5432
J. Phys. Chem. B 1998, 102, 5432-5437
Analysis of Photoinduced Magnetization in a (Co, Fe) Prussian Blue Model Kazunari Yoshizawa,*,† Fumihito Mohri,‡ Gerhard Nuspl,‡,§ and Tokio Yamabe‡ Department of Molecular Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan, and Institute for Fundamental Chemistry, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed: December 19, 1997; In Final Form: April 22, 1998
The electronic properties of a modified (Co, Fe) Prussian blue compound, which exhibits interesting photoinduced magnetic properties, are analyzed. For band structure calculations we set realistic models (for the ground and excited states) that involve interstitial potassium ions, crystal water, and lattice defects. The top of the “t2g” block consists mainly of Fe 3d orbitals, the low-lying “eg(1)” block Co 3d, and the high-lying “eg(2)” block Fe 3d. Because of the influence of crystal water coordinating to the Co ion, the “eg(1)”-block bands split into two parts; one is the “eg(1)(N)” block originating from the Co-NC fragment and another the “eg(1)(O)” block from the Co-OH2 fragment. The energy of visible light causing the magnetization enhancement is close to the computed “t2g(Fe-based)-eg(1)(N)” and “t2g(Fe-based)-eg(1)(O)” separations in the ground-state model. It is reasonable from DOS (density of states) analyses that irradiation with visible light causes an electron transfer from the Fe(II) to the Co(III) ions, through which the magnetization is effectively enhanced. Such an electron transfer is not a d-d transition on the same metal ion so that the Laporte rule may not be applied, or if applied, this rule may be relaxed in the (Co, Fe) Prussian blue.
Introduction Molecule-based magnetic materials have attracted much attention for more than a decade.1-4 Recent studies by several groups5-12 have shown that classic Prussian blue analogues are very attractive for applications to novel magnetic and optical materials. Hashimoto et al.11 reported that a Prussian blue analogue with an elemental composition of K0.4Co1.3[Fe(CN)6]‚ 5H2O exhibits interesting photoinduced magnetic phenomena that are controllable by irradiation of light with different wavelengths. Infrared and Mo¨ssbauer spectra indicate that an electron transfer from the Fe(II) to the Co(III) ions is a possible reason for the photoinduced phenomena. Upon visible light irradiation, the population of Fe(II)(“t2g6”, S ) 0)-CN-Co(III)(“t2g6”, S ) 0) is decreased and that of Fe(III)(“t2g5”, S ) 1/ )-CN-Co(II)(“t 5”, “e 2”, S ) 3/ ) is increased. The 2 2g g 2 increase in paramagnetic component would lead to an observed enhancement of magnetization below Tc of 26 K.11 Interestingly, the photoinduced ferrimagnetic state is reversed to the ground state by irradiation of near-infrared light. The important discovery by Hashimoto et al.11 is that the magnetic interaction throughout the bulk can be controlled by irradiation of light with different wavelengths. For a first insight of the mechanism of the interesting magnetic phenomena, we calculated the band structure of a simplified three-dimensional model with a formula unit of CoFe(CN)11 in which countercations are neglected and the resultant charge is -6.13 The model in our previous study can qualitatively explain the fundamental aspects of the interesting photoinduced magnetic properties. Yamaguchi et al.14 have systematically analyzed the magnetic interactions in various Prussian blue analogues from ab initio UHF (unrestricted Hartree-Fock) and DFT (density functional theory) calculations, †
Kyoto University. Institute for Fundamental Chemistry. § Present address: Institut fu ¨ r Anorganische Chemie der LMU Mu¨nchen, Meiserstr. 1, D-80333 Mu¨nchen, Germany. ‡
