Neutron Diffraction and EXAFS Studies of K - ACS Publications

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Neutron Diffraction and EXAFS Studies of K2x/3Cu[Fe(CN)6]2/3·nH2O Dariusz Wardecki,† Dickson O. Ojwang,† Jekabs Grins, and Gunnar Svensson* Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691, Stockholm, Sweden S Supporting Information *

ABSTRACT: The crystal structure of copper hexacyanoferrate (CuHCF), K2x/3Cu[Fe(CN)6]2/3·nH2O, with nominal compositions x = 0.0 and x = 1.0 was studied by neutron powder diffraction (NPD) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The compound crystallizes in space group Fm3̅m, with a = 10.1036(11) Å and a = 10.0588(5) Å for x = 0.0 and x = 1.0, respectively. Difference Fourier maps for x = 0.0 show that the coordinated water molecules are positioned at a site 192l close to vacant N positions in the −Fe−C−N−Cu− framework, while additional zeolitic water molecules are distributed over three sites (8c, 32f, and 48g) in the −Fe−C−N−Cu− framework cavities. The refined water content for x = 0.0 is 16.8(8) per unit cell, in agreement with the ideal 16 (n = 4). For x = 1.0, the refinement suggests that 2.6 K atoms per unit cell (x = 0.98) are distributed only over the sites 8c and 32f in the cavities, and 13.9(7) water per unit cell are distributed over all the four positions. The EXAFS data for Fe, Cu, and K Kedges are in agreement with the NPD data, supporting a structure model with a linear −Fe−C−N−Cu− framework and K+ ions in the cavities. symmetries are also found for a number of PBAs.28−33 In the cubic framework, a number of M′(CN)6 octahedra are vacant, and the nitrogen sites in the missing CN units are filled by water molecules that bind to the neighboring M ions through their oxygen atoms. In addition, water/alkali ions also occupy voids or cavities formed by the framework (Figure 1). The locations of the water molecules in cubic (Fm3̅m) PB and PBAs have been determined from X-ray and neutron diffraction, and IR spectroscopy studies.14,27,34,35 Two kinds of water molecules have been identified to occupy three different sites: (i) zeolitic water at the site 8c (1/4, 1/4, 1/4) in the center of the cavity and at a site 32f (x, x, x), forming a tetrahedron around the 8c site, and (ii) coordinated water at a site 24e (x, 0, 0) close to the N atom positions. Ideally, the maximum number of water molecules in the unit cell of AxM[M′(CN)6]z·nH2O is 16 (n = 4) for a z value of 2/3, where 8 of them are distributed over the sites 8c and 32f in the cavities, and 8 coordinated to the M cations close to the empty nitrogen positions.34 Despite the extensive studies on PB and PBAs, a more detailed and precise determination of the crystallographic positions for the water molecules and alkali ions is, however, essential. The available neutron studies on water occupancies on Mn3[Co(CN)6]2·12H2O and Cd3[Co(CN)6]2·12H2O, and Fe4[Fe(CN)6]3·xH2O dates back to 1977 and 1980, respectively.27,34 However, the models proposed in those studies do not completely describe the structure of CuHCF. Even though a series of CuHCF compounds with varying compositions and stoichiometries have been investigated, no well-defined

1. INTRODUCTION Today, renewable energy resources such as the sun, wind, and ocean tides are considered as the most attractive power sources for clean and sustainable energy consumption.1 However, the renewable resources suffer a number of limitations, e.g., shortterm transients, load balancing, and frequency regulation.2 To meet the present global energy demand, a new kind of largescale battery with better and cheaper materials is required.3,4 Potassium-ion rechargeable batteries consisting of Prussian blue analogues (PBAs) as electrode materials are promising candidates for large-scale energy applications. Unlike lithium, potassium has more abundant natural reserves.5−8 In recent years, copper hexacyanoferrate (CuHCF) has emerged as a PBA with interesting electrochemical properties. The battery’s chemistry involves insertion/extraction of alkali ions. Environmental friendliness, safe, low operation cost, and simple and scalable synthesis methods are some of its merits.3,9−11 Depending on the presence or absence of alkali cations, PB is often designated as alkali-rich “soluble” or alkali-free “insoluble”.12,13 The former can be expressed as AIFeIII[FeII(CN)6]·nH2O, where A = alkali metal ions and n = 1−5, and the latter as FeIII[FeII(CN)6]3/4·nH2O, where n = 3.5−4. PBAs have a general formula AxM[M′(CN)6]z·nH2O (M and M′ = divalent or trivalent transition metal ions, A = alkali metal ions).14−18 Many investigations have shown that PB and PBAs have several potential areas of applications, e.g., electrochromism,19 waste recovery,20 hydrogen storage,21 electrocatalysis,22 molecular magnets,23 and charge storage.24 PB and PBAs are usually obtained as very fine powders from water-based precipitation reactions.25 They often have cubic structures consisting of a three-dimensional −NC−M′−CN− M−NC− (M = M′ = Fe for PB) cubic framework that forms eight cavities per unit cell.14,15,26,27 However, lower crystal © 2017 American Chemical Society

