CoFe Prussian Blue Analogues under Variable Pressure. Evidence of

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J. Phys. Chem. C 2008, 112, 17709–17715

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CoFe Prussian Blue Analogues under Variable Pressure. Evidence of Departure from Cubic Symmetry: X-ray Diffraction and Absorption Study Anne Bleuzen,*,§ Jean-Daniel Cafun,§ Anne Bachschmidt,† Michel Verdaguer,† Pascal Mu¨nsch,‡ Franc¸ois Baudelet,‡ and Jean-Paul Itie´‡ Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay, UniVersite´ Paris-Sud-Paris 11, UMR 8182 CNRS, Orsay, F-91405 France, Laboratoire de Chimie Inorganique et Mate´riaux Mole´culaires, Unite´ CNRS 7071, UniVersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France and Synchrotron SOLEIL, UR1, L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-YVette Cedex, France ReceiVed: July 3, 2008; ReVised Manuscript ReceiVed: August 29, 2008

Energy dispersive X-ray diffraction and X-ray absorption spectra of two CoFe Prussian blue analogues were collected under variable pressure at room temperature: one of them is essentially composed of CoII and FeIII ions and undergoes a CoII-FeIII f CoIII-FeII pressure-induced electron transfer, whereas the other is essentially composed of CoIII and FeII ions whatever the pressure. In both compounds, the CoIII-FeII phase undergoes a structural distortion upon compression. The alkali cation free CoFe Prussian blue analogue, in which the pressure-induced electron transfer occurs, exhibits a more complicated interplay between structural and electronic events. Key interatomic interactions are proposed to account for the pressure behavior of both CoIII-FeII and CoII-FeIII phases. 1. Introduction Recent years have seen the discovery of unusual and intriguing properties in Prussian blue analogues. All of these properties (magnetic,1 electrochemical,2 photomagnetic,3 negative thermal expansion4) arise from their specific structure and the variety of transition metal ions combinations that may be achieved through coordination chemistry within their threedimensional network. In the well-known face-centered cubic structure of CoFe Prussian blue analogue of chemical formula CxCo4[Fe(CN)6](8+x)/30(4-x)/3,nH2O, 0 represents the intrinsic [Fe(CN)6] vacancies, which are randomly distributed in the solid. C+ is an alkali metal ion. The Wyckoff positions 4a (0, 0, 0) are occupied by Fe ions or the 0 vacancy and the 4b positions (1/ 2, 1/2, 1/2) are occupied by Co ions. A scheme of the unit cell of the alkali cation free CoFe Prussian blue analogue is shown in Figure 1. This unit cell can be divided into eight small cubes called octants. When cesium ions are inserted in the structure, they are randomly located in the octants.5 The switching properties of CoFe Prussian blue analogues are due to the CoII(HS)-FeIII T CoIII(LS)-FeII electron transfer accompanied by the spin state change of the Co ion.3a,6-8 This electronic event occurs with a significant shortening of the Co to ligand bond from 2.08 (CoII(HS)) to 1.91 Å (CoIII(LS)) and the related significant shortening of the cell parameter from 10.30 to 9.96 Å when the electronic switch spreads in a cooperative way in the solid.6,7 If the oxidation and spin states of the stable and metastable states involved in the process are now well characterized,6-8 several questions remain on the atomic arrangement of the Co-NC-Fe linkages, water molecules, and alkali cations in the structure and their role in the * To whom correspondence should be addressed. E-mail: annebleuzen@ icmo.u-psud.fr. § Universite ´ Paris-Sud-Paris 11. † Universite ´ Pierre et Marie Curie. ‡ Synchrotron SOLEIL.