using binuclear transition metal cyanide models. A DFT band structural analysis for the (Mn, Cr) and (Ni, Cr) Prussian blue complexes was recently reported by Eyert et al.15 However, since the actual Prussian blue complexes include crystal water, interstitial potassium ions, vacancies, and lattice defects, we should take these features into account for an extended analysis in this paper. To have a better understanding of the interesting photoinduced magnetic phenomena, we performed band structure calculations for the ground state and the photoexcited state, using realistic models with a formula unit of KCo4[Fe(CN)6]3‚6H2O. Our model structures are based on the crystallographic data of the original (Fe, Fe) Prussian blue compound, since only unit cell parameters have been published for the modified (Co, Fe) Prussian blue compound.11 We will derive fundamental aspects on the photoinduced magnetic properties of the modified Prussian blue compound from extended Hu¨ckel16 band calculations. Although this qualitative molecular orbital method does not include electron-electron interactions, it models general orbital energy trends, orbital interactions, and major charge shifts reasonably well. Model Setting The oxidation number and composition of the Co ions in the Prussian blue analogue with an elemental composition of K0.4Co1.3[Fe(CN)6]‚5H2O are Co(III)1.0 and Co(II)0.3. This compound involves non-negligible lattice defects, as mentioned above. By analogy with the published data for the original (Fe, Fe) Prussian blue,17 we set a simplified model with an elemental composition of K1/3Co(III)4/3[Fe(II)(CN)6]‚2H2O. This model system is not electrically neutral and has a positive charge of +1/3. The formula unit of this model structure is thus KCo4[Fe(CN)6]3‚6H2O with +1 charge. The Co(III) coordination in this model is idealized from the corresponding Fe(III) coordination in the original Prussian blue. In our model structure there are two kinds of Co ions per formula unit. One
S1089-5647(98)00575-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/18/1998
Magnetization in Prussian Blue Co ion per unit cell is octahedrally coordinated by six nitrogen atoms of cyanide, represented as Co(N6), whereas three Co ions are octahedrally coordinated by four nitrogen atoms of cyanide and two oxygen atoms of crystal water, represented as Co(N4O2). Thus, the average Co coordination per unit cell is represented as Co(N4.5O1.5). In the original Prussian blue, the Fe(III) coordination is expressed as Fe(N6-2pO2p) for the 1a position in space group Pm3hm and Fe(N4+(2/3)pO2-(2/3)p) for the 3c position, where p is a fractional occupancy of the Fe(II) ion located at the 1b position. Buser et al.17 prepared three samples with different stoichiometries in which p is 0.267, 0.665, and 0.824. Hence, in sample I,17 for example, the Fe(III) coordinations at the 1a and the 3c positions are Fe(N5.47O0.53) and Fe(N4.18O1.82), respectively. But note that the average Fe(III) coordination per unit cell is Fe(N4.5O1.5), which is the same as that in our model structure, Co(N4.5O1.5). Thus, our model structure is likely to be quite realistic with respect to the coordination sphere of the trivalent metal ions in the Prussian blue structure. This model is much more realistic than our earlier model composed of CoFe(CN)11 cluster units.13 It will enable us to examine the role of crystal water and vacancies in the fundamental properties of the (Co, Fe) Prussian blue analogue. Next, we describe the bond distance estimation. For the original Prussian blue, both single-crystal X-ray diffraction17 and powder neutron diffraction18 data are available. Thus, the published fractional coordinates and the occupancies of the crystallographic sites for Fe4[Fe(CN)6]3‚xH2O (x ) 14-16) are the bases for our model with a formula unit of KCo4[Fe(CN)6]3‚ 6H2O, while the experimental unit cell parameter of a ) 9.96 Å is maintained. Although the neutron diffraction data18 of the original Prussian blue present a full data set including the hydrogen positions, slight modifications are necessary for setting our