ION CONDUCTION IN COBALTOUS COBALTICYANIDE DOPED WITH LITHIUM CHLORIDE

Cobaltous cobalticyanide (a Prussian Blue type of solid with large interstitial cavities) doped with lithium chloride is found to exhibit ~ i + ion conduction. The data on temperature variation of conductivity at different concentrations of LiCl are presented.


Introduction
Prussian Blue (Fe4(Fe(CN)6)3) and related heavy metal hexacyanides form a class of c stalline solids with complex ions whose structure is well under-~t o o d .' ~~~' They have similar facecentered cubic arrangement of metsl cations at the comers of unit cubes linked by-cyanide ions placed along the edges.A peculiar property arising from this structure is that the unit cells are unusually large (lattice constant-?. 1 0 A' ).
As the result the crystal can accommodate foreign molecules and ions as interstitial impurities.11273 We have noted that Pryssian -Blue type compounds doped with Li salts exhibit ionic conduction.Electronic conductivity (30°c) of Prussian Blue is ?. 3 x a-I rn-l.However, it was found that compound with  was mixed with the desired amount of LiC1, the mixture homogenized and then dried in vacuum at 1 4 0 ' ~ to remove all moisture.The dried powder was compacted between carbon electrodes in a glass tube (diameter ?, 0.6 cm) to a pressure of 800 psi until a pellet (length ?, 0.5 cm) was formed.Ends of the tube were sealed with epoxy resin, the sample immersed in a thermostatic oil bath and a.c.(40 Hz) conductivity measured, (Modified Electronic Instruments Conductivity Bridge Model MC -1, operated at 9 V).The d.c.conductivity was also measured by the polarization (blocking electrode) method4?' and found to be of the same order as the a.c.values.The rapid decrease in conductivity with time approaching a limit comparable to intrinsic electronic clearly demonstrated that the charges carried are ionic.

Results and Discussion
Figure 1 gives a plot of I n a vs T ' for different concentrations of LiCl (c, measured as a percentage by wt).In each case the graph is a straight,line showing that the relation, is satisfied, both E and a, are found to depend on the degree of doping (ie, c).The plots of E vs c and a, vs c are shown in Figures 2 and 3 The smaller ~i + ion become mobile and move through the interstices.C1 ions could also have some mobility, but we did not succeed in detecting this experimentally.

Conclusion
Further experimental work is necessary to elucidate the exact nature of ~i + ion transport in this material.As interstitial cavities are large, it is very likely that Cl-ions also have some mobility in this material.Measurement of C1mobility would assist in understanding the mechanism of ionic conduction in Prussian Blue type materials.Unfortunately we did not have facilities for determination of the mobilities of ~i + and Cl-ions separately.
identical structure cobaltous cobalticyanide ( C O ~( C O ( C N ) ~J ~) has wall& electronic conductivity ( 3 0 ' ~) % 4 x lo-' and high stability towards thermal degradation.Consequently this material is more suitable for studying ~i + ion transport in metal hexacyanides doped with lithium salts.In this note we report our observations on ~i + ion conduction in C O ~( C O ( C N ) ~) ~ doped with LiC1.
( C O ( C N ) ~) ~ was prepared by adding potassium cobalticyanide (Aldrich) solution (0.1 M) dropwise t o a solution of cobalt nitrate (0.5 M). (Cobalt nitrate kept in excess to avoid formation of double salts containing potassium).The pink precipitate of C O ~( C O ( C N ) ~) ~ separated by filteration was washed with distilled water until the filterate is free from potassium.The powder was dried in vacuum at 1 4 0 ~~ for several hours t o remove water of hydration.(Anhydrousmaterial has a deep blue colour).The doping with LiCl was done by the following method.C O ~( C O ( C N ) ~) ~ . The conductivity was found to be maximum ( u ~~ N 2.5 x n-lrn-') when c N 34% wt (Figure 4) and minimum value of E also corresponds to this value of c. Again the minimum value of o, happens occur when the level of doping is % 34%.It is possible that the critical point occurs when the interstitial cavities are nearly filled with LiCl.A simple calculation based on estimate of the volume of an interstitial cavity (using following data: lattice constant, ionic radii of c o 2 + , c o 3 + and CN-) and density of solid LiCl indicate that the cavities get completely filled when c .y 41%.The ~i + ion mobility probably results from ionization of LiCl into ~i + and Cl-ions by the crystal field of C O ~( C O ( C N ) $ ) ~.