Received: November 21, 2016 Revised: February 7, 2017 Published: February 8, 2017 1285

DOI: 10.1021/acs.cgd.6b01684 Cryst. Growth Des. 2017, 17, 1285−1292

Crystal Growth & Design

Article

step was twice repeated within a N2-filled glovebox, leading to a product deuterated to ∼80%, as determined by infrared spectroscopy (see Figure S1). The samples used for NPD studies were thus drenched in D2O. Prior to the NPD studies, the samples were loaded in 8 mm vanadium cans and sealed with an indium metal wire. Time-of-flight (TOF) NPD data were collected at room temperature for the x = 0.0 and x = 1.0 samples on the GEM instrument at ISIS, United Kingdom. Diffraction patterns from banks at 35° and 90° and d values > 0.7 Å were used for the Rietveld analysis by means of the TOPAS 4.1 program.45 Prior to the Rietveld refinements, including ∼90 unique reflections, the background for all diffraction patterns was graphically corrected; i.e., in each pattern, ∼50 background points were selected manually and subtracted using a linear interpolation. 2.2. EXAFS Studies. The CuII[FeIII(CN)6]2/3·nH2O compound was obtained by mixing two aqueous solutions. The starting reagents, 0.08 M Cu(NO3)2 (50 mL; Sigma-Aldrich) and 0.04 M K3Fe(CN)6 (50 mL; Sigma-Aldrich), were simultaneously added to 25 mL of distilled water under constant stirring. Potassium ions were then inserted into CuII[FeIII(CN)6]2/3·nH2O by reducing FeIII with 0.1 M K2S2O3 (aq) to give K2x/3CuII[FeIIxFeIII1−x(CN)6]2/3·nH2O precipitates. The precipitates were washed, freeze-dried, and ground to a fine powder. Samples with 0.0 ≤ x ≤ 1.0, where Δx = 0.2, were prepared. Inductively coupled plasma-optical emission spectroscopy (ICP) to confirm the cation concentration (Fe, Cu, and K) and combustion analysis of C, N, and H were performed by Medac Ltd., United Kingdom. The K content for the samples with nominal compositions x = 0.0 and x = 1.0 were determined by ICP to be x = 0.09(5) and x = 0.91(5) per formula unit, respectively. The K content corresponding to x = 0.09(5) originates from the K3Fe(CN)6 (aq) precursor. In the structural study, the nominal compositions have been used. Moreover, the compositions of the samples used for NPD study were assumed to be identical to those used for EXAFS, except for interchange of H by D. More details about synthesis and structure characterization by X-ray powder diffraction (XRD) of the samples are reported elsewhere.44 EXAFS analysis was performed on the x = 0.0 and 1.0 samples. The samples were mixed with BN (30:50 w/w) and pressed into 1 mm thick pellets. EXAFS experiments were performed at room temperature at the wiggler beamline I811 at MAX-Lab, Lund, Sweden. The beamline was equipped with a Si(311) double crystal monochromator, and the higher order harmonics were rejected by detuning the second monochromator crystal. Spectra at the K-edges for Fe and Cu were acquired in transmission mode for x = 0.0 and 1.0, and spectra at the K-edge for K were recorded in fluorescence mode for x = 1.0. The spectra of an iron foil, copper foil, and KCl pellet were recorded simultaneously in each scan and used for energy calibration. All data treatment was performed using the EXAFSPAK package.46 EXAFS signals were extracted from the raw spectra following the standard procedures for pre-edge subtraction, normalization, and background removal using a spline function.47 Amplitudes and phases for multiple scattering (MS) signals were calculated using the FEFF6 program.48 The k-ranges used were 2−14 Å−1 for Fe and Cu, and 2−6 Å−1 for K.