model. Because of the general inaccuracies of powder diffraction techniques and the limits of structure refinement caused by strong correlation effects among C, N, and O in the disordered Prussian blue, it is difficult to determine the O and H positions with high accuracy. Two measurements using different neutron wavelengths show distinct discrepancies in the fractional coordinates.18 Therefore, to estimate the bond distances in our model, we have chosen a data set that is in best agreement with the single-crystal data17 for the C, N, and O crystallographic sites while the Fe and Co positions are fixed. The orientation of the water molecules within the unit cell was preserved, but an unlikely H-O-H bond angle of 140° was replaced with that of 112° of the second data set. In this way, the bond distances in the ground-state model with a ) 9.96 Å were chosen as follows: Fe(II)-C ) 1.89, Co(III)-N ) 1.97, Co(III)-OH2 ) 2.06, C-N ) 1.13, and O-H ) 1.02 Å. The angle of H-O-H was 112°. Results Figure 1 shows the unit cell of KCo4[Fe(CN)6]3‚6H2O printed from the Cerius2 molecular modeling system.19 The six water molecules coordinate to the Co ions. Water molecules in interstitial sites were omitted for clarity because they are unlikely to significantly affect the essential electronic properties of the crystal framework. The Fe ions occupy three positions of the 3d site in the lattice with space group Pm3hm, which can describe the essential feature of the Prussian blue structure.17 The 1b site at (0.5 0.5 0.5) of the unit cell remains vacant, as seen in Figure 1. Therefore, the “empty” octahedral coordination sites of the six Co ions surrounding this vacancy may be filled with water molecules instead. From a crystallographic point of view,
J. Phys. Chem. B, Vol. 102, No. 28, 1998 5433
Figure 1. Model crystal structure of (Co, Fe) Prussian blue analogue with a ) 9.96 Å.
TABLE 1: Extended Hu1 ckel Parametaers for H, C, N, O, K, Fe, and Co Atoms20 orbital
Hii (eV)
ζ1
H 1s C 2s C 2p N 2s N 2p O 2s O 2p K 4s K 4p Fe 4s Fe 4p Fe 3d Co 4s Co 4p Co 3d
-13.6 -21.4 -11.4 -26.0 -13.4 -32.3 -14.8 -4.34 -2.73 -9.10 -5.32 -12.6 -9.21 -5.29 -13.18
1.3 1.625 1.625 1.950 1.950 2.275 2.275 0.874 0.874 1.9 1.9 5.35 2.0 2.0 5.55
c1
ζ2
c2
0.5505
2.00
0.6260
0.5680
2.10
0.6060
a potassium ion is assumed to occupy one position of the 8g interstitial site at (0.25 0.25 0.25). Band calculations were performed with the program package YAeHMOP,20 an extended Hu¨ckel implementation of the tight binding method. The parameters for H, C, N, O, K, Fe, and Co atoms listed in Table 1 were taken from the standard parameter collection of Alvarez.21 Space group Pm3hm of the original Prussian blue compound was adopted for band and DOS (density of states) calculations in the model compound. Following the geometrical method of Ramirez and Bo¨hm,22 a mesh consisting of 35 symmetry-weighted k points in the irreducible part of the Brillouin zone was generated for space group Pm3hm to compute DOS curves. For band calculations, four symmetry points, Γ(0, 0, 0), M(1/2, 1/2, 1/2), X(0, 1/2, 0), and R(0, 1/2, 0),22 were taken into consideration. Figure 2 shows the band structure and the density of states (DOS) for the ground-state model with a formula unit of KCo4[Fe(CN)6]3‚6H2O. The shaded area in DOS1 indicates the contribution of Fe 3d orbitals and that in DOS2 the contribution of Co 3d orbitals of the Co(N4O2) fragment. The symmetry labels “t2g” and “eg” are only valid for molecular orbitals in a regular octahedral environment, but for clarity we use these symbols in quotation marks also for those 3d orbitals that are
5434 J. Phys. Chem. B, Vol. 102, No. 28, 1998
Yoshizawa et al.
Figure 2. Band structure and density of states (DOSs) of (Co, Fe) Prussian blue analogue. The shaded areas in DOS1 and DOS2 indicate the contribution from Fe 3d and Co 3d orbitals, respectively, in which the Co belongs to the Co(N4O2) fragment.
