J. Phys. Chem. C 2008, 112, 19147–19150
19147
Magnetic Properties and Pressure-Induced Ferromagnetism of Cu2(OH)3(CH3COO) · H2O Kentaro Suzuki,‡,| Julien Haines,§ Pierre Rabu,*,† Katsuya Inoue,‡,⊥ and Marc Drillon† Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504 CNRS-ULP, 23 rue du Loess, BP 43, 67034 Strasbourg cedex 2, France, Institute for Molecular Science, Myoudaiji, Okazaki, 444-8585, Japan, and Institut Charles Gerhardt Montpellier UMR 5253 CNRS-UM2-ENSCM-UM1, Equipe PMOF, UniVersite´ Montpellier II Sciences et Techniques du Languedoc, Place E. Bataillon cc 003, 34095 Montpellier cedex 5, France ReceiVed: August 27, 2008; ReVised Manuscript ReceiVed: October 3, 2008
The magnetic properties of the layered compound Cu2(OH)3(CH3COO) · H2O (1) have been investigated in details. The behavior indicates that the CuIIO2 layers are weakly ferromagnetic and order antiferromagnetically below TN ) 12 K. At low temperature the compound exhibits a metamagnetic transition with the threshold field H ) 0.8 T. Moreover, the pressure dependence of the magnetization was studied and a transition of the ground-state from antiferromagnetic to ferromagnetic was induced by applying a pressure of 1.2 GPa. This transition is correlated to structural variation of the exchange pathways. Introduction Hybrid organic-inorganic magnetic compounds have been of continuous interest since the late 90s because they often provide nice model systems for understanding the correlation between the structural and magnetic features. Their lowdimensional character is associated with specific behaviors that differ from those observed in bulky systems. In recent years, we have focused on the family of layered transition-metal hydroxides, M2(OH)3X (M(II) ) Co, Cu, Ni, Mn and X ) NO3-, CH3CO2-, Cl-), with botallackite- or brucite-type structure, which are appropriate for the design of new twodimensional organic/inorganic materials. Indeed, the presence of short metal-metal distances (ca. 0.3 nm), brought about by the µ3 coordination mode of the OH- moieties within the hydroxide layers, results in effective magnetic interactions by exchange coupling along the metal-oxygen-metal pathways, leading to a ferromagnetic, antiferromagnetic, or ferrimagnetic 2D behavior. Moreover, the structure of these layered transition metal hydroxides is flexible and can be tuned by functionalization of the layers. The X- anion located in the interlayer space may be substituted by a large variety of organic molecules (mono- or dicarboxylates, sulfates, sulfonates...) via anionic exchange reactions. The interleaved species can be used as pillar or as connector between the magnetic layers for monitoring the interlayer magnetic coupling as a function of the size and electronic characteristic of the exchange anion. 1-13 Besides playing on the interlayer interaction by means of the exchanged anions, we have shown also that external pressure can have some effect. Indeed, the magnetic study of the ferrimagnets Cu2(OH)3(CH3(CH2)nCOO) · zH2O n ) 9, 11, indicated a decrease of TC under pressure, mainly driven by * To whom correspondence should be addressed. E-mail: pierre.rabu@ ipcms.u-strasbg.fr. † Institut de Physique et Chimie des Mate ´ riaux de Strasbourg. ‡ Institute for Molecular Science. § Universite ´ Montpellier II Sciences et Techniques du Languedoc. | Present address: Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo, 153-8902, Japan. ⊥ Present address: Faculty of Science, Hiroshima University, Kagamiyama, Higashihiroshima, 739-8526, Hiroshima, Japan.
