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Characterization of Prussian Blue analogue: nanocrystalline nickel-iron cyanide
ELSEVIER
Materials Scienceand Engineering B49 (1997) 89-94
Characterization Shigeyuki Department
of Applied
Chemistry,
of Prussian Blue analogue: nanocrystalline nickel-iron cyanide Yamada, Graa’uate
Katsumi School
Kuwabara
of Etlgineer ing, Nagoya
*, Kunihito
University,
Furo-cho,
Koumoto Chikusa-ku,
Nagoya
46401,
Japan
Received 20 July 1996;received in revised form 11 June 1997;accepted 11 June 1997
Abstract
Characterization of nanocrystalline nickel-iron cyanide was carried out from the viewpoints of composition, electrical conductivity, crystal parameters,and modelof crystal structure. The nickel-iron cyanidewassynthesizedby mixing two solutions containing nickel (II) and hexacyanoferrate(III) ions, respectively. The soluble form with potassiumand the insolubleform without potassiumwere obtained by changing the mixing ratio of- nickel/iron in the starting solutions. The representative compositionof the insolubleform was determinedto be Ni(II),[Fe(III)(CN),1,,, . 17H,O. The electrical conductivity was of the order of 10V5 S.rn- ’ at room temperature.The crystal phasewas an NaCl-type cubic with the unit cell of 1.022nm and the averagecrystallite size of 12.9m-n.The structure modelin which nickel and iron statisticallylocated at vacant siteswasproposed, where water moleculesin the lattice alsoexerted influenceon simulationof the X-ray diffraction (XRD) pattern. 0 1997Elsevier ScienceS.A. Kqx~ortz’s:
Prussian Blueanalogue; Nanocrystalline nickel-ironcyanide;Electricalconductivity
1. Introduction
Since thin films of iron-iron cyanide complex or Prussian Blue (PB) have been prepared recently by an electrochemical deposition method [l], the thin films of the complex have been noted from the viewpoint of electrochromic (EC) materials [2,3]. The electrochromism is a physical phenomenon showing reversible color change caused by electrochemical oxidatiomreduction reaction of a material, and material providing electrochromism is the EC material. There are two stable forms of PB, i.e. insoluble and soluble compounds. The insoluble PB has the composition Fe(3 + ),[Fe(II)(CN),], * xH,O (x = 14- 15) [4-61, while the composition of the soluble PB is KFe(3 +) [Fe(II)(CN),] .xH,O [7], where Arabic numerals mean high spin state and Roman numerals mean low spin state of iron ions. The energy levels of the iron ions in the insoluble and soluble PB were analyzed by Moesbauer spectra measurement [8]. At the oxidized state, the charge in PB transfers from the low spin Fe (II) ion to the high spin Fe (3 +) ion by absorption of light * Corresponding author. 0921-5107/97/$17.00 0 1997ElsevierScience S.A. All rightsreserved P~~s0921-5107/97)00118-9
energy. This leads to the blue color. Namely, the blue color of PB is caused by the charge transfer absorption at the wave length of 700 nm [9,10]. In the reduced state, the charge transfer cannot proceed between the
high spin Fe (2 +) ion and the low spin Fe (II) ion, and the material is colorless. The reversible change in color of the PB film is very useful for production of EC devices or EC windows modulating incident light intensity [II]. In addition to the optical property, electrical property of PB is important for devices like EC windows, because if the resistance of the component material is high the energy loss originated from d.c. current pas-
sage through the material will become significant. PB is a p-type semiconductor based on hopping conduction mechanism [12]. The order of the conductivity changes from lop4 S.rn-’ to lo-” S.m-’ depending on the solvated water [13 - 151. The electrical characteristics result from the interesting crystal structure of PB. The essential crystal structure of PB was analyzed first by Keggin and Miles [16]. The unit cell of the soluble PB is face centered cubic with the cell constant of 1.02 nm. Cyanide ions join each of the iron ions to six octahedral nearest neighbor iron ions and the potassium ions in the structure surround the iron ions tetrahedrally.
Besides PB, several metal-iron cyanides belong to the 3d metal cyanide complexes. They are nickel-iron cyanide [17,18], cobalt-iron cyanide [19]? and copper-iron cyanide [20], etc. These 3d metal-iron cyanide complexes have a crystal structure analogous to that of PB. Thin films of the cyanides are usually prepared by anodic oxidation of the 3d metal plates in an aqueous ferricyanide solution [17-201. In the previous paper, we reported the electrochemical redox behavior of the thin nickel-iron cyanide film electrode formed on IT0 glass [21]. The modified electrode revealed electrochemically reversible redox reaction and the film was almost transparent in the oxidation/reduction states. In this paper, we prepared the nickel-iron cyanide complexes by precipitation reaction to estimate composition, to measure the electrical conductivity of the compacted powders, and to analyze the crystal structure of the cyanide complex. 2. Experimental
The nickel-iron sized by following
cyanide reaction
complexes
were synthe-
2Fe(CN)z - + 3Ni2 + ---fNi,[Fe(CN)& The source reagents used were potassium hexacyanoferrate (III), K,Fe(CN), and nickel (II) chloride, NiCl,. The samples were obtained by mixing two aqueous solutions. The composition of the samples was regulated by changing the concentrations of the solutions. In general, the insoluble precipitates formed under the conditions of excess nickel ion compared with hexacyanoferrate ions. The precipitates were washed by using centrifugal machine until chloride ion could not be detected in the supernatant solution and were dried at 60°C for 20 h. In order to measure the water of crystallization, the thermogravimetric analysis (TG) was carried out with a thermal analyzer (Thermoflex CN8078B2 Rigaku). The composition of the samples was examined by using a scanning electron microscope (SEM) (S-510 Hitachi), an energy dispersion X-ray analyzer (EDX) (EMAX- 770 Horiba) and by means of an inductively coupled plasma emission spectroscopic analyzer (ICP) (Plasma Atomcomp Mk (II) Jarrell Ash). Electrical conductivities of the tablets were measured with a d.c. voltage current standard (TYPE 2553 Yokogawa Electric Works) and a digital multimeter (TR-6656 Takeda Riken) at room temperature. The sample powders were pressed at 2 x lo6 MPa m - ’ for 30 s to make compacted tablets of 10 - 2 m in diameter and 2-3 x lo- 4 m thick. The electrode material used was graphite. The cell constants and the crystallite sizes were measured by powder X-ray diffraction (XRD) method (RAD-C Rigaku). The simulation of the crys-
tal structure was program (Rietan copper and the 0.15405 nm. The scanning rate was
carried out with Rietveld analysis ‘94). The cathode ray target was wave length of X-ray used was step scan width was 0.02” and the 0.5” mm-‘.