Figure 1. Scheme of the unit cell of the alkali cation free CoFe Prussian blue analogue of chemical formula Co4[Fe(CN)6]8/3 · 18H2O.

switching properties. The answer to these questions is crucial to elaborate in a rational way new systems with improved properties. Pressure studies in molecular magnetism have proven to be a powerful technique,9 and variable-pressure studies have provided valuable data on interatomic forces and chemical bonding in the solid. We decided therefore to record energy dispersive X-ray diffraction (EDXD) spectra of two different CoFe Prussian blue analogues as a function of pressure. The analogue of chemical formula K0.1CoII4[FeIII(CN)6]2.701.3 · 18H2O (abridged as C0) is essentially composed of CoII(HS) and FeIII ions at room pressure and contains a negligible amount of alkali metal ions. No thermally activated electron transfer occurs in this compound,10 but a piezo-induced CoIIFeIII f CoIIIFeII electron transfer has been evidenced.11 On the contrary, the photomagnetic analogue of chemical formula Cs2CoIII3.3CoII0.7[FeII(CN)6]3.300.7 · 13H2O (abridged as Cs2) is mainly composed of CoIII(LS) and FeII ions at room pressure and contains an important amount of large size cesium ions.12 The study of the pressure behavior of both compounds shows that variable pressure provide essential pieces of information to better

10.1021/jp805852n CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008

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Figure 3. Pressure dependence of the a-cubic cell parameters calculated from the 200 and 220 diffraction lines in the C0 and Cs2 EDXD spectra. Figure 2. Representative room-temperature EDXD spectra of (a) Cs0 and (b) Cs2 at various pressure. Peaks marked by an asterisk (*) correspond to Cs fluorescence.

establish the link between the unusual electronic properties of Prussian blue analogues and their structure. 2. Experimental Section The syntheses of C0 and Cs2 are reported elsewhere.10,12 All measurements were performed on the synchrotron facility DCI at LURE in Orsay, France. A few micrograms of finely powdered sample were loaded into a membrane-type diamond anvil cell with silicone oil as pressure transmitting medium. A few ruby spheres were added to the sample to measure the pressure by the ruby fluorescence method with an error less than (0.1 GPa.13 EDXD spectra were recorded over the 0.5-70 keV energy range on the DW-11A EDXD beamline. The orientation of the detector was fixed at θ ) 3.829° for C0 and θ ) 3.929° for Cs2. The θ values were measured using the EDXD pattern of a Cu foil recorded at ambient pressure and temperature. The energy position, line width, and intensity of the diffraction lines were obtained by fitting the peaks to Gaussian line shape. Two reflections, the 200 and 220, were intense enough to work with up to 6 GPa for C0 and up to 4 GPa for Cs2. X-ray absorption near-edge structures (XANES) were recorded on the energy dispersive D11 beamline of DCI in the transmission mode at the Co and Fe K edges to study the pressure-induced electron transfer in C0. Spectra of metallic (cobalt and iron) foil recorded at ambient pressure and temperature were used for energy calibration. The quality of the spectra at the Co K edge allows EXAFS analysis up to 300 eV after the edge. 3. Results and Discussion 3.1. EDXD Spectra and Lattice Parameter. The roomtemperature EDXD spectra of C0 and Cs2 at various pressures are displayed in Figure 2 over the 15-45 keV energy range. At room pressure the spectra of both compounds exhibit the diffraction lines of the well-known face-centered cubic structure of Prussian blue analogues. The peaks were then indexed within the Fm3m space group. An increase of pressure produces a slight broadening of the diffraction lines and a progressive shift of the peaks toward higher energies indicative of a cell parameter contraction. It is noticeable that (i) the peak shift is stronger for C0 than for Cs2 and (ii) the 220 reflection of Cs2 exhibits a singular broadening upon compression.

The a-cubic lattice parameter was calculated from the energy position of the 200 and 220 reflections as a function of pressure for both compounds (Figure 3). The values calculated from the 200 and 220 reflections of Cs2 (a200(Cs2) and a220(Cs2) progressively decrease and diverge, which indicates that the compression of the sample is accompanied by a distortion of the cubic structure. From room pressure to about 1 GPa the a-cubic lattice parameter of C0 exhibits a strong decrease associated with compression of the sample and the piezo-induced CoII(HS)FeIII f CoIII(LS)-FeII electron transfer (see below). The continuous shift of the diffraction peaks indicates a secondorder transformation. Above 2 GPa, a200(C0) and a220(C0) progressively decrease and diverge and the pressure behavior of C0 resembles that of Cs2. As in Cs2, C0 above 2 GPa is mainly composed of CoIII and FeII ions (see below). Their pressure behavior (Cs2 and C0 above 2 GPa) is assignable to the pressure behavior of CoFe Prussian blue analogues mainly composed of CoIII and FeII ions for which the compression is accompanied by a distortion of the cubic structure. 3.2. Distortion of the CoIII-FeII Phase from the Cubic Structure. The pressure dependence of the a-cubic lattice parameter calculated from the 200 and 220 reflections of both compounds in the CoIII-FeII state (Figure 3) points to a simple flattening or stretching of the cubic cell along the 3-fold axis and a cubic to rhombohedral change upon compression. In such a structural change the interplanar spacings in the rhombohedral F pseudo-cubic space group14 are given by

dhkl )