SCHEME 1
Figure 3. DOS of (Co, Fe) Prussian blue analogue and projection of the contribution from Co 3d orbitals in which the Co belongs to the Co(N6) fragment.
not in a regular octahedral environment. There are four main d blocks: “t2g” at -13 eV, “eg(1)(O)” at -11 eV, eg(1)(N)” at -10 eV, and “eg(2)” at -4 eV. Potassium bands are highlying near 0 to -3 eV. By comparison of DOS1 and DOS2 in Figure 2, we see that the top of the “t2g” bands just below the Fermi level are mainly composed of Fe 3d orbitals, and the two “eg(1)” bands and the “eg(2)” band are mainly composed of Co 3d and Fe 3d orbitals, respectively. Although ligand and Co 3d orbitals contribute to the “t2g” bands, it is important for our discussion below that the contribution of Fe 3d orbitals is in the majority especially in the top of the “t2g” bands. From a closer inspection, the “eg(1)(O)” band appears to come from Co 3d and O 2p orbitals in the Co-OH2 fragment and the eg(1)(N) band from Co 3d and N 2p orbitals in the Co-NC fragment. In the diamagnetic ground state, the “t2g” block is occupied and the three “eg” blocks are empty. The unoccupied bands at -8 to -7 eV, L1 and L2, consist of ligand-based orbitals. The shaded area in Figure 3 indicates the projected DOS of Co 3d orbitals in the locally octahedral Co(N6) fragment. Discussion Photoinduced Magnetization. Let us first make a comparison between Figures 2 and 3. One can see that the contribution of Co(N6) to the total DOS is very small and the contribution of Co(N4O2) is large, since the content of Co(N6) is only 1/3 of that of Co(N4O2) in the unit cell. This fact suggests that in our
model compound the photoinduced magnetic properties originate mainly from the interaction between the Co(N4O2) and Fe(CN)6 moieties. Hence, we will discuss the Co(N4O2) moiety in the following part of this paper. In Figure 2, the band energies of “eg(1)(O)” and “eg(1)(N)” are about 1.3 and 3.0 eV, respectively, higher than the Fermi level (top of the “t2g” block). These energy gaps roughly correspond to the energy of visible light (1.6-3.0 eV) that induces the remarkable enhancement of magnetization below Tc. Thus, an electronic transition for the magnetization enhancement is likely to occur from the “t2g” block toward the two “eg(1)” blocks. Since the top of the “t2g” block just below the Fermi level consists mainly of Fe 3d orbitals and the “eg(1)” blocks consist of Co 3d orbitals, as shown in the DOS1 and DOS2 of Figure 2, the population of Fe(III)(“t2g5”, S ) 1/ )-CN-Co(II)(“t 5”, “e 2”, S ) 3/ ) is increased upon 2 2g g 2 irradiation by visible light. This corresponds to an electron transfer from the Fe toward the Co ions, as indicated in Scheme 1. In the DOS curves of Figure 2, there are two eg(1) blocks, i.e., eg(1)(N) and eg(1)(O), so that we can expect two kinds of electron transfers to occur from the “t2g” block. Following our extended Hu¨ckel calculations, two peaks at 950 and 410 nm are predicted. Although the observed absorption spectrum shows a single broad peak around 550 nm, this peak may consist of two or more components. We think that such a broad absorption band has relevance to crystal water, interstitial potassium ions, vacancies, and lattice defects that are included in the actual Prussian blue complex. Selection Rules for d-d Transitions. For a better insight into the electron transfer from the Fe to the Co ions, let us consider the selection rules for the observed d-d electronic transition. One of the most important selection rules, known as the Laporte rule,23 tells us that the only allowed transitions are accompanied with a change of parity. In the crystal structure of Prussian blue, the local point group symmetry of a single Co(N4O2) cluster is D4h. Since all d orbitals are gerade under the D4h point group, d-d transitions are formally forbidden.23 Of course, the same conclusion is valid for a local Co(N6) cluster with a point group of Oh. However, such a rule that forbids
Magnetization in Prussian Blue
J. Phys. Chem. B, Vol. 102, No. 28, 1998 5435
Figure 4. DOS of the ferrimagnetic state of a model of (Co, Fe) Prussian blue analogue. The shaded areas in DOS1 and DOS2 indicate the contribution from Fe 3d and Co 3d orbitals, respectively.