3. Results and discussion
The compositions of the washed and dried precipitates were examined by two analytical procedures. EDX provided semi-quantitative information on the samples. According to the EDX data, the soluble nickel-iron cyanide complexes containing potassium ions were formed predominantly in the region where the molar mixing ratio of nickel (II) ion to cyanoferrate (III) ion was smaller than 1.5, and the insoluble cyanides containing no potassium ions were obtained when the mixing ratio was larger than 1.5. The ICP technique provided quantitative information on the precipitates. Table 1 shows the representative results after measuring values ten times and taking an average, where the mixing ratio means the molar ratio of nickel (II) ion to ferricyanide ion in the starting solutions. The results of the samples with the mixing ratio smaller than 1.2 were not shown in the table, because sufficient washing of the precipitates could not be attained. These precipitates consisted of very fine particles and was supposed to adsorb a little potassium and chloride ions. This leads to misjudgment of whether these ions belong to the essential lattice component or belong to only the adsorbates. As can be seen in Table 1, the potassium content in the precipitates drastically decreases with the increase of the mixing ratio> while the nickel content increases slightly with increase in the mixing ratio. From the analytical data, we tentatively consider the nickel-iron cyanide composition as Ni,[Fe(CN),], * n H,O at this stage. And then, the final formula was determined to be Ni,[Fe(CN),],i,.sH,O as described in later section. The Table 1 Composition analysis by the method of ICP emission Number
Mixing ratio
R
1
iVi2+/Fe(CN)~= 1.2 Ni’*/Fe(CN)z= 1.5 Ni”/Fe(CN)i= 2.0 NiZ+/Fe(CNfi= 3.0
0.97 k 0.04 2.68 f 0.01 2.00 + 0.01
2 3 4
Ni
0.13 & 0.02 2.81 kO.01
Fe
2.00 f 0.00
0.07 i 0.02 2.85 4 0.01 2.00 f 0.01 0.08 2 0.03 2.86 f 0.01 2,OO+ 0.01
S. Ymnnda
Table 2 Conductivities of the nickel-iron mixing ratios of Ni?+:Fe(CN)aNumber
Mixing ratio
1 2 3 4 5
Ni’+/Fe(CN):Ni*+/Fe(CN):Ni*+/Fe(CN)iNiZ+ iFe(C Ni**/Fe(CN)i-
et 01. jhfateCds
Science
cyanides made with various molar Conductivity (S m- ‘)
= = = = =
0.5 1.0 1.5 2.0 3.0
2.58 x 4.80 x 2.98 x 2.74x 2.91 x
10-x 10-S 10-S IO-’ 1O-5
water content x was observed to be 17 from the analysis of the TG curve. 3.2. Electsical conrhctiaities
Before the measurement of conductivities of the nickel-iron cyanides, the conductivities of PB tablets were preliminarily measured in order to check the suitability of the measuring procedure. The PB conductivities obtained in this study were of the order low5 S ’ m - ‘. According to the reference [12], the conductivity of the anhydrous PB sample vacuum-dried at ambient temperature is around 10 -g S *rn - I, while the sample containing water has a conductivity of the order lOus S.rn-‘. It was concluded from these facts that the measuring procedure was appropriate. Table 2 shows the conductivities of soluble and insoluble nickel-iron cyanides. This table suggests that the conductivity decreases with an increase in the mixing ratio. The experimental result that the conductivity increases with increase in potassium content is also known in the case of PB [14]. The number one sample, in particular, showed the highest conductivity of all. This can be attributed to the effect of the slightly adsorbed impurity ions that remained in the sample as described in Section 3.1. The conductivity of the number two sample seems to have little or no influence of the impurity adsorbed on the surface of the powder. The samples of numbers three to five have values of the order 10m5 S.rn-‘. These results showing that the value varied a little from sample to sample suggests that the conductivity of the cyanide complex is very sensitive to composition, especially the potassium content. The electric conduction in the soluble cyanide complex consists of cationic and electronic conductions. Cations like potassium ions migrate through interstitial sites in the lattice, while electrons move according to the following hopping mechanism. The d electrons of iron ion bound to carbon in the cyano-group are supplied to the Z* orbital of the cyano-group, and then the electrons are nonlocalized over the whole crystal. Furthermore, the d orbital of nickel ion bound to nitrogen in the cyano-group partially overlaps the Z* orbital of the cyano-group. Based on this situation of electrons, the activation energy for the electronic con-
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duction is lowered, and the electrons can transfer by hopping mechnism. The conduction in the insoluble cyanide is exclusively electronic, and the hopping mechanism can be applied. 3.3. Lattice parameters and average crystallite sizes