Figure 1. Schematic representation of the studied PBA structure with space group Fm3̅m and unit-cell axis a ≈ 10.1 Å. When there are Fe(CN)6 vacancies (in the figure, two vacancies are shown, at the top left corner and top face center), coordinated water (O-red, D-light gray) complete the coordination sphere of the surrounding M atoms. The framework atoms and zeolitic water are marked as follows: Fe (dark gray), Cu (bronze), and D2O (light blue). The CN groups are omitted for clarity.

positions are provided for the alkali ions/water molecules in the −Fe−C−N−Cu− framework cavities.36−42 Our interest in K-containing CuHCF (K2x/3Cu[Fe(CN)6]2/3· nH2O) stems from the promising electrochemical results that have been reported for aqueous battery systems with CuHCF compounds used as electrode materials.9,43 The structural changes during K+ ions insertion can be linked to the electrochemical processes. Therefore, we recently reported a study of chemical intercalation of K+ ions in CuHCF. Therein, a number of techniques were employed for the structural characterization; however, the results were not completely unambiguous concerning the positions of the water molecules and K+ ions.44 Here, we report on the synthesis of partly deuterated samples with and without K and a study of their structures using NPD and EXAFS in order to better describe the water substructure and the local environment around Fe, Cu, and K atoms. Many studies on alkali containing PB and PBAs using X-ray spectroscopy have focused on the K-edge of 3d transition metals in the compounds, but to the best of our knowledge, none have used the K K-edge.

2. EXPERIMENTAL METHODS

3. RESULTS AND DISCUSSION 3.1. Structure Refinement. All Bragg peaks could be indexed with a F-centered cubic unit cell with a = 10.1036(11) Å and a = 10.0588(5) Å for x = 0.0 and x = 1.0, respectively. The structure of PB (space group Fm3̅m) was used as starting model for the Rietveld refinement of x = 0.0.27 The positions of the framework atoms were: Fe and Cu at sites 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2), respectively, and C and N at sites 24e (0.185, 0, 0), and 24e (0.302, 0, 0), respectively. In order to localize the water molecules in the structure, a series of difference Fourier maps were generated. Figure 2 shows a difference map for x = 0.0 that includes only the framework atoms (Cu, Fe, C, and N) in the model. The x = 0.0 and 1.0 samples for NPD were only deuterated to ∼80%. In the

2.1. Neutron Studies. The chemicals Cu(NO3)2, K3Fe(CN)6, and deuterium oxide (D2O) were all purchased from Sigma-Aldrich. Two compounds with nominal compositions, CuII[FeIII(CN)6]2/3·nD2O and K2/3CuII[FeII(CN)6]2/3·nD2O, were synthesized. To begin, 6 g of Cu(NO3)2 was dried at 110 °C for 4 h. Solutions of the dried Cu(NO3)2 (0.25 M, 50 mL) and K3Fe(CN)6 (0.125 M, 50 mL) were prepared separately in D2O. The two solutions were then mixed simultaneously in 25 mL of D2O under constant stirring for 2 h at room temperature to give CuII[FeIII(CN)6]2/3·nD2O. Potassium ions were inserted into the CuII[FeIII(CN)6]2/3·nD2O structure by reducing FeIII with 0.1 M K2S2O3, also prepared in D2O. The resulting precipitates were centrifuged for 10 min, washed three times with D2O, and dried overnight under vacuum at 70 °C. In the final step, 10 mL of D2O was added to the dry precipitates and the resulting suspension was kept at 20 °C for 2 h and at 50 °C for 1 h. This last 1286



