AC conductivity and scaling studies of polycrystalline SnF2
Introduction
Fluoride ion conductors, which show high value of ionic conductivity among the anionic conductors due to the small size of the anion, are found to be useful in several solid state ionic devices such as solid state batteries, electrochromic devices and gas sensors [1]. The choice of the material, stannous fluoride for the present study is unique in the sense that unlike in the case of alkaline earth fluorides and lead fluoride, the dielectric and conduction characteristics of SnF2 are not well explored. The good electrical performance of SnF2 results due to the high polarizability of Sn2+ and weak coordination of F− ion [2]. SnF2 forms a number of solid electrolytes by combining with monovalent and divalent fluorides, which are found to be very attractive for various electrochemical applications [3]. SnF2 is known to exist in three polymorphic forms. Among these α-SnF2 (monoclinic) is known to be stable at room temperature. It transforms to γ-SnF2 (tetragonal) in the temperature range of 413–453 K which is stable up to the melting temperature (488 K) and upon cooling γ-SnF2 transforms to β-SnF2 (orthorhombic) through a second order phase transition [4], [5].
In this paper the dielectric and conduction characteristics of SnF2 have been investigated through impedance spectroscopy and the impedance data have been analyzed under conductivity, permittivity and modulus formalisms in order to obtain a better insight of the relaxation properties of the material.
Section snippets
Experimental details
Powder sample of SnF2 (Aldrich, 99%) was used in the present study as procured. For conductivity measurement, fine powder of SnF2 was pelletized between thin layers of ultra fine graphite powder. The pellet was then mounted between two spring-loaded electrodes in an evacuated conductivity setup. The ac impedance data, ǀZǀ and phase angle were recorded using a HP4192A impedance analyzer over the temperature range of 300–463 K.
Impedance analysis
Fig. 1 shows typical complex impedance plots for SnF2 at some selected temperatures. In all the cases the experimental complex impedance plot comprises of a depressed semicircle accompanied by a straight line on the low frequency side, suggesting electrolyte–electrode polarization at the blocking electrodes. Such depression of the semicircle may originate from the presence of distribution in relaxation times within the bulk response [6], [7]. The radius of the semicircle decreases with
Conclusion
The frequency dependent conductivity, permittivity and modulus behavior of SnF2 have been investigated as a function of temperature. There is a decrease in conductivity during α → γ phase transition, associated with an increase in the activation energy for F− ion migration from 0.53 eV (α-phase) to 0.76 eV (γ-phase). The real part of dielectric permittivity shows saturation at higher frequencies and a strong dispersion at lower frequencies. The frequency variation of M″ data has been characterized
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