1 ⁄ aR*

√h2 + k2 + l2 + cos RR*(hk + kl + hl)

(1)

where aR* and RR* are the lattice parameters in reciprocal space. The values of RR* and aR* can be calculated from the experimental determination of d200 and d220 and the following relationships

((( ) ) ⁄ )

RR* ) arcos

d200 2 -2 2 d220

(2)

and

(aR*)-1 ) 2d200 ) d220√8 + 8cos RR*

(3)

where arcos is the arccosine function. The linear pressure dependence of R*(Cs2) and R*(C0) at P > 2 GPa15 (Figure 4a) supports the rhombohedral structure.

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Figure 4. Pressure dependence of (a) RR* for Cs2 (open square) and C0 (filled circle) and (b) 1/aR* for Cs2 (P < 3 GPa) and C0 (P > 3 GPa).

Furthermore, the pressure dependence of aR*(C0) (P > 3 GPa) and aR*(Cs2) (P < 3 GPa) can be well fitted (Figure 4b) with a same Murnaghan-type equation of state at the first order

(aR*)-1 ) (aR*0)-1(1 + B0′P ⁄ B0)-1⁄3B0′

(4)

where B0, B0′, and are, respectively, the bulk modulus, its first derivative, and the inverse of the lattice constant at P ) 0 GPa (B0 ) 43 ( 2 GPa, B0′ ) 3.6 ( 0.6 (aR*0)-1 ) 9.961 ( 0.03 Å). The value of (aR*0)-1 is very close to the cell parameter reported for CoFe Prussian blue analogues mainly composed of CoIII(LS) and FeII ions for which a face-centered cubic structure was considered at room pressure (9.96 ( 0.05).16 Many substances have a B0′ value of about 3.5, close to the one computed here. B0 ) 43 ( 2 is, to our knowledge, the first bulk modulus reported for Prussian blue analogues. This B0 value can be described as intermediate: higher than bulk moduli generally reported for soft materials such as van der Waals compounds or many organic crystals and lower than the bulk moduli reported for hard materials such as diamond, some oxides carbides, and nitrides. This value can be compared to the bulk modulus values reported for iono-covalent semiconductor such as ZnTe. In a precise cubic structure, the four 3-fold axes should be equivalent. However, in Prussian blue analogues the random distribution of vacancies and alkali cations can make the four directions slightly different. Given the pressure dependence of aR* and RR* (Figure 4a and 4b) different compressibility axes have to be considered in the solids: (i) aR* is related to the compressibility along the [100], [010], and [001] directions and then to the compressibility of the CoIII-NC-FeII units, and (ii) RR* is related to the compressibility along the [111] direction associated to the compressibility of the Fe and Co coordination polyhedra. The same pressure dependence of aR* for both compounds (Figure 4b) means that, over the considered pressure ranges, the compressibility of the Co-NC-Fe linkages is the same in Cs2 and C0, although the chemical composition of the samples is significantly different (CoIII2.7CoII1.3[FeII(CN)6]2.701.3 · 18H2O and Cs2CoIII3.3CoII0.7[FeII(CN)6]3.300.7 · 13H2O). The compressibility of the CoIII-NC-FeII linkages depends on neither weak interactions between the CoIII-NC-FeII linkages and other species such as alkali cations or water molecules nor the amount of remaining CoII(HS) species within the CoFe bimetallic network (i.e., 32.5% in C0 and 17.5% in Cs2 of the total Co ions). In both compounds (i) the majority CoIII-NC-FeII linkages form the backbone of the structure and (ii) the chemical bonds are very close within all the CoIII-NC-FeII linkages regardless of the chemical composition of the solids. Such a (aR*0)-1

