ELECTRONIC TRANSPORT IN CUPRIC FERROCYANIDE 'ACTIVATED BY INTERSTITIAL WATER MOLECULES K. TENNAKONE

Coordinated and free interstitial water molecules in cupric ferrocyanlde is found to excite electronic conduction with different activation energies. Theoretical argument is given to explain the observations.


Introduction
Ferrocyanldcs and ferricyanides of heavy metals form crystallinc solids of similar structure with intcrcsting properties.'. 2. s 3 '9 '' All thcse materials have face-centred cubic arrangement of metal cations at thc corners of unit cubes linked by cyanide ions placed along the edges.In a fcrrocyanide the carbon atom of CN-is coordinated t o Fe2+ and the nitrogen atom is coordinated t o the other metal ion.Feriicyanidcs have the samc type of bondings with ~e * + repiaced by ~e ~+ .
A property arising from this structure is that the unit cells arc unusually largc (lattice constant % 10AO) and as a result the crystal can accommodate forcign molecules, generally water as interstitial impuritics.ll 2 * '9 s' ' , 7 7 It is known that coordinated as well as free wratcr moleculcs could exist within thc intcrsticcs of prussian blue type of compounds.X-ray structural analysis indicates that thc coordinated water molcculcs are located ncar thc metal cations and the free water molecules gets hydrogen bonded t o the coordinate ones.' Ho\vevcr, on thermodynamical grounds thc author has argued that a sm,all fraction of free \cater molcc~~lcs could also gct located within the ferro-or ferricyanlde ions.l o The author anrl co-workers.'.9*1".11-129'' have noted ------that most fcrro-and ferric~'ulides exllibits electronic conduction. in the presence o f interstitial water with the following characteristics : (1) Conductivity, dccrcascs rapidly once a critical temperature T c (1 10-140 C) is exceeded. (2) The thcrinnl activation cncrg!. ol' charge carriers generally have the same order of magnitude 0.23-0.35el' (Tablc 1) (3) Unless the concentration of free water molecules is largely in excess of the bound ones, the majority charge carriers in ferrocyanides are electrons and those in ferricyanides are holes (Table 1).The author h"as shown that although it is thermodynamically more favourable for H20 molecules to get coordinated near the metallic ions a small fraction of ferro-br ferricyanide ions could also get hydrated.''The hydrated ferrocyanide (ferricyanide) ions (Figure 1) donate (accept) electron to (from) the conduction (valence) band, *. 9 7 ''9 12, l 3 i.e., In this note we report our observations on electronic transport properties of Cu Fe(CN)6.2H20, that deviate significantly from the general pattern describe 8 "earlier.In contrast to the other ferrocyanides we have examined, the thermal activation energy of this material is 0.57 eV and the major charge carriers are found to be holes.Theoretical arguments are given to explain the observations.

Experimental
Measurements were made with compressed pellets as well as single crystals.The material for making compressed pellets were made by the decomposition of copper\ sulphate with potassium ferrocyanide in the presence of dilute nitric acid (acid prevents the contamination of the product with cupric hydroxide resulting from hydrolysis).The precipitate, was washed w s h deionized water and dried in vacuum at 40°C.Chemical analysis confirmed that it corresponds to stoichiometric C U ~F ~( C N ) ~ .2H20.It was noted that unlike other heavy metal ferrocyanides the bound water molecules cannot be removed by prolonged drying in vacuum at temperatures below 1 10" C. Thermal gravimetric analysis also indicated that the hydration number of the material is 2 (Figure 2).Single crystals of cupric ferrocyanide were prepared by diffusing very dilute solutions ( x m o l dm-3 ) of K4 Fe(CN)6 and CuS04 into a pellet of agar jelly (% 10 cm in length) at the middle of a glass tube (diameter 'L 2 cm) from opposite sides.Crystals (?J 2mmx2mmx2mm) were produced in a period of 6 months.The crystals were dried in vacuum at 40°C and pressed between stainless steel electrodes in glass tubes.The ends of the tube were sealed with epoxy resin, immersed in a thermostatic oil bath and the D.C. resistivity was determined using a bridge type resistance meter.The D.C. current-voltage characteristics were tested.Measurements were also carried out with samples where the concentration of H 2 0 was in excess of the coordinated molecules.No significant differences were noted between the conductivity properties of single crystals and compacted peIlets provided the pellets were made by compressing the powder to pressure exceeding % 20 kbar.
We have not succeeded in devising methods for exact measurements of carrier mobilities.The sign of major charge carriers were determined from thermoelectric tests.

Results and Discussion
The plots of log p vs T-' are indicated in Figure 3.In Cu2 Fe(CN), .2H20 ( ~& u r e 3 Curve I ) , it is seen that until a critical temperature Tc = 11 1°C is reached, p decreases with T according to the relation, with E 0.5 7 eV and p o * 1 .0 3 ~1 0-3 SL cm. when T > Tc, p increases very rapidly with T. The behaviour of samples containing water in excess of the two coordinated molecules is different.Figure 3, Curves 2 and 3 show the plots of log p vs T-' when the water content corresponds to the approximate compositions Cu, Fe(CN), .2.4 H,O and Cu2 Fe(CN), .6H20 respectively.The characteristic feature of the Curve 2 is that it fits into two straight line portions with activation energies 2~ 0.30 eV, T C 72" and 0.57 eV, T > 72°C.In the Curve 3, where H, 0 concentration is much higher the activation energy remain constant at r~ 0.3 eV until a critical temperature is reached.D.C current-voltage characteristics are indicated in the Figure 4. Samples containing only coordinated molecules of water give linear plots (Figure 4 Curve 1).Here the charge carriers are entirely electronic and thermoelectric test show that the material is -p-type.Figure 4 Curve 2 is the I-V characteristic for a sample with the approximate composition Cu, Fe(CN),.GH,O.Here polarization effects are clearly evident and the shape of the plot suggests that three types charge carrierselectron holes 2nd ions-are prcscnt. 3% '+ The ionic carriers are probably protons or H i 0 ions.I t is not difficult to understand why electrical transport properties of cupric ferrocyanide is different from other ferrocyanides we have examined (Table 1).Cuft ion is a good electron acceptor.Cuu/Cu+ redox potential in the crystal field is reduced by water molecules.Thus Cu" ion could accept electrons from the valence band.In other ferrocyanides (Table 1) the activation energy for a process similar to this at the metal cation is probably much higher.When excess water is present some of the ferrocyanide ions could also get hydrated and thus donate electrons to the conduction band with an activation energy 0.3 eV as discussed earlier.If the concentration of excess water is small, it is natural to expect that the former process would predominate as the temperature increases.Thus the difference between the three curves in Figure 3 can be understood.Again the above hypothesis explains why both types of charge carriers appear when there is free interstitial water.

Figure 1 .FerrocyanidesFe
Figure 1.D~agrarns showing the octahedral linkage of CH-ions into (a) ~e ~+ ion in ferrocyanide (b) ~e ~+ ion in a ferricyanide ( i d is a metal ion).When the spherical region denoted by the circle is filled with H2 0 molecules (a) and (b) behave as ferro-and ferrlcyanide ions in aqueous solution.