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Mohammed Hassan, Rfi Rafiuddin, "Ionic Conductivity and Phase Transition Behaviour in 4AgI-(1-)-2CuI System", Physics Research International, vol. 2008, Article ID 249402, 4 pages, 2008. https://doi.org/10.1155/2008/249402
Ionic Conductivity and Phase Transition Behaviour in 4AgI-(1-)-2CuI System
Samples of general formula 4AgI-(1-)-2CuI, , have been prepared and investigated by XRD, DSC, and temperature-dependent conductivity studies. X-ray diffractograms showed the presence of binary system consisting of AgI and in the sample . Cu-substituted samples showed very similar diffractograms to that of the pure compound which indicates that no effect for the substitution on the nature of the binary system. DSC curves showed the presence of phase transition whose temperature increased with ratio in the system. Ionic conductivity measurements confirmed the occurrence of the phase transition and showed that the high temperature phase is superionic conducting, whose conductivity increases with the increasing amount in the system.
Superionic conductors are solid compounds in which electric current is carried by charged atoms, that is, by ions. The passage of current is thus associated with mass transfer. Such ionic conductors are sometimes called “solid electrolytes,” by analogy to liquid electrolyte solutions, and have permitted development of a new scientific discipline, namely, solid state electrochemistry. The associated technology is termed solid-state ionics, in contrast to solid-state electronics.
AgI is the most investigated superionic conductor which has high ionic conductivity in its a-phase stable above . Below silver iodide exists in several modifications based on zinc blend and wurtzite structures, both of which favour ionic diffusion via face sharing polyhedra. Many attempts have been made to stabilize this high conducting phase at room temperature by substitution leading to a growing group of silver ion conductors in which the transitions temperatures to the superionic phase are lower or higher than that of the a-AgI.
is one such system which has been studied extensively owing to the improved transport properties of AgI. The phase diagram of the system was studied by many workers [1–4]. The recent work by Hull et al.  has shown that the system contains a superionic phase of composition stable at . This phase has the ionic conductivity of . The structure of the phase has been resolved and was found to have an fcc Structure sublattice with majority of cations (over 90%) located in octahedral 4(b) cavities and the remainder within tetrahedral 8(c) interstices. Extensive work has been done to study the effect of substitution of mobile cation on the ionic conductivity of AgI-based superionic conductors. However, not much work seems to have been done on the substitution of immobile cations. The present work is an attempt to study the effect of substitution of immobile cation by on the ionic conductivity, phase transition temperature, and dielectric constant of system. Two ions replace each ion; therefore, the extra ion is expected to occupy interstitial position and participate in the conduction process thereby enhancing the ionic conductivity of the system.
AgI was prepared by the precipitation from ammonical silver nitrate solution by the addition of ammonium iodide solution. was prepared by the precipitation from lead nitrate solution by potassium iodide. CuI was taken from Ottockemi, India, with stated purity of 99%. Appropriate amounts of the starting materials were mixed to produce the series where . The materials were then heated for 20?hours at 480?K with intermittent grinding.
Pellets for conductivity and capacitance measurements were prepared by pouring different molar ratio mixtures into stainless steel die and pressed under the pressure of 4?tonnes/ with the help of a hydraulic press. All the samples were annealed at 310?K for 6?hours to eliminate any grain boundary effects. The pellet was mounted on stainless steel sample holder between two copper leads using two polished platinum electrodes. The copper leads were electrically insulated from the holder by Teflon sheets. The electrical conductivity and capacitance of samples were measured in the temperature range of 300?K–470?K using Gen Rad 1659 RLC Digibridge at a single frequency of 1?KHz. The heating rate was maintained at /min.
Impedance measurements were performed using HIOKI3532-50 LCR meter in the frequency rang of 40?Hz–5?MHz. DSC scanning was traced by Perkin Elmer instrument using alumina as a reference. XRD were recorded using RIGAKU D/MAX-B diffractometer with radiation.
3. Results and Discussion
3.1. X-Ray Diffraction and DSC
X-ray diffractograms of the pure and substituted samples are shown in Figure 1. Two phases can be identified in the diffractogram of the pure compound, namely, AgI and . This agreed with the previous studies which reported that the fcc high temperature phase whose formula is dissociates to its primary compounds at temperatures below the phase transition temperature . The substituted samples show very similar diffractograms to those of the pure system. Substitution by does not seem to affect the crystal structure of the final mixture and ions appeared to occupy the voids in the lattices of the binary system, since no peaks related to CuI can be identified in the diffractograms.