Fig. 1 shows the XRD profiles of the nickel-iron cyanides obtained at various mixing ratios. The profiles correspond to an NaCl-type structure. All the diffraction peaks are rather broad, and both the diffraction angles and the intensities appear to be almost independent of the mixing ratios. This result suggests that the lattice constants and the average crystallite sizes vary little with the change of the mixing ratio. The crystallographic values calculated by using the XRD profiles are shown in Table 3. The lattice parameter decreased with increasing molar mixing ratio. As described above, the samples of low mixing ratios are of the soluble form and contain potassium ions in the lattice, while the samples of high mixing ratios are of the insoluble form and contain very little or no potassium ions. It is clear from these results that the lattice parameter depends on the potassium ion content. Unlike the lattice constant, the average crystallite size increased little by little with increase in the mixing ratio. In the case of PB, when the mixing ratios of ferrous ion to ferricyanide ion in the starting solutions were 1 and 1.6, the lattice constants and the average crystallite sizes were 1.027 and 1.016 nm, and 9.77 and 9.46 nm, respectively [22]. Comparing the values of the nickeliron cyanides with those of PB, it can be seen that the lattice parameters are comparable with each other and are in the same tendency with the mixing ratio, while the crystallite sizes of the nickel-iron cyanides are a
10
20
30
40
so
60
28 (deg.) Fig. 1. XRD patterns of nickel-iron cyanides. The mixing ratios Ni2+ /Fe(CN)%- in the starting solutions are: (a) 0.5; (b) 1; (c) 1.5; (4 2; and (e) 3.
92
S. Yumnda
er al. ,‘Moterials
Table 3 Lattice constants and average crystallite sizes of the nickel-iron Number
Mixing ratio
1
Ni’+(Fe(CN)aNi’+/Fe(CNj:NT-+/Fe(CNxW+/Fe(CN)zNi*+/Fe(CN)~-
2 3
4 5
I~.~. = 0.5 = 1.0 = 1.5 = 20 = 310
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B49 (1997)
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cyanides made with various molar mixing ratios of Ni’+:Fe(CN)zAverage crystallite size (nm)
Lattice constant (nm) ,.- i-l 1.0240 1.0232 1.0219 I.0214
10.1
11.3 12.9 12.9 13.0
1.0215
little larger than those of PB and are in the inverse tendency with the mixing ratio. In any case, the crystallites are very small and have sizes of only approx. ten times of the lattice constants. It may be hard to prepare the cyanide precipitates with larger crystallite sires by the method of mixing aqueous solutions. 3.4. Models
and Engineering
structure
As described in previous sections, the nickel-iron cyanide has an NaCl-type structure (Fig. 1), and the component ratio of nickel to iron is about 3:2 (Table I). Therefore, the unit cell of the cyanide must contain an excess of nickel ions or a lack of iron ions, because the ideal NaCl-type crystal has the same number of two component elements. The structure analysis of single crystal PB was carried out with comparatively low symmetric space group of Pm3m and the excellent result was obtained by Buser et al. [S]. Considering these experimental results? two kinds of structure models with space group of Fm3m were proposed as shown in Fig. 2(a) and (b). The first model (Fig. 2(a)) is based on the composition Ni,[Fe(CN),], nH,O, where besides four nickel ions occupying sites at face centers and at corners of the unit cell, two nickel ions in the bulk of the unit cell statistically distribute at a pair of sites (l/4, l/4, l/4) and (3/4, 314, 3/4), (3/4, l/4, l/4) and(l/4, 3/4, 3/4), (l/4, l/4, 314) and (314, 314, l/4), or (314, l/4, 314) and (l/4, 3/4, l/4). The second model (Fig. 2(b)) is based on the composition Ni,[Fe(CN),],,, 17H,O. This structure includes vacant sites for both nickel and iron ions, locating statistically as illustrated in the figure. Tables 4 and 5 summarize the potential and thermal parameters used for simulation of the crystal structure analysis of the nickel-iron cyanide. These parameters were selected with trial and error procedures referring to the data used for the structural analysis of PB [5,6]. The most notable difference between the two models is the matter of whether nickel ions exist on the 8c site or not. Two simulation patterns corresponding to the models of Fig. 2(a) and (b) are shown in Fig. 3(a) and (b) together with the experimental XRD pattern of the nickel-iron cyanide powder. As can be seen clearly in the profiles (a) and (b), the diffraction line for the (220)
plane is stronger in (a) than that in (b), and then the simulation pattern of (b) agrees with the real pattern better than that of (a). Water molecules in the crystal lattice exert considerable influence on the diffraction profile, although the positions for water have not been shown in Fig. 2. The main feature of the profiles illustrated in Fig. 3(a) and 3(b) are the results including effects of diffractions by water molecules as well as by nickel, iron, and cyanide groups. The number of water molecules included in the unit cell was around 17, obtained from thermal analysis. The water molecules are statistically distributed on the positions 8c, 24e, and 32f. As in the case where the Fe (III) atom composes the octahedron Fe (III). C,
(a)
bh[Fe(CN)s]em-nH20 (b) Fig. 2. Crystal structure models of nickel-iron cyanide: (a) is based on the composition Ni,[Fe(CN)&’ 1 1H20; and (b) is based on the refined composition Ni,[Fe(CN),],,, . 17H,O.