Figure 5. (a) Scheme of the alternate stacking of the Fen and Con planes along the [111] direction (alkali cations and [Fe(CN)6] vacancies are omitted for clarity), and (b) scheme of the closest environment of the cesium cation (gray sphere) in one octant of the rhombohedral structure.

behavior is in agreement with strong CoIII to ligand and FeII to ligand covalent bonds expected for the 3d6 low-spin electronic configuration of both transition metal ions in linear CoIIINC-FeII units already proposed for this state in previous work.17 The “weak points” of the structure in this state are the coordination polyhedra of the transition metal ions. In both compounds the RR* values (Figure 4a) are above 90°, which means that RR in the direct space is less than 90° and the cubic cell is slightly stretched along the [111] 3-fold axis. The [111] direction is a hard compression axis. Along this axis, the compounds are made of alternate layers of Fe ions and Co ions linked through the cyanide bridges as shown in Figure 5a. RR* is then also related to the interlayer distance. The chemical species present in the interlayer space are different in C0 and Cs2: water molecules in the former compound and water molecules and bulky alkali metal ions in the latter. This difference can account for the different pressure dependence of RR* for C0 and Cs2. Extrapolation of RR*(Cs2) at P ) 0 GPa (90.14 ( 0.03°) is clearly above the expected 90° for a zero-pressure cubic structure.18 This small but significant deviation from 90° indicates that the structure of Cs2 is already rhombohedral at room pressure, and yet Cs2 mainly contains CoIII(LS) and FeII(LS) ions which are both expected to be strongly stabilized in octahedral symmetry due to their (t2g)6(eg)0 electronic configuration and are then expected to favor a cubic structure. The rhombohedral structure of Cs2 could be explained by the existence of interactions between the cesium ions and the bimetallic cyanide network. The closest neighborhood of the cesium ion located at the center of an octant in the rhombohedral structure is composed of six cyanide bridges as schematized in Figure 5b, with cesium to cyanide distances in

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Figure 6. Variable-pressure XANES spectra of C0 at (a) the Co K edge and best linear combinations (bold lines) using the spectra of C0 and Cs2 at room pressure as reference spectra and (b) the Fe K edge.

Figure 7. Pressure dependence of the a-cubic cell parameter (circle) compared to the (a) pressure dependence of the percentage of CoII ion in C0 (square) and (b) pressure dependence of the intensity of the first peak of the FT of the EXAFS signal (square) (see text).

the expected distance range for cesium to ligand bonds, around 3 Å. The alkali metal ions would be 6-fold coordinated and their interactions with the cyanide bridges strong enough to produce the small distortion of the cubic structure at room pressure. The RR* reciprocal lattice parameter of Cs2 strongly increases upon compression. The stretching of the octants upon compression significantly shortens the alkali metal ion to cyanide shortest bonds in the (111) plane (Figure 5b). Such a volume’s reduction of the alkali metal ion site could induce a shift of the bulky alkali metal ion, making it come out of the {220} diffraction family planes, which could account for the singular broadening of the 220 diffraction line of Cs2 (see above). 3.3. Piezo-Induced Reversible Electron Transfer in C0. XANES spectra of C0 were recorded at the Co (Figure 6a) and Fe (Figure 6b) K edges as a function of pressure. The shift of the absorption maxima from 7725 to 7728 eV (Co K edge) and from 7131 to 7130 eV (Fe K edge) directly evidence the piezoinduced CoII(HS)-FeIII f CoIII(LS)-FeII electron transfer.7,8,10 Given the profile of the spectra a pressure-induced CoII(HS)-