the decrease of in-plane interactions, which can be caused by small variations of the Cu-O-Cu bond angles within the layers.14 Layered Cu(II) derivatives synthesized by exchange reaction are usually obtained from the copper hydroxyacetate, Cu2(OH)3(CH3COO) · H2O (1), substituting acetate moieties by various anions like n-alkylcarboxylates, sulfonates, or sulfates. Whereas, the magnetic properties of the exchanged compounds were thoroughly investigated, no detailed magnetic study of the starting compound 1 was published so far.15 We thus report here the magnetic behavior of 1 in standard conditions and under hydrostatic pressure, showing a pressure-induced transition from antiferromagnetic to ferromagnetic 3D ordering. High-pressure powder X-ray diffraction experiments were carried out to understand precisely the origin of the magnetic transition, through a close analysis of the correlation between the structure and magnetic behavior. Experimental Methods Synthesis. The title compound 1 was synthesized as bluegreen crystalline powder reacting a solution of Cu(CH3COO)2 · 6H2O in deionized water (0.1 mol · L-1, 20 mL) and an aqueous solution of NaOH (0.1 mol · L-1, 30 mL) under argon at 60 °C. The chemical formula was checked by elemental analysis for C and H contents; the metal was evaluated by thermal weigh loss at 600 °C at which temperature 1 transformed in CuO. Analysis: Cu2(OH)3(CH3COO) · H2O Exp (calcd): C 9.38 (9.41), H 2.81 (3.14), Cu 50.55 (49.8). Powder X-ray Diffraction. High-pressure, angle-dispersive X-ray diffraction measurements were performed using a leverarm-type diamond anvil cell. The powder sample was loaded, in a 200 µm hole drilled in a tungsten gasket preindented to a thickness of 80 µm. Small ruby single crystals were loaded with the sample for pressure determination by the shift of its R1 fluorescence line.16 Glycerol was used as a pressure transmitting medium. The diffraction patterns were obtained on an imaging plate placed at 143.69 mm from the sample. Zr filtered radiation from a Mo microfocus X-ray tube, collimated to about 100 µm by X-ray capillary optics, was used for the measurements. Exposure times were typically 48-60 h. The 2θ dependence
10.1021/jp807636u CCC: $40.75 2008 American Chemical Society Published on Web 11/07/2008
19148 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Suzuki et al.
Figure 2. Suceptibility vs T variation as χ ) f(T) and χT ) f(T) for 1.
Figure 1. Structure of 1, showing the stacking along c of the copper(II) hydroxide layers interleaved with acetate (OAc) and water molecules and the copper(II) chains within the pseudotriangular planes. (Crystallographic data were taken from Masciocchi et al.)15 The H atoms are omitted. The circled numbers correspond to the seven different exchange interactions Ji, i ) 1-7.
of the X-ray diffracted intensity was obtained by integration of the diffraction pattern. All figures in parentheses correspond to estimated standard deviations. Full profile fitting of the diffraction data was performed using the program FULLPROF.17 The starting material was studied at ambient pressure in a 0.3 mm X-ray capillary tube using the same diffractometer. SQUID Measurements. The magnetic properties of 1 were investigated on powder samples with a Quantum Design MPMSXLSQUID magnetometer between 1.8 and 300 K. The magnetic field strength was varied from -5 to +5 T. All magnetic data were corrected for diamagnetism of the sample and of the sample holder. For magnetic measurements under pressure, ca. 10-15 mg of a powder sample of 1 was mounted with a fluorine oil as pressure transmitting medium in a small high-pressure clamp cell made of Cu-Ti alloy (Yamaha, YCuTM),18,19 and the magnetic measurements were performed with the same SQUID equipment. An ac field of 20 Hz and 3 Oe was used. The pressure was calibrated from the superconducting transition of Pb in a separate measurement, with good reproducibility. Results and Discussion The ambient pressure X-ray diffraction pattern of 1 (Figure S1) fully agrees with that expected from the crystallographic study.15 It crystallizes in the monoclinic space group P21/m, with a ) 5.6025(5) Å, b ) 6.1120(6) Å, c ) 18.747(3) Å and β ) 91.012(9)°, Z ) 4. One is a structural analogue of the layered mineral Botallackite, with hydrogen bonded water molecules intercalated between the Cu2(OH)3(CH3COO) sheets. The structure consists in the stacking along the c axis of Cu2(OH)3O layers, each containing a planar triangular array of copper(II) atoms bridged by µ3-OH or µ3-O of the acetate anions (see Figure 1). The interlayer distance is 1/2 × c × sin β ) 9.34 Å.
Figure 3. Μ ) f(H) variation for 1 recorded at 1.8 K.