S. Yarnada
et al. /Materials
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and Engineerhzg
B49 (1997)
Table 4 Positional and thermal parameters of the nickel-iron cyanide for calculating XRD simulation pattern of the model (a) in Fig. 2 Atom Ni(II)I Ni(II)2 Fe(III) C N 0
Position 4a SC
4b 24e 24e SC
p
x
Y
Z
B (nm2)
4 2 4 24 24 5
0 0.25 0.5 0.311 0.1985 0.25
0 0.25 0.5 0 0 0.25
0 0.25 0.5 0 0 0.25
0.007 0.0035 0.007 0.035 0.035 0.08
p shows the occupancies, and B shows the isotropic thermal vibration parameters.
with carbon atoms of the cyanide group, Ni (II) atom forms the octahedron Ni (II).N,O, with oxygen atoms of water molecules and nitrogen atoms of the cyanide group. Next, a part of the water molecules are coordinated to the water octahedrally bonded to Ni (II) atom, and then occupy the 32f positions. Finally, some water enters the zeolitic site of 8c [6]. Comparing the observed XRD pattern with the simulated patterns in Fig. 3 once more, the nickel-iron cyanide complex seems to have the crystal structure shown in Fig. 3(b). The density of the cyanide calculated using the refined composition and lattice parameters was 1.674 x 10” kg*me3. This value agreed well with the observed density 1.60 x lo3 kg. m- 3. The reliability factor, R, was calculated to be around 8.
4. Conclusion The nickel-iron cyanide was characterized from several points of view. The nickel-iron cyanide complexes were prepared by liquid phase reactions between aqueous potassium hexacyanoferrate (III) and nickel (II) chloride solutions. The composition of the precipitates after washing and drying varied with the varying mixing ratio of nickel (II) ion to hexacyanoferrate (III) ion in the starting solutions. As the mixing ratio inTable 5 Positional and thermal parameters of the nickel-iron cyanide fat calculating XRD simulation pattern of the model (b) in Fig. 2 atom
position
p
x
Y
=
Ni(I1) Fe(II1) C N 01 H 02 03
4a 4b 24e 24e 24e 96k
4 s/3 16 16 8 16 5 4
0 0.5 0.311 0.1985 0.193 0.25 0.25 0.33
0 0.5 0 0 0 0.06 0.25 0.33
0 0.007 0.5 0.007 p:o35 0 0 0.035 0 -0:035 0.06 0.06 0.25 0.08 0.33 o:os
SC
32f
B (nm2)
In both of these models, the lattice constants are as follows: a = b = c=1.022 nm, r=~=~=90”
89-94
93
I’j,,l”‘,/,,,,/,‘,I/,,,,/,,,‘,,,lr I
h 3 ti K 5 f -‘E
I,,,
10
7.0
l,,l,!,,,,/,,,,,,,,,,,,,,,,,,
30
40
50
60
70
80
90
2e(deg.)
Fig. 3. XRD patterns of nickel-iron cyanide. (a) Simulation pattern of structure model in Fig. 2(a), and (b) simulation pattern of structure model in Fig. 2(b). The top pattern is the observed one.
creased, the potassium content of the precipitate decreased and the nickel content slightly increased. We have proposed temporarily at this stage that the experimental formula is Ni(II),[Fe(III)(CN),]~ . nH,O. The electrical conductivity measured by the d.c. method was around 10 - ’ S .rn - l. The lattice constant of the precipitate decreased and the crystallite size increased with increasing molar mixing ratio. The lattice constant of the insoluble cyanide was 1.022 run and the average crystallite size was 12.9 nm. The nickel-iron cyanide had the NaCl-type structure with space group of Fm3m. Two models of the crystal structure were investigated using the potential and thermal parameters, and finally the simulation based on the refined composition WFeKNM813 .17H,O agreed well with the observed XRD data. The proposed structure included vacant sites for nickel and iron ions locating statistically. A part of water of crystallization was also distributed statistically.
References [I] V.D. Neff, J. Electrochem. Sot. 125 (1978) 886. 121 K. Honda, WI. Fujita, H. Ishida, R. Yamamoto, K. Ohgaki, J. Electrochem. Sot. 135 (1988) 3151. [3] K. Ho, T.G. Rukavina, C.B. Greenberg, J. Electrochem. Sot. 141 (1994) 2061. [4] A. Ludi, H.U. Gudel, Struct. Bonding (Berlin) 14 (1973) 1.
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[5] H.J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, Inorg. Chem. 16 (1977) 2704. [6] F. Herren, P. Fischer, A. Ludi, W. Halg, Inorg. Chem. 19 (1980) 956. 173 M.B. Robin, P. Day, Adv. Inorg. Chem. Radiochem. 10 (1967) 247. [8] K. Itaya, T. At&d, S. Toshima, T. Shinohara, J. Phys. Chem. 86 (1982) 2415. [9] K. Itaya, I. Uchida, Inorg. Chem. 25 (1986) 389. [IO] M.B. Robin, Inorg. Chem. 1 (1962) 577. [ll] K. Kuwabara, Solid state electrochromic devices, in: M.Z.A. Munshi (Ed.), Handbook of Solid State Batteries and Capacitors, World Scientific, New Jersey, 1995, pp. 555-577. [12] K. Tennakone, W.G.D. Dharmaratne, J. Phys. C: Solid State Phys. 16 (1983) 5633.