Figure 5. DOS of the original Prussian blue. The shaded areas in DOS1 and DOS2 indicate the contribution from Fe(II) 3d and Fe(III) 3d orbitals, respectively.
d-d transitions is applied to an intra-atomic d-d transition on a single atom. As discussed above, from a close inspection of the DOS curves for KCo4[Fe(CN)6]3‚6H2O, the transition from the “t2g” to the “eg(1)” blocks can be regarded as an interatomic electron transfer from the Fe to the Co ions. Thus, it is not a d-d transition on the same metal center. We therefore think that this selection rule may not be applied, or if applied, it may be significantly relaxed in the (Co, Fe) Prussian blue. A strong, broad absorption band observed around 550 nm is probably due to the “t2g” f “eg(1)” transitions. Moreover, another important selection rule states that any electronic transition for ∆S * 0 is formally forbidden. Although spin-orbit coupling can break this spin selection, the absorption coefficient for such a forbidden transition is quite weak in general. Since the observed absorption band around 550 nm is strong, it is likely to be a spin-allowed transition with ∆S ) 0. Thus, we think that the observed magnetic enhancement induced by irradiation of visible light should be a consequence of multistep electronic processes after an initially occurring “t2g” f “eg(1)” transition with ∆S ) 0. Ferrimagnetic State. Let us next consider the photoexcited magnetic state and the mechanism for its rededuction of magnetization. We use a ferrimagnetic-state model of the (Co, Fe) Prussian blue. In the ferrimagnetic state, we can expect
that the Fe ions exist as a low-spin (S ) 1/2) species, Fe(III)(LS), and the Co ions as a high-spin (S ) 3/2) species, Co(II)(HS). These species are antiferromagnetically coupled to each other to lead to a ferrimagnetic state. Thus, the Fe-C and Co-N bond distances in the ferrimagnetic state should be significantly changed from those of the ground state discussed above. Infrared spectroscopic measurements strongly suggest a significant change in the coordination sphere of the Fe-CNCo moiety.11a Unfortunately, structural parameters for the ferrimagnetic state of Prussian blue complexes have not yet been reported so that we estimated the Fe-C and Co-N bond distances in the ferrimagnetic state according to Shannon’s effective ionic radii.24 The estimated bond distances are as follows: Fe(III)(LS)-C ) 1.83, Co(II)(HS)-N ) 2.17, Co(II)(HS)-OH2 ) 2.26, and C-N ) 1.13 Å. Both the Co-N and Co-O bond lengths are elongated by 0.2 Å from those of the ground-state model. The resulting lattice constant becomes 10.26 Å. The DOS curves for the ferrimagnetic-state model of the (Co, Fe) Prussian blue are shown in Figure 4. The shaded area in DOS1 indicates the contribution of Fe 3d orbitals and that in DOS2 the contribution of Co 3d orbitals of the Co(N4O2) fragment, as in Figure 2. The “t2g-eg(1)(N)” and “t2g-eg(1)(O)” separations are small compared with those of the ground state shown in Figure 2. This result is quite reasonable because
5436 J. Phys. Chem. B, Vol. 102, No. 28, 1998 in the ferrimagnetic-state model the Co-N and Co-O bond distances are 0.2 Å longer than those of the ground-state model. The ligand-field strength of the nitrogen and oxygen affecting the central Co ion is thus weakened to significantly decrease the “t2g-eg(1)(N)” and “t2g-eg(1)(O)” separations. As a consequence, one expects that the high-spin (S ) 3/2) state of the Co(II) ion should be greatly stabilized because of the lowlying “eg(1)” blocks and exchange interactions25 among the three electrons with the same spin at one Co(II) ion. In particular, the low-lying eg(1)(O), which originates from the coordination of crystal water, contributes to the stability of the ferrimagnetic state. From a close inspection of the DOSs in Figure 4, the energy of near-infrared light (λ ) 1319 nm), which causes magnetization reduction, roughly corresponds to the “t2g(Co-based)-eg(1)(O)” separation. Since this electronic transition is a d-d transition on the same Co ion, it is in principle forbidden under D4h symmetry. Electron-vibration (vibronic) interactions may relax this rule, but such an unrealistic electronic transition is not the origin of the magnetic reduction. The ferrimagnetic state is supposed to be a metastable state that lies slightly above the diamagnetic ground state. Such a metastable spin state can be reversibly converted to the ground state by various kinds of perturbations such as heat and light.26,27 In fact, the photoinduced magnetization is reduced by irradiation by near-infrared light (λ ) 1319 nm)11b,c as well as by blue light (λ ) 450 nm).11a In particular, irradiation of near-infrared light should induce molecular vibrations and will lead to lattice distortions. Thus, near-infrared irradiation is probably an effective perturbation causing the photoreduction of the magnetization of the (Co, Fe) compound. Comparison with Original (Fe, Fe) Prussian Blue. Let us finally look at the electronic structure of the original (Fe, Fe) Prussian blue. In the original Prussian blue with a formula unit of Fe(III)4[Fe(II)(CN)6]3‚6H2O, photoinduced magnetic properties have not been known so far. To consider the reason, we analyzed the DOS of the original Prussian blue using the structural data from single-crystal X-ray diffraction.17 The left and right parts of Figure 5 show the projected DOSs of Fe (II) and Fe(III), respectively, as well as the total DOS. The general profile of the total DOS curve is very similar to that of the (Co, Fe) Prussian blue shown in Figure 2. However, the orbital composition of the “t2g” bands just below the Fermi level is different from each other. In the (Co, Fe) Prussian blue, the dominant contribution in the top of the “t2g” block is 3d orbitals of the Fe(II) ion locating at the 3d position of space group Pm3hm. On the other hand, in the original Prussian blue the top of the “t2g” block is composed mainly of 3d orbitals of the Fe(III) ion locating at the 1a and 3c positions; the contribution of Fe(II) 3d orbitals is relatively small in the vicinity of the Fermi level. Therefore, an electron transfer from the Fe(II) to the Fe(III) ions is unlikely to occur in the original Prussian blue because of the small difference in the band structures. Summary and Conclusions The photoinduced magnetic properties of a (Co, Fe) Prussian blue analogue have been investigated with the extended Hu¨ckel crystal orbital method by using realistic models with a formula unit of KCo4[Fe(CN)6]3‚6H2O. Although this method does not explicitly include electron-electron interactions, it models general orbital energy trends, orbital interactions, and major charge shifts well. Our models for the diamagnetic ground state and the ferrimagnetic state involve interstitial potassium ions, crystal water, and lattice defects. The two models are different
Yoshizawa et al. especially in the coordination spheres of the Co ion; the Co-N and Co-O bond lengths of the ferrimagnetic-state model are set to be 0.2 Å longer than those of the ground-state model according to Shannon’s effective ionic radii. The energy of visible light causing the magnetization enhancement in K0.4Co1.3[Fe(CN)6]‚5H2O is close to the calculated “t2g-eg(1)(N)” and “t2g-eg(1)(O)” band gaps in the ground-state model. It is confirmed from detailed DOS analyses that the top of the “t2g” block is mainly composed of Fe 3d orbitals and the “eg(1)” blocks are mainly of Co 3d. Thus, the electronic transition from the “t2g” to the “eg(1)” blocks can be regarded as an interatomic electron transfer from the Fe to the Co ions so that the Laporte selection rule should be relaxed in the (Co, Fe) compound. The photoexcited high-spin state of the Co ion is stabilized both by the low-lying “eg(1)s” resulting from the weak ligand field at the Co ion and by exchange interactions between