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Mn3[Co(CN)6]2·12H2O,34 there is no indication of water positioned at the site 4a (0, 0, 0). In the refined model, the water molecules at each site in the cavities were treated as pseudo-atoms with an average scattering length of b{D2 O} = b{O} + 2(0.8b{D} + 0.2b{H})

where b{O} = 5.81 fm, b{D} = 6.67 fm, and b{H} = −3.74 fm are scattering lengths of O, D, and H atoms, respectively.49 This approach takes into account different orientations of the water molecules in the cavity and seems to be reasonable since the exact positions of D for the zeolitic water could not be found from the analyzed diffraction data. According to the model by Herren et al. for PB, there are two water positions in the cavity, at site 8c (1/4, 1/4, 1/4), and a site 32f (x, x, x).27 However, assessment of the difference Fourier maps revealed an additional position at a site 48g (x, 1/4, 1/4), which was also included in the refinement. The partially occupied oxygen sites 32f and 48g form a tetrahedron and an octahedron, respectively, both with atoms ∼0.8 Å from the center 8c site. The model for PB further assumes that the O atoms of the coordinated water occupy a site 24e (x, 0, 0) with defined positions of D atoms at a site 96k (x, x, z). A Rietveld refinement with this model showed significant discrepancy between the observed and calculated powder diffraction patterns and yielded a χ2 value of ∼4. In order to improve the model, the coordinated water molecules were positioned at 192l (x, y, z), akin to the model presented by Ludi et al.15 To limit the number of refined parameters and to make the refinement more stable, the coordinated water molecules were modeled using a rigid body with fixed O−D distances of 0.96 Å and D−O−D angles of 106°. During the refinement, the Euler angles and the position of the water molecules were allowed to vary. To stabilize the refinement, the atomic displacement parameters of the CN groups, coordinated water, and pseudo-

Figure 2. Difference Fourier map generated with the model including only the framework atoms (Cu, Fe, C, and N). The unit cell with one cube octant selected is shown. The three positions of zeolitic water (O1, O2, O3), and coordinated water (O4) are marked as red spheres.

following tables and text, D designates accordingly a 80:20 ratio of deuterium, D, to hydrogen, H. The map shows no residual scattering intensities at the positions of the framework atoms. However, there are distinct peaks in the cavities, which can be assigned to zeolitic water, and smaller ones around the CN group positions, attributed to coordinated water completing the coordination sphere around the Cu atoms, whenever vacant Fe(CN)6 units are present. Further evaluation of the Fourier maps showed that the intensity in the cavity is an overlap of three peaks, which are linked to different positions of the partially occupied zeolitic water molecules. Contrary to what Beall et al. reported for

Figure 3. Observed (red), calculated (black), and difference (blue) powder patterns for the K2x/3Cu[Fe(CN)6]2/3·nH2O, (a) x = 0.0 and (b) x = 1.0 samples. 1287



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Table 1. Atomic Coordinates and Isotropic Thermal Parameters for K2x/3Cu[Fe(CN)6]2/3·nH2O, x = 0.0d atom

site

x

Fe Cu C N O1 D1b D2b D2O1c D2O2c D2O3c

4a 4b 24e 24e 192l 192l 192l 8c 32f 48g

0 /2 0.185(1) 0.302(1) 0.046(1) 0.026(1) 0.127(1) 1 /4 0.202(4) 0.176(7) 1

z

occ.

Biso (Å2)

0 /2 0 0 0.296(2) 0.218(2) 0.277(2) 1 /4 0.202(4) 1 /4

0.68(1) 1 0.68(1) 0.68(1) 0.046(1) 0.046(1) 0.046(1) 0.24(4) 0.10(2) 0.06(1)

1.1(1) 3.1(3) 2.6(1)a 2.6(1)a 3.4(5) 3.4(5) 3.4(5) 8(1) 8(1) 8(1)

y 0 /2 0 0 0.099(2) 0.049(2) 0.146(2) 1 /4 0.202(4) 1 /4 1

1

a

Isotropic equivalent of u11 = 0.021(1), u22 = u33 = 0.046(1). bb{D} = 0.8b{D} + 0.2b{H}. cb{D2O} = b{O} + 2(0.8b{D} + 0.2b{H}). dSpace group Fm3̅m, a = 10.1036 (11) Å, Rexp = 0.8%, Rwp = 0.9%, χ2 = 1.4, RBragg = 0.2%.

Table 2. Atomic Coordinates and Isotropic Thermal Parameters for K2x/3Cu[Fe(CN)6]2/3·nH2O, x = 1.0d atom

site

x

Fe Cu C N O1 D1b D2b D2O1c K1 D2O2c K2 D2O3c

4a 4b 24e 24e 192l 192l 192l 8c 8c 32f 32f 48g

0 /2 0.189(1) 0.307(1) 0.023(3) 0.015(3) 0.108(3) 1 /4 1 /4 0.200(3) 0.200(3) 0.175(5) 1

z

occ.