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[13] S. Ganguli, M. Bhattacharya, J. Chem. Sot. Faraday Trans. 1 79 (1983) 1513. [14] B.J. Feldman, R.W. Murray, Inorg. Chem. 26 (1987) 1702. [15] M.B. Robin, Inorg. Chem. 1 (1962) 337. [16] J.F. Keggin, F.D. Miles, Nature 137 (1936) 577. [17] S. Sinha, B.D. Humphrey, A.B. Bocarsly, Inorg. Chem. 23 (1984) 203. [I81 B.D. Humphrey, S. Sinha, A.B. Bocarsly, J. Phys. Chem. 88 (1984) 736. [19] 2. Gao, G. Wang, P. Li, Z. Zhao, Electrochim. Acta 36 (1992) 147. [ZO] L.M. Siperko, T. Kuwana, J. Electrochem. Sot, 130 (1983) 369. [21] S. Yamada, K. Kuwabara, K. Koumoto, Thin Solid Films 292 (1997) 227. [22] A. Nagatsu, Thesis of Nagoya University, 1993.
Materials Scienceand Engineering B49 (1997) 89-94
Characterization Shigeyuki Department
of Applied
Chemistry,
of Prussian Blue analogue: nanocrystalline nickel-iron cyanide Yamada, Graa’uate
Katsumi School
Kuwabara
of Etlgineer ing, Nagoya
*, Kunihito
University,
Furo-cho,
Koumoto Chikusa-ku,
Nagoya
46401,
Japan
Received 20 July 1996;received in revised form 11 June 1997;accepted 11 June 1997
Abstract
Characterization of nanocrystalline nickel-iron cyanide was carried out from the viewpoints of composition, electrical conductivity, crystal parameters,and modelof crystal structure. The nickel-iron cyanidewassynthesizedby mixing two solutions containing nickel (II) and hexacyanoferrate(III) ions, respectively. The soluble form with potassiumand the insolubleform without potassiumwere obtained by changing the mixing ratio of- nickel/iron in the starting solutions. The representative compositionof the insolubleform was determinedto be Ni(II),[Fe(III)(CN),1,,, . 17H,O. The electrical conductivity was of the order of 10V5 S.rn- ’ at room temperature.The crystal phasewas an NaCl-type cubic with the unit cell of 1.022nm and the averagecrystallite size of 12.9m-n.The structure modelin which nickel and iron statisticallylocated at vacant siteswasproposed, where water moleculesin the lattice alsoexerted influenceon simulationof the X-ray diffraction (XRD) pattern. 0 1997Elsevier ScienceS.A. Kqx~ortz’s:
Prussian Blueanalogue; Nanocrystalline nickel-ironcyanide;Electricalconductivity
1. Introduction
Since thin films of iron-iron cyanide complex or Prussian Blue (PB) have been prepared recently by an electrochemical deposition method [l], the thin films of the complex have been noted from the viewpoint of electrochromic (EC) materials [2,3]. The electrochromism is a physical phenomenon showing reversible color change caused by electrochemical oxidatiomreduction reaction of a material, and material providing electrochromism is the EC material. There are two stable forms of PB, i.e. insoluble and soluble compounds. The insoluble PB has the composition Fe(3 + ),[Fe(II)(CN),], * xH,O (x = 14- 15) [4-61, while the composition of the soluble PB is KFe(3 +) [Fe(II)(CN),] .xH,O [7], where Arabic numerals mean high spin state and Roman numerals mean low spin state of iron ions. The energy levels of the iron ions in the insoluble and soluble PB were analyzed by Moesbauer spectra measurement [8]. At the oxidized state, the charge in PB transfers from the low spin Fe (II) ion to the high spin Fe (3 +) ion by absorption of light * Corresponding author. 0921-5107/97/$17.00 0 1997ElsevierScience S.A. All rightsreserved P~~s0921-5107/97)00118-9
energy. This leads to the blue color. Namely, the blue color of PB is caused by the charge transfer absorption at the wave length of 700 nm [9,10]. In the reduced state, the charge transfer cannot proceed between the
high spin Fe (2 +) ion and the low spin Fe (II) ion, and the material is colorless. The reversible change in color of the PB film is very useful for production of EC devices or EC windows modulating incident light intensity [II]. In addition to the optical property, electrical property of PB is important for devices like EC windows, because if the resistance of the component material is high the energy loss originated from d.c. current pas-
sage through the material will become significant. PB is a p-type semiconductor based on hopping conduction mechanism [12]. The order of the conductivity changes from lop4 S.rn-’ to lo-” S.m-’ depending on the solvated water [13 - 151. The electrical characteristics result from the interesting crystal structure of PB. The essential crystal structure of PB was analyzed first by Keggin and Miles [16]. The unit cell of the soluble PB is face centered cubic with the cell constant of 1.02 nm. Cyanide ions join each of the iron ions to six octahedral nearest neighbor iron ions and the potassium ions in the structure surround the iron ions tetrahedrally.