the three electrons at the Co ion, as indicated in Scheme 1. The band electronic structure of the original (Fe, Fe) Prussian blue is slightly different from that of the modified (Co, Fe) compound in the Fe 3d orbital component especially at the Fermi level. The interesting photoinduced magnetic properties observed in the (Co, Fe) compound is ascribed to the “t2g” and “eg(1)” blocks that are composed of 3d orbitals of different metal ions. Acknowledgment. This study was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. Thanks are also due to “Research for the Future” Program from the Japan Society for the Promotion of Science (JSPS-RFTF96P00206) for its support of this work. G.N. expresses his gratitude to the JSPS for financial support, which made his stay at the Institute for Fundamental Chemistry possible. References and Notes (1) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (2) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 385. (3) Gateschi, D. AdV. Mater. 1994, 6, 635. (4) Proceedings of the Fifth International Conference on MoleculeBased Magnets; Itoh, K., Miller, J. S., Takui, T., Eds.; Molecular Crystals and Liquid Crystals; Gordon and Breach: Philadelphia, 1997; Vol. 305, pp 1-1106. (5) Gadet, V.; Mallah, T.; Castro, I.; Verdaguer, M.; Veillet, P. J. Am. Chem. Soc. 1992, 114, 9213. (6) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554. (7) Feraly, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (8) Verdaguer, M. Science 1996, 272, 698. (9) (a) Entley, W. R.; Giolami, G. S. Inorg. Chem. 1994, 33, 5165. (b) Entley, W. R.; Girolami, G. S. Science 1995, 268, 397. (10) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 271, 49. (11) (a) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (b) Hashimoto, K.; Sato, O. Kagaku to kogyo (Tokyo) 1996, 49, 1662 (in Japanese; title translation, Chemistry and Chemical Industry). (c) Sato, O.; Einaga, Y.; Iyoda. T.; Fujishima, A.; Hashimoto, K. J. Electrochem. Soc. 1997, 144, L11. (d) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Phys. Chem. B 1997, 101, 3903. (12) Gu, Z.-Z.; Sato, O.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1996, 47, 18189. (13) (a) Yoshizawa, K.; Miyajima, H.; Mohri, F.; Yamabe, T. Abstracts of Papers (in Japanese), 72nd Spring Meeting of the Chemical Society of Japan, Tokyo; Chemical Society of Japan: Tokyo, 1997; Abstract 1PB018. (b) Yoshizawa, K.; Mohri, F.; Miyajima, H.; Yamabe, T. Inorg. Chem., submitted. (14) Nishino, M.; Kubo, S.; Yoshioka, Y.; Nakamura, A.; Yamaguchi, K. Mol. Cryst. Liq. Cryst. 1997, 305, 109. (15) Eyert, V.; Siberchicot, B.; Verdaguer, M. Phys. ReV. B 1997, 56, 8959. (16) (a) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. (b) Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 2179.
Magnetization in Prussian Blue (17) Buser, H. J.; Schwarzenbach, D.; Petter, W.; Ludi, A. Inorg. Chem. 1977, 16, 2704. (18) Herren, F.; Fischer, P.; Ludi, A.; Ha¨lg, W. Inorg. Chem. 1980, 19, 956. (19) Cerius2, Version 3.0; Molecular Simulation Inc.: San Diego, California, 1997. (20) Landrum, G. A. YAeHMOP (Yet Another Extended Hu¨ ckel Molecular Orbital Package), Version 2.0; Cornell University: Ithaca, New York, 1997. (21) Alvarez, S. Tables of Parameters for Extended Hu¨ ckel Calculations; Universitat de Barcelona: Barcelona, Spain, 1993.
J. Phys. Chem. B, Vol. 102, No. 28, 1998 5437 (22) Ramirez, R.; Bo¨hm, M. C. Int. J. Quantum Chem. 1988, 34, 571. (23) See, for example, the following. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th ed; Haper Collins College: New York, 1993. (24) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751. (25) Spin-crossover phenomena on a one-dimensional iron(II) chain are discussed in terms of orbital interactions. Yoshizawa, K.; Miyajima, H.; Yamabe, T. J. Phys. Chem. B 1997, 101, 4383. (26) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85, 219. (27) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024.