There are three nonequivalent CuII ions, Cu1, Cu2, and Cu3 aligned in rows in the a,b planes and lying in distorted octahedral environment of oxygen atoms of the hydroxyl or acetate moieties (Figure 1). Considering the distance between neighboring copper atoms, in the range 2.98-3.23 Å, two alternating exchange pathways (J1, J2) can be identified within the Cu1 chains running along b, whereas only one (J3) can be considered in the adjacent mixed Cu2, Cu3 chains. Between these chains, the Cu2 and Cu3 ions are linked by two interactions with the Cu1 ion on one side and two others on the other side (J5, J6 and J4, J7). The Cu2-Cu1 and Cu3-Cu1 distances are all different thus raising the number of in-plane interactions up to 7. The pathways defining these interactions J1-J7 are given in Figure 1. The temperature dependence of the magnetic susceptibility of 1 in the range 1.8-295 K is shown in Figure 2. Between 295 and 50 K, the χT product increases very slightly obeying the Curie-Weiss law with a Curie constant C ) 0.84 emu · K · mol-1, in agreement with two CuII ions in octahedral symmetry with S ) 1/2 spin and g ) 2.12,17 and a positive Weiss temperature (θ ) +8.3 K). Below a small hump around 50 K, the χT product increases up to a maximum of 0.97 emu · K · mol-1 at TN ) 4.84 K, where a sharp maximum of the susceptibility is observed, indicating a weak ferromagnetic behavior of the CuII planes. The subsequent decrease to zero can be related to an antiferromagnetic coupling between the layers. This behavior is consistent with the low-temperature magnetization versus field curve presented in Figure 3. The S shape is characteristic of an antiferromagnet exhibiting a metamagnetic transition at a threshold field of 8000 Oe without hysteresis. It is worth noticing that the observed ferromagnetic behavior is the result of seven in-plane interactions, the sign and intensity of which being likely significantly different. The same situation was reported in the parent copper(II) hydroxonitrate which is
Properties of Cu2(OH)3(CH3COO) · H2O
Figure 4. Temperature variation of the magnetic moment of 1 under 1 bar (squares) and 1.2 GPa (open circles) isostatic pressure.
Figure 5. Magnetization versus field curves of 1 under 1 bar (squares) and 1.2 GPa (circles) isostatic pressure.
an antiferromagnet in the whole temperature range, with six different in-plane interactions.20 The fact that the observed ferromagnetic behavior results here from unbalanced interactions is also suggested by the low value of the magnetization at high field, M ) 1.04 µB that is almost half of that expected for S ) 1/2 copper ions.21 The occurrence of a small hump around 50 K instead of a regular increase of χT with decreasing temperature may result from such complex situation. We have already demonstrated that the grafting of n-alkyl carboxylates instead of acetate ions stabilizes ferrimagnetic ordering. This was related by Awaga and co-workers to the “chemical pressure” induced by the alkyl chains onto the copper hydroxide layers modifying the local structure and hence the geometry of the exchange pathways between neighboring CuII ions.10 We thus recorded the magnetic susceptibility of 1 under external pressure to do a comparison with such internal pressure effect. The temperature dependence of the magnetic moment and the field dependence of the magnetization at 2 K, at ambient pressure and 1.2 GPa are shown in Figures 4 and 5. For P ) 1 bar, the behaviors are very similar to those observed in normal conditions (without pressure cell). When applying an isostatic pressure of 1.2 GPa, a dramatic change is observed with a divergence of M/H below 20 K. Accordingly, the M(H) curve in Figure 5 exhibits a steep increase at low field, characteristic of a ferromagnetic-like behavior. The magnetic transition is further confirmed by the occurrence of an out of phase signal in the ac susceptibility (insert of Figure 5), suggesting a 3D ferromagnetic-like ordering at 2 K. Thus, a pressure induced transition from an antiferromagnetic to a ferromagnetic state is found for the first time in
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Figure 6. Pressure dependence of the cell parameters a, b, c, and β of 1.
