Ionic conductivity of suzuki phases

Solid State Ionics 1 (1980) 223-230 0 North-Holland Publishing Company

IONIC CONDUCTIVITY N. BONANOS*

and

OF SUZUKI PHASES

E. LILLEY

Materials Science Group, School of Engirzeering and Applied Sciences, Falmer, Brighton BNI 9QT, UK Received

14 January

University of Sussex,

1980

The metastable structure 6NaCl. CdC12, known as Suzuki phase, has been prepared and studied as an ionic conductor because of its high cation vacancy concentration (12.5%). Electrical measurements over a range of frequencies (10 to 10’ Hz) have been interpreted in terms of circuit equivalents using plots of the complex resistivity and modulus. In polycrystalhne pellets of Suzuki phase, two processes are observed, both having an activation energy of 0.90 eV. They are explained as conduction in the bulk and across gaps in the grain boundaries. The conductivity of 6NaCl CdClz is consistent with a model containing some disorder in the cation lattice.

1, Introduction Suzuki phases are a family of compounds of which the prototype is 6NaCl . CdCI2. Their structure is similar to that of NaCl, but with a superlattice extending over eight unit cells of NaCl [ 11. A diagram of the structure is given in fig. 1. The sodium ions at the face centres of the NaCl unit cell are retained, while the corners are alternate-

* Now at the Wolfson Unit for Solid State Ionics, Department Science,

Imperial

College,

London,

of Metallurgy

and Materials

UK.

Fig. 1. Arrangement of the ions in the (100) plane of the 6NaCI CdCla unit cell. Small solid circles are Cd2+ ions, small open circles Na+ ions, large open circles Cl- ions and solid squares present vacancies. Arrows indicate anion displacements.

re-

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N. Bonanos, E. Lilley /Ionic conductivity of Suzuki phases

ly vacant, or occupied by cadmium ions. Vacancies account for 4 out of the 32 cation sites. Of the 32 chlorine ions, the 8 that are Na-coordinated are in the normal NaCl positions, while the remaining 24 are displaced towards the divalent ion. which has a net positive charge. The high concentration of cation vacancies (12.5%) make Suzuki phases interesting for study as solid electrolytes. To date, there are, however. no ionic conductivity measurements reported in the literature. Suzuki phases have been observed and studied as precipitates from solid solutions of divalent halides at levels up to 1000 ppm of divalent cation. These studies give information about point defects in alkali halides in a manner discussed by Lidiard [2]. From two such studies there is some evidence for ionic conduction in Suzuki phase. Firstly, Chapman [3] has found an excess conductivity in heavily doped NaCl/CdCl, crystals in the precipitation region of the ionic conductivity plot. Secondly, Laredo [4] has observed ITC peaks which she interprets as due to conducting Suzuki precipitates. Neither of these studies gives quantitative information on the conductivity of Suzuki phases. Clearly it would be desirable to have a large single crystal of 6NaCl . CdCl, but this is difficult, if not impossible to obtain, because these phases are metastable. Suzuki, in fact did extract small single crystals of 6NaCl CdCl, from a polycrystalline mass of solidified NaCl/ CdCl,, but this result does not seem to have been repeated [5]. Recently we have reported our own efforts to prepare Suzuki phases, using a variety of techniques [6]. Although it did not prove possible to grow single crystals, polycrystalline samples were prepared and studied by X-ray diffraction. In the present study electrical measurements have been carried out on such samples and have been interpreted using circuit equivalents and complex plane techniques.

2. Experimental Polycrystalline 6NaCl .CdCl, containing a volume fraction of a few percent of NaCl and CdCl, was prepared by a technique of rapid quenching from the melt described previously [6]. This was necessitated because 6NaCl .CdCl, is a metastable peritectic phase and thus may not be grown by the usual solidification techniques. The material thus produced was in the form of flakes of 100 /J thickness, which were suitable for X-ray analysis, but not for electrical measurements, being irregularly shaped and severely cracked. The flakes in the “as quenched” condition were studied with a vacuum Guinier focusing camera in the transmission mode. Samples were analysed by atomic absorption spectrometry for any loss of cadmium during the quenching process. For electrical measurements, well formed, thin pieces of the flakes were ground in a mortar and cold-pressed at a pressure of 1000 kg cm-2. The faces of the pellets were coated with an adherent film of gold deposited by vacuum evaporation. The pellets were mounted in a measurement rig [7] the leads being screened and kept as short as possible; i.e. 10 cm for frequencies up to 10 MHz. The rig was evacuated to

N. Bonanos, E. Lilley /Ionic

Fig. 2. Simple electrical

circuit

conductivity

representing

of Suzuki phases

225

a two phase layer model.

