorption, ESR, DC conductivity and dielectric properties have been investigated. Optical absorption and ESR studies have indicated that iron ions exist in Fe2+ state in addition to Fe3+ state. DC conductivity is increased up to 0.9 mol% of Fe2O3 and beyond that the conductivity is found to decrease. The analysis of the DC conductivity results indicated that there is a mixed

Acknowledgement

One of the authors K. Srilatha wishes to thank UGC, Govt. of India and the Management of St. Theresa's College, Eluru for sanctioning study leave to carry out this work.

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