Besides PB, several metal-iron cyanides belong to the 3d metal cyanide complexes. They are nickel-iron cyanide [17,18], cobalt-iron cyanide [19]? and copper-iron cyanide [20], etc. These 3d metal-iron cyanide complexes have a crystal structure analogous to that of PB. Thin films of the cyanides are usually prepared by anodic oxidation of the 3d metal plates in an aqueous ferricyanide solution [17-201. In the previous paper, we reported the electrochemical redox behavior of the thin nickel-iron cyanide film electrode formed on IT0 glass [21]. The modified electrode revealed electrochemically reversible redox reaction and the film was almost transparent in the oxidation/reduction states. In this paper, we prepared the nickel-iron cyanide complexes by precipitation reaction to estimate composition, to measure the electrical conductivity of the compacted powders, and to analyze the crystal structure of the cyanide complex. 2. Experimental
The nickel-iron sized by following
cyanide reaction
complexes
were synthe-
2Fe(CN)z - + 3Ni2 + ---fNi,[Fe(CN)& The source reagents used were potassium hexacyanoferrate (III), K,Fe(CN), and nickel (II) chloride, NiCl,. The samples were obtained by mixing two aqueous solutions. The composition of the samples was regulated by changing the concentrations of the solutions. In general, the insoluble precipitates formed under the conditions of excess nickel ion compared with hexacyanoferrate ions. The precipitates were washed by using centrifugal machine until chloride ion could not be detected in the supernatant solution and were dried at 60°C for 20 h. In order to measure the water of crystallization, the thermogravimetric analysis (TG) was carried out with a thermal analyzer (Thermoflex CN8078B2 Rigaku). The composition of the samples was examined by using a scanning electron microscope (SEM) (S-510 Hitachi), an energy dispersion X-ray analyzer (EDX) (EMAX- 770 Horiba) and by means of an inductively coupled plasma emission spectroscopic analyzer (ICP) (Plasma Atomcomp Mk (II) Jarrell Ash). Electrical conductivities of the tablets were measured with a d.c. voltage current standard (TYPE 2553 Yokogawa Electric Works) and a digital multimeter (TR-6656 Takeda Riken) at room temperature. The sample powders were pressed at 2 x lo6 MPa m - ’ for 30 s to make compacted tablets of 10 - 2 m in diameter and 2-3 x lo- 4 m thick. The electrode material used was graphite. The cell constants and the crystallite sizes were measured by powder X-ray diffraction (XRD) method (RAD-C Rigaku). The simulation of the crys-
tal structure was program (Rietan copper and the 0.15405 nm. The scanning rate was
carried out with Rietveld analysis ‘94). The cathode ray target was wave length of X-ray used was step scan width was 0.02” and the 0.5” mm-‘.
3. Results and discussion
The compositions of the washed and dried precipitates were examined by two analytical procedures. EDX provided semi-quantitative information on the samples. According to the EDX data, the soluble nickel-iron cyanide complexes containing potassium ions were formed predominantly in the region where the molar mixing ratio of nickel (II) ion to cyanoferrate (III) ion was smaller than 1.5, and the insoluble cyanides containing no potassium ions were obtained when the mixing ratio was larger than 1.5. The ICP technique provided quantitative information on the precipitates. Table 1 shows the representative results after measuring values ten times and taking an average, where the mixing ratio means the molar ratio of nickel (II) ion to ferricyanide ion in the starting solutions. The results of the samples with the mixing ratio smaller than 1.2 were not shown in the table, because sufficient washing of the precipitates could not be attained. These precipitates consisted of very fine particles and was supposed to adsorb a little potassium and chloride ions. This leads to misjudgment of whether these ions belong to the essential lattice component or belong to only the adsorbates. As can be seen in Table 1, the potassium content in the precipitates drastically decreases with the increase of the mixing ratio> while the nickel content increases slightly with increase in the mixing ratio. From the analytical data, we tentatively consider the nickel-iron cyanide composition as Ni,[Fe(CN),], * n H,O at this stage. And then, the final formula was determined to be Ni,[Fe(CN),],i,.sH,O as described in later section. The Table 1 Composition analysis by the method of ICP emission Number
Mixing ratio
R
1
iVi2+/Fe(CN)~= 1.2 Ni’*/Fe(CN)z= 1.5 Ni”/Fe(CN)i= 2.0 NiZ+/Fe(CNfi= 3.0
0.97 k 0.04 2.68 f 0.01 2.00 + 0.01
2 3 4
Ni
0.13 & 0.02 2.81 kO.01
Fe
2.00 f 0.00
0.07 i 0.02 2.85 4 0.01 2.00 f 0.01 0.08 2 0.03 2.86 f 0.01 2,OO+ 0.01
S. Ymnnda
Table 2 Conductivities of the nickel-iron mixing ratios of Ni?+:Fe(CN)aNumber
Mixing ratio
1 2 3 4 5
Ni’+/Fe(CN):Ni*+/Fe(CN):Ni*+/Fe(CN)iNiZ+ iFe(C Ni**/Fe(CN)i-
et 01. jhfateCds
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cyanides made with various molar Conductivity (S m- ‘)
= = = = =
0.5 1.0 1.5 2.0 3.0
2.58 x 4.80 x 2.98 x 2.74x 2.91 x
10-x 10-S 10-S IO-’ 1O-5
water content x was observed to be 17 from the analysis of the TG curve. 3.2. Electsical conrhctiaities
Before the measurement of conductivities of the nickel-iron cyanides, the conductivities of PB tablets were preliminarily measured in order to check the suitability of the measuring procedure. The PB conductivities obtained in this study were of the order low5 S ’ m - ‘. According to the reference [12], the conductivity of the anhydrous PB sample vacuum-dried at ambient temperature is around 10 -g S *rn - I, while the sample containing water has a conductivity of the order lOus S.rn-‘. It was concluded from these facts that the measuring procedure was appropriate. Table 2 shows the conductivities of soluble and insoluble nickel-iron cyanides. This table suggests that the conductivity decreases with an increase in the mixing ratio. The experimental result that the conductivity increases with increase in potassium content is also known in the case of PB [14]. The number one sample, in particular, showed the highest conductivity of all. This can be attributed to the effect of the slightly adsorbed impurity ions that remained in the sample as described in Section 3.1. The conductivity of the number two sample seems to have little or no influence of the impurity adsorbed on the surface of the powder. The samples of numbers three to five have values of the order 10m5 S.rn-‘. These results showing that the value varied a little from sample to sample suggests that the conductivity of the cyanide complex is very sensitive to composition, especially the potassium content. The electric conduction in the soluble cyanide complex consists of cationic and electronic conductions. Cations like potassium ions migrate through interstitial sites in the lattice, while electrons move according to the following hopping mechanism. The d electrons of iron ion bound to carbon in the cyano-group are supplied to the Z* orbital of the cyano-group, and then the electrons are nonlocalized over the whole crystal. Furthermore, the d orbital of nickel ion bound to nitrogen in the cyano-group partially overlaps the Z* orbital of the cyano-group. Based on this situation of electrons, the activation energy for the electronic con-