Cu2(OH)3(CH3COO) · H2O. This is merely due to the distortion of the inorganic network, similarly to the effect of the chemical pressure in the case of the long chain alkylcarboxylate analogues (see above). Nevertheless, for the latter, the distortion is localized at given coordination sites while in the present case a more homogeneous deformation is expected under isostatic pressure. To validate our hypothesis and to clarify the influence of the pressure on the geometry of the exchange pathways, we have investigated the structure by powder X-ray diffraction study under pressure. As already emphasized by Masciocchi et al.,15 it was difficult to obtained very nice diffraction patterns of 1 and our best result is similar to that of this reference work with the differences mainly being due to preferred orientation effects. Nevertheless, our results confirmed that the β angle value is around 92° (Figure 6) and the structure model of Masciocchi et al. could be applied for analyzing our diffraction patterns. On this basis, the powder X-ray diffraction profiles recorded at different pressures were fitted starting from the corresponding atomic positions (fixed in the refinement) and varying the cell parameters taking into account the calculated intensities. Interatomic distances and angles were calculated in each case. As shown in Figure 6, the compressibility in the ab plane is almost isotropic, whereas it is ∼2.5 times larger in the c direction indicating a reduction of the interplane distance from 9.34 to 8.97 Å. The values obtained when relaxing to ambient pressure are very close to the initial values thus indicating a good reversibility of the pressure induced changes. The variation of the geometry of the different exchange pathways is given in the Supporting Information. All the nearest neighbor interactions occur via double oxo bridges which vary significantly with applying pressure. Upon increasing pressure, the distances diminish and the Cu-O-Cu angles increase. Interestingly, the pathways along the b direction, i.e., along the Cu1-Cu1 and Cu2-Cu3 chains (J1, J2, and J3) remain relatively symmetrical whereas applying pressure raises the dissymmetry of the interchain pathways (J4-J7). Dealing with the incidence on the magnetic properties, both experimental and theoretical findings on symmetrical copper(II) hydroxo-based compounds indicate that the increase of the bridge angle favors an antiferromagnetic coupling.20,22,23 The threshold value for the ferro- to antiferromagnetic transition was found around 98°. Because of the dissymmetry of the double oxo-bridges, the change of sign of the interaction occurs likely for different bond angles in the present compound. Experimental and density functional theory (DFT) studies of the parent antiferromagnet Cu2(OH)3(NO3) has shown the existence of both ferromagnetic and antiferromagnetic
19150 J. Phys. Chem. C, Vol. 112, No. 48, 2008 competing in-plane interactions, the associated bond angle values being generally lower than in the present case, at least for one of the oxo bridge relating adjacent copper centers.20,24,25 The important point here is that the pressure dependence of the angles shows a reinforcement of the antiferromagnetic character, whatever the interaction. Moreover, the shortening of the interatomic in-plane and interplane distances is expected to favor the orbital overlap and hence to increase the intensity of the interactions. Thus, it supports our hypothesis that the whole magnetic behavior of these layered hydroxide systems is governed by the competition between several in-plane interactions. In compound 1, applying pressure changes the imbalance between the pair exchange couplings leading to a metastable ferromagnetic state. Conclusions We have described here the magnetic behavior of the copper hydroxyacetate (1). At ambient pressure, the compound 1 is an antiferromagnet, with weakly ferromagnetic Cu(II) layers and antiferromagnetic interplane coupling, promoting a metamagnetic system. Under pressure of 1.2 GPa, a transition to a metastable ferromagnetic state is observed. This transition is reversible and was correlated to the reversible distortion of the inorganic network. An accurate investigation of the pressure dependence of the cell parameters providing information on the geometry (distance, angle) of the Cu-O-Cu exchange pathways was realized by using high pressure powder X-Ray diffraction. It was deduced on the basis of the structural features that pressure significantly modifies the relative efficiencies of the seven different in-plane competing interactions. This experimental result gives important insight on the mechanisms responsible of the magnetic behavior of layered transition metal hydroxides and more generally compounds with competing interactions, the magnetic state of which being rather difficult to define a priori. One understands better why insertion-grafting into these layered structures can influence drastically their magnetic behavior. The flexibility of this hybrid organic-inorganic compound is comparable to that of 1D and 3D molecular magnetic systems several of which experience a shift of their temperature ordering under pressure.26-29 The crossover from antiferro- to ferromagnetism was however rarely reported in the literature.19 As a complementary study, a DFT modeling of the present 2D system under pressure is envisaged for closer investigation of the structure- properties relationships. Acknowledgment. The authors thank Dr. A. Derory for his help in magnetic measurements. This work was also supported by a Grant-in-Aid for Scientific Research (A) (No. 18205023), MEXT, Japan.