lop5 Torr and the samples were given preliminary baking out treatments at temperatures of 100 to 350°C. It has been established that 6NaCI .CdCl, is kinetically stable up to a temperature of 400°C [6]. Measurements were made at several temperatures and over a frequency range of 10 to lo7 Hz by means of a General Radio 1616 audio frequency bridge and a Wayne-Kerr B 602 radio frequency bridge. A Solartron JM 1600 transfer function analyser was used for some low frequency measurements. The electrical measurements were interpreted with the aid of a circuit equivalent of the sample. The circuit chosen is shown in fig. 2 and is the one most often used to describe the electrical response of a mixture of two conductive phases. It was proposed by Maxwell [8] and has been discussed by Volger [9]. Values of the components of this circuit were obtained from two types of plot, the complex impedance and the complex modulus plot. These have the property of resolving the R-C elements of a series connected array. Plots for the circuit shown in fig. 2 are composed of two semicircles with a certain amount of melding (overlap), which depends on the ratio of the time constants (Rc) of the two elements. The semicircle diameters on the impedance plot are equal to the resistances R 1 and Rz, while on the modulus plot the diameters are Cc1 and CFI. Thus, if the resistances are comparable, but the capacitances are very different, the two semicircles on the impedance plot are visible, while in the modulus plot, the smaller semicircle may not be resolved. Such is the case with our polycrystalline Suzuki phase pellets. The opposite is found when the two capacitances are comparable, but the resistances are very different, as shown by Hodge et al. [IO].

3. Results and discussion Experimental plots of the complex modulus and resistivity are shown in figs. 3 and 4 for two pellets of 6NaCl. CdCl,. The modulus plots consist of single arcs which approximate to depressed semicircles. The effect of temperature is to alter the distribution of points on the plots but not their shape. Strictly, to account for this behaviour requires a circuit consisting of an array of R-C elements of varying time constants [ 1 I]. In the present work we have ignored this refinement in order to extract a unique value of the dielectric constant from the modulus plots. From the inverse of the real axis intercepts of fig. 3a and similar plots for other pellets, values of the dielectric constant ranging between 5.4 and 6.4 are obtained. These are close to the value of 5.9 reported for NaCl [12].

226

N. Bonanos, E. Lilley /Ionic conductivity of Suzuki phases

101

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Fig. 3. Complex and (b) 101°C.

modulus

,

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,

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phase pellet (pellet A) at two temperatures

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The complex resistivity plots of the pellets are composed of two melded semicircles, as can be seen clearly in fig. 4, and therefore may be represented by tircuits of the type shown in fig. 2. Fig, 5 shows complex resistivity plots for a second pellet at three different temperatures. In fig. SC (3OO”C), measurements up to the maxi-

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N. Bonanos, E. Lilley /Ionic

conductivity

of Suzuki phases

221

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Fig. 5. Complex resistivity plots for a Suzuki phase pellet (pellet B) at three different temperatures (a) lOO”C, (b) 200°C and (c) 300°C. Plot (d) is a numerical simulation using values ofR and C determined from plot (a).

mum available frequency of 10 MHz were still in the region of the lower frequency semicircle and thus the high frequency part is missing. On the same plot, 6 orders of magnitude lower in frequency appears a small spike. From the high values of measured capacitance it is clearly due to an interfacial effect and it is not resolved on the corresponding modulus plot. By extending our measurement frequencies to the millihertz range, using the Solartron digital transfer function analyser, we established that the impedance can be expressed by the relation Z* = (4 + iR) m-n where A and B are comparable and n is less than unity. Precisely this type of behaviour has been described by Raistrick et al. for solid lithium silicates [ 131. Although it seems to be due to a diffusional effect, its exact nature is not yet understood. In figs. 5aa5c the two semicircles are not as obvious as in fig. 4. However, in fig. 5d a numerical simulation based on a two-element circuit shows that plots similar to fig. Sa can be generated. The method used to obtain the individual resistances from the complex resistivity plots was to determine the total resistance (R1 + R2) from the low frequency real axis intercept, and adjust R, and R2 within this constraint until a good simulated plot could be generated. The task was made easier by the fact that the shape of the experimental plots, and hence the resistance ratio, is unaffected by temperature. The two capacitances C, and C2, which are temperature independent were obtained from the semicircle maxima, at which the relation wRC = 1 is satisfied. The values of resistivities obtained from the plots at different temperatures were converted to the conductivity temperature product UT and plotted in an Arrhenius plot, fig. 6. The conductivity data o,T obtained from the high frequency semicircle lie on a straight line for pellets A and B, showing that the process involved is not

228

I% Bonanos, E. Lilley /Ionic conductivity of Suzuki phases LOO 300 k"","

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200

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Fig. 6. Ionic conductivity plot of pellets A and B. 02 represents the conductivity of the bulk Suzuki phase. 01 represents the grain boundary impedance expressed in units of conductivity.