NC-FeIII f CoIII(LS)-CN-FeII linkage isomerization can be ruled out in this case.19 In order to quantify the pressure-induced electron transfer, the Co K-edge spectra of C0 were reproduced by linear combinations of reference spectra.7,8,20 Quantitative analysis confirms a total electron transfer involving all the iron cations above 2 GPa: CoII(HS)4[FeIII(CN)6]2.7 (P ) 0 GPa) f CoIII(LS)2.7CoII(HS)1.3 [FeII(CN)6]2.7 (P > 2 GPa).11a Above 2 GPa, the Co K-edge spectrum of C0 is close to the one of Cs2 at room pressure (Figure 6a). Both compounds mainly contain CoIII(LS) species. Back to room pressure, the initial spectra are recovered. The piezo-induced electron transfer is reversible. 3.4. Comparison of the Pressure-Induced Structural and Electronic Events in C0. At room pressure C0 is essentially composed of CoII(HS) and FeIII ions. Its structure is, within the error, face-centered cubic. When the piezo-induced electron transfer is completed at P ) 2 GPa, C0 is mainly composed of CoIII(LS) and FeII ions, and the structure becomes rhombohedral as shown above. Between 0 and 2 GPa several effects superimpose. Among them (1) compression of the low-pressure

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Figure 8. Co K-edge spectrum of C0 over the (a) 0-1, (b) 1-1.8, and (c) 2.1-3.3 GPa pressure ranges and FT of the EXAFS signal over the (d) 0-1, (e) 1-1.8, and (f) 2.1-3.3 GPa pressure ranges.

CoII-FeIII phase and compression of the high-pressure CoIIIFeII one (2) the CoII(HS)-FeIII f CoIII(LS)-FeII electron transfer and Co ion spin state change accompanied by the strong shortening of the Co to ligand bond. The decrease of the cell parameter associated with the electron transfer seems to spread rather sluggishly over the 0-1.2 GPa pressure range, whereas the Co oxidation state change becomes really detectable above 1 GPa and spreads over the 1-2 GPa pressure range (Figure 7a). The curves do not superimpose. The percentage of CoII ions calculated from the experimental cell parameter value taking into account the compressibility of the CoII-FeIII low-pressure phase and the compressibility of the CoIII-FeII high-pressure one does not superimpose either (Supporting Information). As the pressure was carefully measured before and after each measurement by the ruby fluores-

cence method, this discrepancy indicates that the pressure dependence of the cell parameter is not only linked to the pure compression of the low- and high-pressure phases and the fractions of CoII and CoIII ions. The strong decrease of the cell parameter at low pressure is not associated with the CoII(HS) to CoIII(LS) transformation. It is therefore due to a supplementary structural event. In order to get more information on this structural event, the XAS spectra at the Co K edges were carefully studied. The Co K-edge XANES spectrum and the Fourier transform modulus of the EXAFS spectrum are shown in Figure 8a-d at various pressures. Over the 0-1 GPa pressure range the Co K-edge XANES spectra (Figure 8a) are very close with an absorption maximum located at 7725 eV. No contribution of CoIII(LS) ions is