Suzuki et al. Supporting Information Available: Powder X-ray diffraction pattern of 1 showing profile refinement results and variation of the geometry of the seven exchange pathways with pressure. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Demessence, A.; Rogez, G.; Rabu, P. Actu. Chim. 2006, 293, 17– 21. (2) Demessence, A.; Rogez, G.; Rabu, P. Chem. Mater. 2006, 18, 3005– 3015. (3) Laget, V.; Hornick, C.; Rabu, P.; Drillon, M.; Ziessel, R. Coord. Chem. ReV. 1998, 178-180, 1533–1553. (4) Rabu, P.; Drillon, M. AdV. Eng. Mater. 2003, 5 (4), 189–210. (5) Rabu, P.; DrillonM.; Awaga, K.; Fujita, W.; Sekine, T. In Magnetism: Molecules to Materials; Miller, J. S., Drillon,M. , Eds.; WileyVCH: Weinheim, 2001; Vol. II, pp 357-395. (6) Poul, L.; Jouini, N.; Fie´vet, F. Chem. Mater. 2000, 12, 3123–3132. (7) Taibi, M.; Ammar, S.; Jouini, N.; Fie´vet, F.; Molinie´, P.; Drillon, M. J. Mater. Chem. 2002, 12, 3238–3244. (8) Fujita, W.; Awaga, K. Inorg. Chem. 1996, 35, 1915–1917. (9) Fujita, W.; Awaga, K. J. Am. Chem. Soc. 1997, 119, 4563–4564. (10) Fujita, W.; Awaga, K.; Yokoyama, T. Inorg. Chem. 1997, 36, 196– 199. (11) Huang, Z.-L.; Drillon, M.; Masciocchi, N.; Sironi, A.; Zhao, J.-T.; Rabu, P.; Panissod, P. Chem. Mater. 2000, 12, 2805–2812. (12) Rujiwatra, A.; Kepert, C. J.; Claridge, J. B.; Rosseinsky, M. J.; Kumagai, H.; Kurmoo, M. J. Am. Chem. Soc. 2001, 123, 10584–10594. (13) Shimizu, H.; Okubo, M.; Nakamoto, A.; Enomoto, M.; Kojima, N. Inorg. Chem. 2006, 45 (25), 10240–10247. (14) Drillon, M.; Panissod, P.; Rabu, P.; Souletie, J.; Ksenofontov, V.; Gu¨tlich, P. Phys. ReV. B 2002, 65, 104404. (15) Masciocchi, N.; Corradi, E.; Sironi, A.; Moretti, G.; Minelli, G.; Porta, P. J. Solid State Chem. 1997, 131, 252–262. (16) Mao, H. K.; Xu, J.; Bell, P. M. J. Geophys. Res. 1986, 91, 4673. (17) Rodriguez-Carvajal, J. Commission on Powder Diffraction (IUCr) Newsletter 2001, 26, 12–19. (18) Hosokoshi, Y.; Mito, M.; Tamura, M.; Takeda, K.; Inoue, K.; Kinoshita, M. ReV. High Pressure Sci. Tech. 1998, 7, 620. (19) Suzuki, K.; Hosokoshi, Y.; Inoue, K. Mol. Cryst. Liq. Cryst. 2002, 379, 247–252. (20) Ruiz, E.; Llunell, M.; Cano, J.; Rabu, P.; Drillon, M.; Massobrio, C. J. Phys. Chem. B 2006, 110 (1), 115–118. (21) Carlin, R. L. Magneto-Chemistry; Springer-Verlag: Berlin, 1986. (22) Crawford, V. H.; Richardson, H. W.; Wasson, J. R.; Hodgson, D. J. Inorg. Chem. 1976, 15, 2107–2110. (23) Ruiz, E.; Alemany, P.; Alvarez, S.; Cano, J. J. Am. Chem. Soc. 1997, 119 (6), 1297–1303. (24) Massobrio, C.; Pouillon, Y.; Rabu, P.; Drillon, M. Polyhedron 2001, 20, 1305–1309. (25) Pillet, S.; Souhassou, M.; Lecomte, C.; Rabu, P.; Drillon, M.; Massobrio, C. Phys. ReV. B 2006, 73, 115116. (26) Ohba, M.; Kaneko, W.; Kitagawa, S.; Maeda, T.; Mito, M., J. Am. Chem. Soc. 2008. (27) Laukhin, V.; Martinez, B.; Fontcuberta, J.; Amabilino, D. B.; Minguet, M.; Veciana, J. J. Phys. Chem. B 2004, 108 (48), 18441–18445. (28) Ohba, M.; Kaneko, W.; Kitagawa, S.; Maeda, T.; Mito, M. J. Am. Chem. Soc. 2008, 130 (13), 4475–4484. (29) Tancharakorn, S.; Fabbiani, F. P. A.; Allan, D. R.; Kamenev, K. V.; Robertson, N. J. Am. Chem. Soc. 2006, 128 (28), 9205–9210.
JP807636U