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duction is lowered, and the electrons can transfer by hopping mechnism. The conduction in the insoluble cyanide is exclusively electronic, and the hopping mechanism can be applied. 3.3. Lattice parameters and average crystallite sizes
Fig. 1 shows the XRD profiles of the nickel-iron cyanides obtained at various mixing ratios. The profiles correspond to an NaCl-type structure. All the diffraction peaks are rather broad, and both the diffraction angles and the intensities appear to be almost independent of the mixing ratios. This result suggests that the lattice constants and the average crystallite sizes vary little with the change of the mixing ratio. The crystallographic values calculated by using the XRD profiles are shown in Table 3. The lattice parameter decreased with increasing molar mixing ratio. As described above, the samples of low mixing ratios are of the soluble form and contain potassium ions in the lattice, while the samples of high mixing ratios are of the insoluble form and contain very little or no potassium ions. It is clear from these results that the lattice parameter depends on the potassium ion content. Unlike the lattice constant, the average crystallite size increased little by little with increase in the mixing ratio. In the case of PB, when the mixing ratios of ferrous ion to ferricyanide ion in the starting solutions were 1 and 1.6, the lattice constants and the average crystallite sizes were 1.027 and 1.016 nm, and 9.77 and 9.46 nm, respectively [22]. Comparing the values of the nickeliron cyanides with those of PB, it can be seen that the lattice parameters are comparable with each other and are in the same tendency with the mixing ratio, while the crystallite sizes of the nickel-iron cyanides are a
10
20
30
40
so
60
28 (deg.) Fig. 1. XRD patterns of nickel-iron cyanides. The mixing ratios Ni2+ /Fe(CN)%- in the starting solutions are: (a) 0.5; (b) 1; (c) 1.5; (4 2; and (e) 3.
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Table 3 Lattice constants and average crystallite sizes of the nickel-iron Number
Mixing ratio
1
Ni’+(Fe(CN)aNi’+/Fe(CNj:NT-+/Fe(CNxW+/Fe(CN)zNi*+/Fe(CN)~-
2 3
4 5
I~.~. = 0.5 = 1.0 = 1.5 = 20 = 310
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cyanides made with various molar mixing ratios of Ni’+:Fe(CN)zAverage crystallite size (nm)
Lattice constant (nm) ,.- i-l 1.0240 1.0232 1.0219 I.0214
10.1
11.3 12.9 12.9 13.0
1.0215
little larger than those of PB and are in the inverse tendency with the mixing ratio. In any case, the crystallites are very small and have sizes of only approx. ten times of the lattice constants. It may be hard to prepare the cyanide precipitates with larger crystallite sires by the method of mixing aqueous solutions. 3.4. Models
and Engineering
structure
As described in previous sections, the nickel-iron cyanide has an NaCl-type structure (Fig. 1), and the component ratio of nickel to iron is about 3:2 (Table I). Therefore, the unit cell of the cyanide must contain an excess of nickel ions or a lack of iron ions, because the ideal NaCl-type crystal has the same number of two component elements. The structure analysis of single crystal PB was carried out with comparatively low symmetric space group of Pm3m and the excellent result was obtained by Buser et al. [S]. Considering these experimental results? two kinds of structure models with space group of Fm3m were proposed as shown in Fig. 2(a) and (b). The first model (Fig. 2(a)) is based on the composition Ni,[Fe(CN),], nH,O, where besides four nickel ions occupying sites at face centers and at corners of the unit cell, two nickel ions in the bulk of the unit cell statistically distribute at a pair of sites (l/4, l/4, l/4) and (3/4, 314, 3/4), (3/4, l/4, l/4) and(l/4, 3/4, 3/4), (l/4, l/4, 314) and (314, 314, l/4), or (314, l/4, 314) and (l/4, 3/4, l/4). The second model (Fig. 2(b)) is based on the composition Ni,[Fe(CN),],,, 17H,O. This structure includes vacant sites for both nickel and iron ions, locating statistically as illustrated in the figure. Tables 4 and 5 summarize the potential and thermal parameters used for simulation of the crystal structure analysis of the nickel-iron cyanide. These parameters were selected with trial and error procedures referring to the data used for the structural analysis of PB [5,6]. The most notable difference between the two models is the matter of whether nickel ions exist on the 8c site or not. Two simulation patterns corresponding to the models of Fig. 2(a) and (b) are shown in Fig. 3(a) and (b) together with the experimental XRD pattern of the nickel-iron cyanide powder. As can be seen clearly in the profiles (a) and (b), the diffraction line for the (220)
plane is stronger in (a) than that in (b), and then the simulation pattern of (b) agrees with the real pattern better than that of (a). Water molecules in the crystal lattice exert considerable influence on the diffraction profile, although the positions for water have not been shown in Fig. 2. The main feature of the profiles illustrated in Fig. 3(a) and 3(b) are the results including effects of diffractions by water molecules as well as by nickel, iron, and cyanide groups. The number of water molecules included in the unit cell was around 17, obtained from thermal analysis. The water molecules are statistically distributed on the positions 8c, 24e, and 32f. As in the case where the Fe (III) atom composes the octahedron Fe (III). C,
(a)
bh[Fe(CN)s]em-nH20 (b) Fig. 2. Crystal structure models of nickel-iron cyanide: (a) is based on the composition Ni,[Fe(CN)&’ 1 1H20; and (b) is based on the refined composition Ni,[Fe(CN),],,, . 17H,O.