17714 J. Phys. Chem. C, Vol. 112, No. 45, 2008 detectable around 7728 eV. At 1 GPa the CoIII ions contribution hardly appears around 7728 eV (inset in Figure 8a). Quantitative analysis of the XANES spectrum taken at 1 GPa gives a percentage of 85% of CoII(HS) ion. Below 1 GPa, C0 is essentially composed of CoII(HS) ions. Over the 0-1 GPa pressure range the intensity of the absorption band slightly decreases and the band slightly narrows upon increasing pressure. The Fourier transform modulus (FT) of the EXAFS signal (Figure 8d) exhibits three peaks attributed to the three first neighbors shells around the Co ion (N and O, C, Fe). Over the 0-1 GPa pressure range the intensity of the peaks significantly decreases. The first shell can be analyzed in terms of single scattering. The significant decrease of its intensity upon increasing pressure is well reproduced by an increase of the Debye-Waller factor. This change reflects an increase of structural disorder in the first neighbor shell. The pressure dependence of the Co K-edge XANES and the FT of the EXAFS signal over the 0-1 GPa confirm the occurrence of a structural event which is not associated with an important CoII(HS)FeIII f CoIII(LS)FeII charge transfer. Over the 1-1.8 GPa pressure range the Co K-edge XANES spectrum (Figure 8b) and FT of the EXAFS signal (Figure 8e) strongly vary. Growth of the higher energy contribution at the edge and the shift of the first peak of the FT of the EXAFS signal toward shorter distance are assignable to the CoII(HS)FeIII f CoIII(LS)FeII charge transfer. Over the 2.1-4 GPa pressure range the Co K-edge XANES spectrum and FT of the EXAFS signal do not vary any more. The electron transfer is completed. The pure compression of the sample and the rhombohedral distortion of the structure are not detectable by X-ray absorption spectroscopy at the Co K edge. The structural event that occurs over the 0-1 GPa pressure range is associated with a significant decrease of the intensity of the first peak of the FT of the EXAFS signal. The pressure dependence of the intensity of the first peak of the FT of the EXAFS signal and the pressure dependence of the cell parameter are compared in Figure 7b. Up to 1 GPa, the pressure dependences of both values superimpose, which indicates that both effects are correlated. The strong decrease of the cell parameter of C0 upon increasing pressure is not the consequence of the CoII(HS) f CoIII(LS) transformation. It is associated with the structural event, which induces the decrease of the intensity of the FT peaks. At room pressure C0 is composed of (i) rigid [FeIII(CN)6] entities highly stabilized in octahedral symmetry with short Fe to ligand bond length (1.92 Å)7,8 (the FeIII crystal field parameter is about 4.35 eV21 in [FeIII(CN)6] complexes) and quasi-linear Fe-CN units and (ii) CoII ions linked to water molecules and rigid [Fe(CN)6] entities with long CoII to ligand bond length (2.1 Å)7 and a very weak crystal field parameter 10Dq(CoII) (0.55 eV),17b reflecting very weak Co to ligand interactions. The “weak points” of the structure under pressure is the articulation between CoII-N and NCFeIII. We suggest therefore that the strong decrease of the cell parameter accompanied by an increase of disorder in the CoII ion first neighbor shell is due to the progressive tilt of the rigid [FeIII(CN)6] entities around the FeIII ion position in a cooperative way upon increasing pressure. This cooperative tilt leads to narrowing of the CoII-NCFeIII angles and shortening of the Co to Fe distance. Such a structural event is in agreement with the second-order phase transition experimentally observed. It is also in agreement with the absence of significant broadening and intensity decrease of the diffraction peaks, indicating that the transition metal ions keep the same

Bleuzen et al. crystallographic positions in the course of the structural transformation. On the other hand, the tilt of the [FeIII(CN)6] entities is different in different sites (tilt axis and/or tilt angle), inducing an increased disorder on the position of the N, O, and C atoms as shown by the increase of the Debye-Waller factor in the first neighbor shell of the Co ions. Bent CoII-NC-FeIII linkages were already proposed for the CoII(HS)FeIII state of C0 in a previous work.17b Above 1 GPa the important tilt of the [Fe(CN)6] entities under applied pressure necessarily induces new interactions within the solid which trigger the electronic event accompanied by linearization of the CoIII-NC-FeII linkage. Above 2 GPa the pressure dependence of the RR*rhombohedral reciprocal lattice parameter becomes linear. The only remaining effects are compression of the CoIII-NC-FeII linkages and distortion of the CoIII and FeII coordination polyhedra. 4. Conclusions Variable-pressure studies of Prussian blue analogues are still rather scarce.11,19,22 We show in this work that they provide essential pieces of information to establish the link between the very peculiar electronic properties of CoFe Prussian blue analogues and their structure. The variable-pressure long-range order study shows that the CoIII-FeII photomagnetic state of CoFe Prussian blue analogues is likely to deviate from the well-known face-centered cubic structure. This suggests that applying pressure enhances the interactions between bulky alkali metal ion and the CoIII-FeII cyanide bimetallic network, which tunes the pressure-induced structural distortion. By combining X-ray absorption and X-ray diffraction, we show the interplay between structural and electronic events in the alkali cation free CoFe Prussian blue. Work is in progress to study the effect of an external pressure on Prussian blue analogues containing variable amount and nature of alkali metal ions. Acknowledgment. We thank the CNRS, Universite´ Paris 11, Universite´ Paris-Sud-Paris 11 and Universite´ Pierre et Marie Curie-Paris 6, and DFG Molekular Magnetismus SPP for financial support. Supporting Information Available: Comparison of the pressure dependence of the CoII ions percentage either obtained by quantitative analysis of the X-ray absorption spectra or calculated from the experimental cell parameter value and using sensible compressibity values for the low-pressure phase. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701–702. (b) William, R. E.; Girolami, G. S. Science 1995, 268, 397–400. (c) Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaisserman, J.; Seuleiman, M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.; Gelly, G.; Lomenech, C.; Rosenman, I.; Veillet, P.; Cartier, C.; Villain, F Coord. Chem. ReV. 1999, 190-192, 1023–1047. (2) (a) Rosseinsky, D. R.; Lim, H.; Zhang, X.; Jiang, H.; Wei Chai, J. Chem. Commun. 2002, 2988–2989. (b) Gimenez-Romero, D.; Agrisuelas, J.; Garcia-Jareno, J.-J.; Gregori, J.; Gabrielli, C.; Perrot, H.; Vicente, F. J. Am. Chem. Soc. 2007, 129, 7121–7126. (3) (a) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704–705. (b) Tokoro, H.; Ohkoshi, S.; Hashimoto, K. Appl. Phys. Lett. 2003, 82, 1245–1247. (c) Tokoro, H.; Matsuda, T.; Hashimoto, K.; Ohkoshi, S. J. Appl. Phys. 2005, 97, 508–510. (4) (a) Margadonna, S.; Prassides, K.; Fitch, A. N. J. Am. Chem. Soc. 2004, 126, 15390–15391. (b) Goodwin, A. L.; Chapman, K. W.; Kepert, C. J. J. Am. Chem. Soc. 2005, 127, 17980–17981. (c) Chapman, K. W.; Chupas, P. J.; Kepert, C. J. J. Am. Chem. Soc. 2006, 128, 7009–7014.