drastically affected by variations in composition or microstructure. This is to be expected if the high frequency semicircle is due to the majority (Suzuki) phase. The plots of UT,from the low frequency semicircle also give straight lines, but this time the data for pellet A are lower than that for pellet B by a factor of approximately 5. This is consistent with the hypothesis that the low frequency semicircle is due to the minority (NaCl, CdCl,) phases, since these might be expected to vary from sample to sample. Moreover, the capacitance C, in pellet A is found to be approximately five times lower than that in pellet B, indicating a higher impurity content. The slopes of all four plots are, within the limits of experimental error, identical. A least squares fit to u1 T and o2T for pellet B gives slopes which represent activation energies of 0.89 and 0.91 eV, The data for pellet A is poorer. showing greater scatter,

N. Bonanos, E. Liiley /Ionic conductivity of Suzuki phases

229

but follows the same pattern. Given the layer model, which treats the two phases present in the pellets as electrically connected in series as in fig. 2, it is surprising to find that both elements have the same activation energy. In fact, Bauerle [ 141 found two relaxation processes having the same activation energy in zirconia, which he attributed to conduction in the bulk and at a blocking grain boundary phase giving rise to a constriction resistance. Likewise Lilley and Strutt [ 151 found two relaxation processes with the same activation energy in polycrystalline 0”-alumina which they interpreted in a similar manner. These data may be similarly interpreted, with u2 due to the Suzuki phase and aI due to conduction across gaps in the grain boundary phase. Clearly u1 is not a true conductivity, but simply a resistance related to the total specimen geometry. Using the 02T data from pellet B we can describe the conductivity of Suzuki phase as a function of temperature UT= 2.7 X lo6 exp(--0.91/kT)

,

where k is the Boltzmann constant expressed in eV. This may be compared to the expression determined by Chapman and Lilley [ 161 for NaCl in the extrinsic region: aT=n

X 1.2 X IO7 exp(-0.7l/kT),

where n is the mole fraction of free cation vacancies. Taking n = 0.125 UT= 1.5 X lo6 exp(-0.7llkT).

The main difference between 6NaCl .CdCl, and NaCl seems to be that the Suzuki phase has a higher activation energy. This can be explained if an energy of disorder is included in the measured activation energy. If the Suzuki phase is only slightly disordered, one might expect linear Arrhenius plots, as found experimentally. Previous high temperature experimental studies [6,17] showed little effect of temperature on X-ray diffracted intensities, suggesting that little change in order was taking place. In conclusion, although the Suzuki phase 6NaCl .CdCl, contains 12.5% cation vacancies, the resulting conductivity is not very high because of the high activation energy for motion. Other Suzuki phases, based on LiI, if they exist, might have a much higher ionic conductivity.

Acknowledgement

The authors wish to thank Professor R.W. Cahn for his interest in this project and for many useful contributions. We would also like to thank the SRC for financial support.

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230

conductivity

of Suzuki phases

References [l] [2] [3] [4] [5] [6] [7] [8] [9]

[lo] [l l]

[ 121 [13]

[ 141 [15] [16] [ 171

K. Suzuki, J. Phys. Sot. Japan 16 (1961) 67. A.B. Lidiard, Ionic conductivity, Handbuch der Physik 20 (1957) 246. J.A. Chapman, Ph.D. Thesis, University of Sussex (1972). D. Figueroa, E. Laredo and M. Puma, Solid State Commun. 25 (1978) 509. R.W. Spengler and R. Kaiser, Phys. Stat. Sol. 66b (1974) 107. N. Bonanos and E. Lilley, Mat. Res. Bull. 14 (1979) 1609. J.E. Strut& M.W. Weightman and E. Lilley, J. Phys. E9 (1976) 683. J.C. Maxwell, A treatise on electricity and magnetism, 2nd Ed. (Clarenden Press, Oxford, 1881). J. Volger, Progress in Semiconductors 4 (1960). I.M. Hodge, M.D. Ingram and A.R. West, J. Electroanal. Chem. 74 (1976) 125. P.B. Macedo, R. Bose, V. Provenzano and T.A. Litovitz, in: Amorphous materials, eds. R.W. Douglas and B. Ellis (Wiley-Interscience, New York, 1972). A. von Hippel, Dielectric Materials and Applications (MIT, Massachusetts/Wiley, New York, 1961). I.D. Raistrick, C. Ho and R.A. Huggins, J. Electrochem. Sot. 123 (1976) 1469. J.E. Bauerle, J. Phys. Chem. Solids 30 (1969) 2657. E. Lilley and J.E. Strutt, Phys. Stat. Sol. 54a (1979) 639. J.A. Chapman and E. Lilley, J. Phys. C9 34 (1973) 455. A.I. Sors, Ph.D. ‘Thesis, University of Sussex (1972).