Symmetry switch of cobalt ferrocyanide framework by alkali cation exchange 56 Materials Science : Structure Nanoporous coordination polymers are attracting increasing interest from materials scientists, because they can be utilized in various devices, for example, Li ion batteries, electrochromic devices, hydrogen storage devices, molecular sensors and molecular filters. Among nanoporous materials, Prussian blue analogues, represented as A x M [Fe(CN) 6 ] y zH 2 O (where A and M are an alkali metal and a transition metal, respectively), form a nanoporous three- dimensional (3D) framework ( - M - NC - Fe - CN - M - ). In Fig. 1, we schematically show the framework structure, together with the guest alkali cation A + . A significant feature of the compounds is the controllability of the guest concentration ( x ) by an electrochemical method [1] or by adjusting external voltage on all solid devices [2,3]. Imanishi et al . reported lithium intercalation into the host framework as well as the charge-discharge behavior in a series of cyano-bridged transition metal compounds with M = V, Mn, Co, Ni and Cu. The average discharge potential for the V compound is about 3.6 V, corresponding to the reduction of the V 3+ ion, while those for the Mn, Co, Ni and Cu compounds are around 3.3 V, corresponding to the reduction of the Fe 3+ (CN) 6 unit. The charge capacity of the Cu compound reaches 140 mAhg -1 , which is comparable to that of the actually used material, LiCoO 2 (capacity, 140 mAhg -1 ). Thus, the compounds are quite promising as positive electrode materials. In order to extract better electrode performance from the compounds, an exhaustive structural investigation is needed to clarify the interaction between the guest cations and the host framework. Now, let us discuss the interrelation between the ionic size of A + and the interval (~ 0.5 nm) of the framework (see Fig. 1). The ionic radius r of A + increases with atomic number: r = 0.092 nm for Li + , r = 0.182 nm for Na + , r = 0.151 nm for K + , r = 0.161 nm for Rb + , and r = 0.174 nm for Cs + . Here, we emphasize that the transition metal ions are bridged by CN groups, and hence the ‘frame’ is rather thick. Recently, Moritomo et al . [4] have investigated the cation channel in the cobalt ferrocyanide framework. They confirmed that the Na + ions go through the square window of the framework, while the larger ions, i.e., the K + and Rb + ions, take a detour channel along the vacancies. The cation dependent behavior is well reproduced by the cation potential, which is calculated by ab initio total energy calculation with use of a WIEN2k program. The cation dependence of the channel suggests that the nanopores of the host framework are rather large for the Na + and Li + ions, but fit well for the K + , Rb + and Cs + ions. We investigated the crystallographic symmetry of the cobalt ferrocyanide framework while changing A + . The cobalt ferrocyanide with larger cations, i.e., A + = K + , Rb + , and Cs + belongs to the face-centered cubic lattice ( F m _ 3m; Z=4), while the cobalt ferrocyanide with smaller Na + ions belongs to the rhombohedral lattice ( R _ 3m; Z=2). We suspect that the displacement of the Na + ion is the origin for the rhombohedral distortion. In the nanopore, the Na + ion is surrounded by our [Fe(CN) 6 ] 4– ions and four Co 2+ ions, and hence, feels an asymmetric ( T d symmetry) electrostatic potential. Then, the local potential minimum is not at the central position of the cube, but at the tetrahedral sites toward the four Fe(CN) 6 ] 4− ions. Then, a cooperative displacement of the Na + ion can cause the rhombohedral distortion. W e f u r t h e r f o u n d t h a t a s i m p l e d i p p i n g Li + 0.92 Å Na + 1.18 Å K + 1.51 Å Cs + 1.74 Å Rb + 1.61 Å 5Å Guest Host Fig. 1. Schematic pictures of the guest alkali cations and host framework for Prussian blue compounds. 