DSC curves of the pure and substituted samples are shown in Figure 2. The pure compound shows the expected thermal arrest at the temperature which was reported to be originated from the formation of the fcc superionic phase . Substituted compounds showed gradual increase in the phase transition temperaturewith ratio without any saturation observed within the concentration range studied. The a-ß transition temperature in AgI has also increased upon incorporating in its lattice . No Other peaks were observed in the DSC curve of these samples other than a very weak arrest which was detected at in the sample which might have resulted from insignificance decomposition of the superionic phase at high concentrations of . The absence of any thermal arrest before the phase transition temperature indicates that the binary system persists below this temperature without any other phase formation.
The variation in the phase transition temperature with the incorporation of substituent ion can be attributed to two reasons: (i) the distortion of the lattice due to the “wrong” sized substituent and (ii) the increased defect concentration in the lattice lead to the stronger defect-defect interaction which affects the phase transition temperature.
The relation between the defect-defect interaction and phase transition temperature is given  by where is the energy difference between the interstitial position and the lattice sites, represents the defect-defect interaction parameter and , , and are the vibration frequencies of the ions at interstitial positions and lattice sites, and are the number of interstitials and original lattice sites per unit volume and is Boltzmann's constant. However, the crucial role in affecting the phase transition temperature arises from lattice distortion which is proportional to size mismatch between the host and guest cation .
3.2. Electrical Conductivity
Complex impedance plots of the investigated samples are shown in Figure 3. They are typical plots of ionic conductors showing a semicircle at high frequency side and a spike at lower frequency for all of the samples. Two overlapped semicircles are shown in case of sample, the one at higher frequency resulted from bulk resistance while that at low frequency results from grain boundary resistance contribution. The relaxation times of the two contributions are very close in the other sample which results in a depressed semicircle. The spike in the lower frequency range is attributed to the blocking electrodes due to ion migration. The appearance of the spikes is an indication that the conduction in these materials is ionic in nature .
The temperature dependence of ionic conductivity is given by the Arrhenius expression, where is the pre-exponential factor and is the activation energy of ionic motion.
Arrhenius plots of the samples are shown in Figure 4. Ionic conductivity measurements showed higher phase transition temperature in Cu-substituted samples which is in agreement with the DSC results.
The ionic conductivity decreased gradually with increasing ratio in the low temperature region while significant enhancement is observed at the high temperature phase. , which is less mobile than , accumulates in the vacancies available in the lattice of AgI and partially blocks ions motion through these vacancies leading to the overall decrease in the ionic transport. While in the high temperature region, the conductivity results from the hopping of the interstitial ions, hence the presence of does not block the migration of these ions but enhances the ionic conduction through its mobility in the lattice.
The activation energies calculated from the slope of Arrhenius plot in the low and high temperature regions are presented in Table 1. Significant enhancement in the activation energies of Cu-substituted samples is observed in the low temperature region. This directly reflects the increase of the potential barrier which has to be overcome by the mobile ion to hope from one position to another. The increase in activation energy in the substituted samples is due to the restricted movement of ions due to the substitution. The comparable values of the activation energies for the various substitutions suggest that an identical hopping mechanism is responsible for the transport in all the samples .
The activation energies in the high temperature region are much lower than those in the low temperature region which is explained by the greater disorder possessed by the high temperature phase. In general, Cu-substituted samples show higher activation energies compared to the pure one. In spite of the enhancement of conductivity which results from the participation of in the conduction process, replacement of the larger size cation () by the smaller one () leads to the contraction of the lattice of the high temperature phase and hence decreasing the bottle-neck size through which ion hopping takes place. Therefore, higher thermal energy is required by ion to overcome this potential barrier. Activation energy decreases in the high temperature region which is explained by the increasing importance of conduction at the higher concentrations of the ion.
The electrical conductivity and phase transition behaviour in the system were investigated. Formation of the fcc superionic phase at temperatures larger than in the pure and substituted compounds was confirmed by DSC as well as electrical conductivity measurements. X-ray measurements showed the presence of binary system at lower temperatures with no effect of the substitution. The ionic conductivity enhanced markedly in the high temperature phase with the incorporation of . Unfortunately, this enhancement comes at the expense of the increasing phase transition temperature beyond that of the unsubstituted system.
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Copyright © 2008 Mohammed Hassan and Rfi Rafiuddin. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.