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Table 4 Positional and thermal parameters of the nickel-iron cyanide for calculating XRD simulation pattern of the model (a) in Fig. 2 Atom Ni(II)I Ni(II)2 Fe(III) C N 0
Position 4a SC
4b 24e 24e SC
p
x
Y
Z
B (nm2)
4 2 4 24 24 5
0 0.25 0.5 0.311 0.1985 0.25
0 0.25 0.5 0 0 0.25
0 0.25 0.5 0 0 0.25
0.007 0.0035 0.007 0.035 0.035 0.08
p shows the occupancies, and B shows the isotropic thermal vibration parameters.
with carbon atoms of the cyanide group, Ni (II) atom forms the octahedron Ni (II).N,O, with oxygen atoms of water molecules and nitrogen atoms of the cyanide group. Next, a part of the water molecules are coordinated to the water octahedrally bonded to Ni (II) atom, and then occupy the 32f positions. Finally, some water enters the zeolitic site of 8c [6]. Comparing the observed XRD pattern with the simulated patterns in Fig. 3 once more, the nickel-iron cyanide complex seems to have the crystal structure shown in Fig. 3(b). The density of the cyanide calculated using the refined composition and lattice parameters was 1.674 x 10” kg*me3. This value agreed well with the observed density 1.60 x lo3 kg. m- 3. The reliability factor, R, was calculated to be around 8.
4. Conclusion The nickel-iron cyanide was characterized from several points of view. The nickel-iron cyanide complexes were prepared by liquid phase reactions between aqueous potassium hexacyanoferrate (III) and nickel (II) chloride solutions. The composition of the precipitates after washing and drying varied with the varying mixing ratio of nickel (II) ion to hexacyanoferrate (III) ion in the starting solutions. As the mixing ratio inTable 5 Positional and thermal parameters of the nickel-iron cyanide fat calculating XRD simulation pattern of the model (b) in Fig. 2 atom
position
p
x
Y
=
Ni(I1) Fe(II1) C N 01 H 02 03
4a 4b 24e 24e 24e 96k
4 s/3 16 16 8 16 5 4
0 0.5 0.311 0.1985 0.193 0.25 0.25 0.33
0 0.5 0 0 0 0.06 0.25 0.33
0 0.007 0.5 0.007 p:o35 0 0 0.035 0 -0:035 0.06 0.06 0.25 0.08 0.33 o:os
SC
32f
B (nm2)
In both of these models, the lattice constants are as follows: a = b = c=1.022 nm, r=~=~=90”
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I’j,,l”‘,/,,,,/,‘,I/,,,,/,,,‘,,,lr I
h 3 ti K 5 f -‘E
I,,,
10
7.0
l,,l,!,,,,/,,,,,,,,,,,,,,,,,,
30
40
50
60
70
80
90
2e(deg.)
Fig. 3. XRD patterns of nickel-iron cyanide. (a) Simulation pattern of structure model in Fig. 2(a), and (b) simulation pattern of structure model in Fig. 2(b). The top pattern is the observed one.
creased, the potassium content of the precipitate decreased and the nickel content slightly increased. We have proposed temporarily at this stage that the experimental formula is Ni(II),[Fe(III)(CN),]~ . nH,O. The electrical conductivity measured by the d.c. method was around 10 - ’ S .rn - l. The lattice constant of the precipitate decreased and the crystallite size increased with increasing molar mixing ratio. The lattice constant of the insoluble cyanide was 1.022 run and the average crystallite size was 12.9 nm. The nickel-iron cyanide had the NaCl-type structure with space group of Fm3m. Two models of the crystal structure were investigated using the potential and thermal parameters, and finally the simulation based on the refined composition WFeKNM813 .17H,O agreed well with the observed XRD data. The proposed structure included vacant sites for nickel and iron ions locating statistically. A part of water of crystallization was also distributed statistically.
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