Biso (Å2)

0 /2 0 0 0.285(3) 0.196(3) 0.290(3) 1 /4 1 /4 0.200(3) 0.200(3) 1 /4

0.68(2) 1 0.68(2) 0.68(2) 0.044(2) 0.031(2) 0.031(2) 0.16(3) 0.125 0.04(1) 0.05 0.06(1)

0.6(2) 3.2(3) 2.6(1)a 2.6(1)a 7(1) 7(1) 7(1) 7(1) 7(1) 7(1) 7(1) 7(1)

y 0 /2 0 0 0.095(5) 0.064(5) 0.139(5) 1 /4 1 /4 0.200(3) 0.200(3) 1 /4 1

1

a Isotropic equivalent of u11 = 0.027(1), u22 = u33 = 0.044(2). bb{D} = 0.8b{D} + 0.2b{H}. cb{D2O} = b{O} + 2(0.8b{D} + 0.2b{H}). dSpace group Fm3m ̅ , a = 10.0588(5) Å, Rexp = 0.7%, Rwp = 1.0%, χ2 = 1.7, RBragg = 0.3%.

only due to the presence of K atoms on these sites, i.e., that the summed occupancies of water molecules and K atoms are the same, calculations presented in the Supporting Information show that the K atoms should be distributed as follows: 1 K on the 8c site and 1.6 K on the 32f site. Using these constraints in the refinement, the site occupancies for the water molecules can be refined. The final parameters are presented in Table 2. The refined water site occupancies correspond to a sum of 13.9(7) water molecules per unit cell. It should be noted here that the samples used here were drenched in D2O as mentioned earlier, which is expected to provide for a maximum occupancy of the available sites. The 0.4% difference in the unit cell constant between x = 0.0 (a = 10.1036 (11) Å) and x = 1.0 (a = 10.0588 (5) Å) can be accredited to the decrease of the Fe−C, Cu−N, and C−N bonds forming the framework: ΔFe−C = 0.004 Å, ΔCu−N = 0.013 Å, and ΔC−N = 0.007 Å; see Table 3. When K+ ion is inserted into the structure, [FeIII(CN)6]3− is reduced to

atoms representing the zeolitic waters were constrained. The resulting model with 45 independent parameters gives a good fit to the data, as shown in Figure 3a. The model yields 16.8(8) water molecules per unit cell (n = 4.2) and a χ2 value of ∼1.6. The final structural parameters for x = 0.0 are presented in Table 1. The refined 192l position of the coordinated water molecule is close to the site 24e (x, 0, 0) of the CN ligands, which can be an indication that the O atom and one of the D atoms are compensating for the anisotropic character of the C and N atoms; i.e., the peaks in the Fourier map interpreted as water molecules are in fact residual peaks from the anisotropic atomic displacements of the CN groups.15,34 However, in our model, anisotropic atomic displacement parameters were used for the C and N atoms and still the refinement yielded the expected number of coordinated water molecules (∼8). Moreover, the refined position of the second D atom gives an average distance of ∼1.5 Å to the nearest O atom of the zeolitic waters in the cavity, which is close to what is expected for a hydrogen bond. To find the atomic coordinates of atoms in the sample x = 1.0, the same model as for x = 0.0 was used. Upon placing only D2O molecules on sites 8c, 32f, and 48g, a notable decrease was observed in the site occupancy factors (sofs) for sites 8c and 48g in comparison with those for x = 0.0 (see Table S6). The decrease in sofs can be accredited to the presence of K atoms on these sites. According to ICP measurements, there are, for x = 1.0, 2.6 K atoms per unit cell.44 Assuming that the differences in water contents on the sites 8c and 32f between x = 0.0 and 1.0 are

Table 3. Selected Bond Lengths (Å)

1288

bond

x = 0.0

x = 1.0

Fe−C Cu−N C−N Cu−O(192l) K(8c)−C/N K(32f)−C/N

1.929(1) 1.962(1) 1.163(1) 2.341(2)

1.925(1) 1.949(1) 1.156(1) 2.375(1) 3.603(1) 2.945(4)



