Rom. J. Biophys. 2002 12(3-4):97-101
STRUCTURAL AND ELECTROCHEMICAL CHARACTERISTICS OF RbAg4I5
SOLID ELECTROLYTE
GEORGETA TARALUNGA*, ELEONORA MARIA RUS**
*University of Agricultural Sciencies and Veterinary
Medicine,
**Faculty of Chemistry and Chemical Engineering, “
Abstract.
RbAg4I5 solid electrolyte is a superionic conductor with
the highest ionic conductivity (0.3 S/cm) at room temperature, having many
applications in medical devices. In this paper we present our method for
obtaining RbAg4I5 and its structural and electrochemical
properties.
Key words:
RbAg4I5 solid electrolyte, superionic conductor,
silver-iodine battery and biomedical devices.
INTRODUCTION
The solid electrolytes are
materials with a high ionic conductivity and negligible electronic mobility.
These materials are used in solid state batteries with applications in
implantable electronic instrumentation such as cardiac pacemakers,
physiological monitoring/telemetry package, etc.
The solid electrolyte RbAg4I5
belongs to the compounds-group of general formula M+Ag4I5
(where M+ = Rb+, K+, NH4+)
having an exceptionally high ionic conductivity (about 10–1 S/cm) at
room temperature. For a time these were referred to, rather inappropriately, as
“superionic conductors”. These compounds were discovered by Bradley and
independently by Owens.
Generally, the structures of
solid electrolytes are not close-packed, but contain two or three networks of
passageways intercalated in crystalline structures. For these materials the
silver ion is a mobile species and its transport into lattice takes place by a
defect mechanism [3–5].
The crystallographic
structure of RbAg4I5 was determined by X-ray diffraction.
There are three crystalline modifications of RbAg4I5
labeled as the a, b and g phases in order of decreasing temperature. The a form has a cubic crystal lattice of P41 32 (O7)
or P43 32 (O6) symmetry, b form has a
rhombohedral crystal lattice of R 32 (D37) symmetry and g modification has a hexagonal structure of loss P 321 (D32)
symmetry.
There are four units of RbAg4I5
in a cubic unit cell with a = 11.24 Ĺ, the refinement structure, by least
squares, belongs to space group P41 32(O7). The number of
sites available for mobile ions is much larger than the number of mobile ions
themselves. For example, in RbAg4I5 the 6 silver ions in each
unit cell are distributed over 56 available sites. The high conductivity is due
to a combination of a high concentration of mobile ions and a
low activation energy for ionic motions from site to site.
MATERIALS AND METHOD
The solid electrolyte RbAg4I5
was prepared by an original method [1, 2]. In order to obtain the electrolyte
samples we prepared silver iodide from diluted solution (8–10%) of AgNO3
and HI. The obtained precipitate AgI was washed until pH = 6 and dried into
vacuum box some days at room temperature and moreover 6 hours at 32–40 °C into
the vacuum drying stove. This substance was structurally characterized by X-ray
diffraction (Fig. 1).
Fig. 1. – X-ray diffraction pattern of a-AgI.
RbAg4I5
samples were obtained by isothermal crystallization from acetone solution
containing a mixture of RbI and AgI in a molar ratio
1:2. We obtained RbAg4I5 but impurified with Rb2AgI3
which favours the decomposition of wished substance. To prevent that, we added
in acetone solution, in small portions, AgI until saturation. The samples were
crystallized at 20°. The structural characterization of the prepared was
performed by X-ray diffraction. To determine the electrochemical properties,
the RbAg4I5 powder was pressed at 2500 kgf/cm2
into a stencil with 8 mm diameter. The contact electrodes were achieved by
pressing a mixture of silver powder and solid electrolyte in a weight ratio
1:1.
Fig. 2. – X-ray dilTraction pattern of a- RbAg4I5.
The ionic conductivity was
determined by means of Radelkis conductometer at a frequency of 1 kHz.
RESULTS AND DiSCUSSION
From X-ray diffraction
pattern of obtained powder (Fig. 1) it comes out that only specific diffraction
lines of a-Agl form are present. In the X-ray spectrum, shown in
Figure 2, are found the characteristic diffraction lines of a- RbAg4I5 solid electrolyte. Also, the
gravimetric analysis demonstrated that the prepared substance corresponds to
stoichiometric formula.
Because RbAg4I5
is the most stable of its compounds-group, the ionic conductivity of the sample
was performed from liquid nitrogen temperature until room temperature.
Experimental data are shown in Figure 3 and represent the mean values.
It is noticed that at two
values of temperature (122 K and 209 K) modifications of the shape slope take place.
From 120 K to 125 K ionic conductivity was grown suddenly from 5.12·10–8
S/cm at 1.33·10–5 S/cm. Precisely at 122 K g-b phase transformation takes place, therefore the passing from the tidy
phase to the untidy phase, when it appears the possibility of one fast
diffusion of silver ions into the solid electrolyte. At 209 K it can be seen a
modification of the shape of Arrhenius line, which corresponds to b-a phase transformation.
Fig. 3. – Dependence of RbAg4I5
conductivity on temperature.
Between the three
crystallographic modifications a- RbAg4I5
is the most suitable form for high ionic mobility. The specific conductivity
value of a- RbAg4I5 determined for our
sample is 0.106 S/cm at 20 °C. From the slope of Arrhenius lines the active
energies of silver ions diffusion were calculated, thus the found values are 13.45 kJ/mol for a form
and 20.23 kJ/mol for b phase. These values of active energy are in accord to
the literature data.
CONCLUSIONS
On the basis of the
experimental data we can conclude that the method proposed by the authors is
simple and allows the obtaining of RbAg4I5 solid
electrolyte in pure state.
The a-RbAg4I5 form is the most stable crystallographic
modification and presents a high ionic conductivity.
This electrolyte can be used
in solid state batteries of common applications in medical devices.
REFERENCES
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2 BOLLA, Cs., ILDIKO SZEKELY, A. BORBELY-KERI, L. ONICIU, GEORGETA TARALUNGA, Pile galvanice miniaturizate, in: Producerea, Transportul si Utilizarea Energiei, vol. XIV, pp. 101–105, Cluj‑Napoca, 1995.
3. BRADLEY, N., P.D. GREENE., Solid with high ionic conductivity in group 1 halide systems. irons. Faraday Soc., 1967, 63, 424–429.
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