57 procedure can exchange the guest alkali cation between K + and Na + , and switch the symmetry of the framework between cubic and rhombohedral o n e s [ 5 ] . F i g u r e 2 s h o w s t h e X - r a y p o w d e r diffraction patterns for K 1.88 Co[Fe(CN) 6 ] 0.97 • 3.8H 2 O (1) , Na 1.52 K 0.04 Co[Fe(CN) 6 ] 0.89 • 3.9H 2 O (2) and Na 0.04 K 1.48 Co[Fe(CN) 6 ] 0.88 • 2.6H 2 O (3) , obtained at BL02B2 beamline. 2 was obtained by dipping 1 i n 1 m o l d m - 3 o f N a C l s o l u t i o n , a n d 3 w a s obtained by dipping 2 in 1 mol dm -3 KCl solution. The wavelength of the X-ray was 0.50243 Å, which was calibrated by the lattice constant of standard CeO 2 powder. We performed Rietveld analysis to determine the lattice symmetry and lattice constant. 1 shows a face-centered-cubic structure with a lattice constant a = 10.057(7) Å. In contrast, the diffraction pattern of 2 was different from that of 1 . We could index the pattern with a rhombohedral structure: a = 10.340(2) Å and α = 91.32°. Again, 3 shows a face-centered-cubic structure with a lattice constant a = 10.082(3) Å. The elemental analysis by ICP-AES spectrometry after respective dipping procedures revealed nearly complete cation exchange at least for 8 cycles (Fig. 3). We investigated the valence states of 1 and 2 using infrared (IR) spectra and the magnetic susceptibility. The CN stretching peaks of [Fe II (CN) 6 ] were observed at 2079.9 cm -1 in 1 and at 2086.6 cm -1 in 2 . The products of the magnetic susceptibility ( χ M ) and temperature ( T ) plots are observed at 300 K and are almost equal to the expected spin-only values of 3.0, i.e., 3.06 ( 1 ) and 3.02 cm 3 K mol -1 ( 2 ). These data indicate that the valence states are the same, i.e., Co II hs -Fe II ls . Thus, the symmetry switch observed in the present system has a purely structural origin. We further investigated the diffuse reflectance spectra. The cation exchange alters the position of the absorption band from 520 nm ( 1 ) to 610 nm ( 2 ). The rather sharp 610 nm band in 2 can be ascribed to the d-d transition of Co II hs . The rhombohedral distortion of the framework can dipole-activate such a d-d transition. In summary, we found a reversible symmetry switch of the cobalt ferrocyanide framework by alkali cation exchange. The symmetry change switches the optical properties of the compound. We suspect that the displacement of the Na + ion is the origin of rhombohedral distortion. The Li + substitution effect on structural properties is the key to the comprehensive understanding of the guest-host interaction of the Prussian blue analogues. Intensity (arb. units) 5 2 θ (deg.) 10 200 200 220 400 2-20 220 400 420 420 4-20 15 1 20 2 3 Alkali Cation Content Cycles 0.0 0.5 1.0 1.5 2.0 0 2 4 6 8 Na K Fig. 2. X-ray powder diffraction patterns for 1 (upper), 2 (middle), and 3 (bottom) measured at BL02B2 beamline. X-ray wavelength was 0.50243 Å. Fig. 3. Reversible alkali cation exchanges. The Na and K contents determined by ICP-AES are plotted after respective cation exchanges. Yutaka Moritomo* and Tomoyuki Matsuda Graduated School of Pure and Applied Science, University of Tsukuba *E-mail: moritomo@sakura.cc.tsukuba.ac.jp References [1] N. Imanishi et al. : J. Power Source 81-82 (1999) 530. [2] Y. Moritomo and T. Shibata: Appl. Phys. Lett. 94 (2009) 043502. [3] T. Shibata and Y. Moritomo: Appl. Phys. Ex. 2 (2009) 105502. [4] Y. Moritomo et al. : Appl. Phys. Ex. 2 (2009) 085001. [5] T. Matsuda, J.E. Kim and Y. Moritomo: J. Am. Chem. Soc. 132 (2010) 12206.
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