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[FeII(CN)6]4− and extra electrons are introduced into the Fe− C−N−Cu system, both of which affect the π-bonding system. However, the shrinkage of the unit cell cannot only be explained by the smaller size of [FeII(CN)6]4− compared to [FeIII(CN)6]3− as suggested by various authors,9,25,50 since the decrease of both ΔCu−N and ΔC−N is larger than ΔFe−C. It is most probable that the K+ ion also has an effect on the πback-bonding system, removing charge from antibonding πorbitals.51 The presence of Fe(CN)6 vacant sites suggests a mixed coordination sphere for Cu atoms. However, the Cu−O(192l) bond is significantly longer, ΔCu−O(192l) = 0.034 Å, for x = 1.0 compared to x = 0.0. The elongation of the Cu−O(192l) bond can be a compensation for the shortening of the Cu−N bond, but also for the presence of K−O(192l) bonds in x = 1.0. The observed differences in Cu−N and Cu−O distances emanates from the different bond properties of N and O atoms of the CN ligands and coordinated water molecules, respectively. The interatomic distances observed for x = 0.0 and 1.0 are in agreement with what is reported for similar compounds in the literature.26,31,41,42,52 3.2. EXAFS Analysis. All modeling was performed with the assumption of a linear atomic Fe−C−N−Cu chain, as suggested from the NPD results, and the initial atomic positions were taken from there as well. In such a model, a high atomic focusing effect is expected;53 hence, multiple scattering (MS) up to 4-leg paths were considered. Figures 4a−

Figure 5. (a) EXAFS functions at the Cu K-edge for x = 0.0, (b) corresponding FT, (c) EXAFS functions at the Cu K-edge for x = 1.0, (d) corresponding FT.

refinement. The final parameters are presented in Table 4. Attempts of using models with nonlinear Fe−C−N−Cu chains to fit the EXAFS signals yielded reduced calculated EXAFS amplitudes and poorer fits. Table 4. Selected Interatomic Distances and Debye−Waller Factors sample x = 0.0

x = 1.0

bond

distance (Å)

σ (Å )

distance (Å)

σ2 (Å2)

Fe−C Fe−N Fe−Cu Fe−O/K Cu−N Cu−C Cu−O/K Cu−Fe K−C/N

1.916(1) 3.071(1) 5.049(2) 4.54(1) 1.970(3) 3.125(1) 4.44(11) 5.039(2)

0.0021(1) 0.0037(1) 0.0060(1) 0.012(2) 0.0026(4) 0.0063(2) 0.0063(2) 0.0053(1)

1.891(1) 3.053(1) 5.017(2) 4.50(2) 1.960(3) 3.122(2) 4.43(11) 5.005(2) 3.237(5)

0.0018(1) 0.0037(1) 0.0065(1) 0.012(2) 0.0025(4) 0.0065(2) 0.0123(2) 0.0054(1) 0.0069(48)

2

2

The FTs for x = 0.0 and x = 1.0 resemble each other, as expected considering the similarity of the structures. Figures 4b,d and 5b,d show three dominant peaks observed in the FT for all spectra. The three peaks in Figure 4b,d can be ascribed to Fe−C, Fe−N, and Fe−Cu interactions, respectively, whereas the ones in Figure 5b,d can be assigned to Cu−N, Cu−C, and Cu−Fe, respectively. Furthermore, the prominence of Fe−N, Fe−Cu, Cu−C, and Cu−Fe shells in the FTs validate a linear Fe−C−N−Cu configuration. The poor data quality of the K K-edge excluded a quantitative model with interactions at larger distances and/ or MS. However, some information can still be gained. The SS fit to the data collected for K K-edge showed that the nearestneighbor distance is 3.237(5) Å, which can be assigned to both K−C and K−N interactions. This is strong evidence that the K+

Figure 4. (a) EXAFS functions at the Fe K-edge for x = 0.0, (b) corresponding FT, (c) EXAFS functions at the Fe K-edge for x = 1.0, (d) corresponding FT.

d and 5a−d show the best fit of experimental and calculated EXAFS functions and the corresponding Fourier transforms (FTs) on Fe and Cu K-edges for samples x = 0.0 and x = 1.0. The same model, including four single scattering (SS) components Fe−C, Fe−N, Fe−Cu, and Fe−O/K, was used in both cases. Additionally, the EXAFS function included three MS components (2 for Fe−N and 1 for Fe−Cu) with distances and the Debye−Waller parameters (σ2) constrained to the SS components. The coordination numbers were fixed during the 1289