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J. Phys. Chem. C, Vol. 112, No. 45, 2008 17715 (16) Escax, V.; Bleuzen, A.; Itie´, J.-P.; Mu¨nsch, P.; Varret, F.; Verdaguer, M. J. Phys. Chem. B 2003, 107, 4763–4767. (17) (a) Bleuzen, A.; Escax, V.; Ferrier, A.; Villain, F.; Verdaguer, M.; Mu¨nsch, P.; Itie´, J.-P. Angew. Chem., Int. Ed. 2004, 43, 3728–3731. (b) V. Escax, V; Champion, G.; Arrio, M.-A.; Zacchigna, M.; Cartier dit, Moulin.; Bleuzen, A. Angew. Chem., Int. Ed. 2005, 44, 4798–4801. (18) The powder X-ray diffraction pattern of Cs2 was collected at room pressure with a X’pert Philips diffractometer. A silicon plate was used for the angle calibration. The R* value (90.13 ( 0.03°) was calculated from equation (2) and the angle position of the 200 and 220 diffraction lines which were obtained by fitting the peaks to Gaussian line shape. (19) Coronado, E.; Gimenze-Lopez, M. C.; Levchenko, G.; Romero, F. M.; Garcia-Baonza, V.; Milner, A.; Paz-Pasternak, M. J. Am. Chem. Soc. 2005, 127, 4580–4581. (20) Escax, V.; Bleuzen, A.; Cartier dit Moulin, C.; Villain, F.; Goujon, A.; Varret, F.; Verdaguer, M. J. Am. Chem. Soc. 2001, 123, 12536–12543. (21) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984. (22) (a) Awaga, K.; Sekine, T.; Okawa, M.; Fujita, W.; Holmes, S. M.; Girolami, G. S. Chem. Phys. Lett. 1998, 293, 352–356. (b) Egan, L.; Kamenev, K.; Papanikolaou, D.; Takabayashi, Y.; Margadonna, S. J. Am. Chem. Soc. 2006, 128, 6034–6035. (c) Papanikolaou, D.; Kosaka, W.; Margadonna, S.; Kagi, H.; Ohkoshi, S.; Prassides, K. J. Phys. Chem. C 2007, 111, 8086–8091. (d) Sava, A.; Enachescu, C.; Stancu, A.; Boukheddaden, K.; Codjovi, E.; Maurin, I.; Varret, F. J. Optoelectron. AdV. Mater. 2003, 5, 977–983. (e) Kim, E. J.; Ohishi, Y.; Moritomo, Y.; Kato, K.; Takata, M.; Ohkoshi, S. Phys. ReV. B 2008, 77, 012101.

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