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the NPD, which furthermore allows for refinement of a collective thermal displacement parameter for the water molecules and K atoms. The local structure around the metal atoms was extracted from EXAFS data in terms of Fe/Cu−C, Fe/Cu−N, Fe−Cu, and K−CN distances. A direct comparison between the bond distances from EXAFS analysis for x = 0.0 and x = 1.0 is consistent with that of crystallographic data. The local structure around Fe is Fe(CN)6, and the Fe−C−N−Cu chains are linear. The CN groups are the closest neighbors to K atoms. Overall, EXAFS and NPD results strongly suggest that the K atoms are located in the cavities, and that the x = 0.0 and x = 1.0 crystal structures are very similar.

ions are located in the cavity of the framework structure and supports the model from NPD, where K atoms were found to be at sites 8c and 32f. The fit with four different SS components is shown in Figure 6. More details about the refinement procedure are available in the Supporting Information.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01684. Results of the infrared measurements, K occupancy calculation from NPD, and refinement parameters from EXAFS study (PDF)

Figure 6. EXAFS data for x = 1.0: (a) k3-weighted EXAFS function at the K K-edge obtained in the 2−6 Å−1 k space, (b) corresponding FT of the EXAFS spectra.

■

In order to have a comparison with the crystallographic data, Table 4 shows a summary of the structural parameters obtained from the EXAFS analysis. The Fe−C (∼1.89−1.92 Å), Cu−N (∼1.96−1.97 Å), C−N (∼1.16−1.17 Å), and K−C/N (∼3.24 Å) distances obtained from the EXAFS analysis are comparable with those obtained by the NPD study as well as with what is reported for similar compounds.54,55 Both EXAFS and NPD analyses show that the average K−C/ N distance is ∼3.24 Å, supporting a model with K+ ions in the cavities of the framework.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +08-164505. Fax: +08-152187. E-mail: gunnar. [email protected]. ORCID

Dickson O. Ojwang: 0000-0001-9304-8975 Gunnar Svensson: 0000-0003-0598-4769 Author Contributions †

All authors have given approval to the final version of the manuscript. D.W. and D.O.O. contributed equally.

4. CONCLUSIONS A key motivation for studying the crystal structures of PBAs like K2x/3Cu[Fe(CN)6]2/3·nH2O, x = 0.0 and 1.0, is to understand their influence on the electrochemical performance. Since the nonframework alkali cations contribute to the charging/discharging processes, it is important to describe their local environment, i.e., how they interact with the water molecules and the framework atoms. To this end, we have complemented results from NPD measurements with those from EXAFS. The NPD study confirms the presence of two kinds of water molecules: (i) zeolitic water distributed over three positions in the cavities, 8c, 32f, and 48g, and (ii) coordinated water bonded to Cu atoms close to empty CN positions when Fe(CN)6 vacancies are present. The difference Fourier maps show that, unlike in the previous studies of PB and PBAs, the positions of the coordinated water at a site 192l are slightly shifted with respect to the CN group positions at site 24e. For x = 0.0, our model gives 16.8(8) water molecules per unit cell which is close to the ideal 16 (n = 4). The K+ ions for x = 1.0 are found to be distributed over sites 8c and 32f, together with zeolitic water, while the 48g site is found to be only occupied by water. The refinement suggests 2.6 K atoms (x = 0.98) and 13.9(7) waters per unit cell. It should be remarked that, in our previous study by synchrotron XRPD data, the three sites (8c, 32f, and 48g) were indeed identified, but it was concluded on the basis of the fit with experimental data that the K atoms were likely to reside on the 32f and 48g sites, and not as in this study on the 8c and 32f sites. However, it is clear that the water positions and changes in site occupancies can be determined with a higher precision from

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by the ‘‘Consortium for Crystal Chemistry, C3” within the Röntgen Ångström cluster, Swedish Research Council VR, diary number: 2011-6512. The authors acknowledge the Knut and Alice Wallenberg Foundation and MAX-Lab synchrotron radiation source, Lund, Sweden, for funding and beamtime allocation for the beamline I811 under the proposal 20140424. We wish to thank Dr. Stefan Carlson for his technical support at the beamline. Also, we thank Dr. Ron Smith for his help with data collection at the GEM beamline at ISIS, United Kingdom.

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Neutron Diffraction and EXAFS Studies of K - ACS Publications

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