© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 21, JUNE 1, 2000
LETTERS Pressure-Induced High Spin State in [Fe(btr)2(NCS)2]‚H2O (btr ) 4,4′-bis-1,2,4-triazole)† Yann Garcia, Vadim Ksenofontov, Georg Levchenko, Georg Schmitt, and Philipp Gu1 tlich* Institut fu¨ r Anorganische Chemie und Analytische Chemie, UniVersita¨ t Mainz, Staudingerweg 9, D-55099 Mainz, Germany ReceiVed: February 7, 2000; In Final Form: March 28, 2000
Application of hydrostatic pressure (e 10.5 kbar) on the two-dimensional spin transition compound [Fe(btr)2(NCS)2]‚H2O (btr ) 4,4′-bis-1,2,4-triazole) results in an unexpected stabilization of the HS state. On release of the pressure, the HS state is found to be partially trapped. After thermal relaxation of the metastable HS state obtained by the LIESST effect (light-induced excited spin state trapping), a pure LS state is obtained in contrast to the pressure experiments. This different behavior supports a structural phase transition as the likely basis of the pressure-induced HS state.
The spin state of iron(II) spin transition (ST) materials can be varied by several external perturbations, such as temperature, pressure, and light, from high-spin (HS, S ) 2) to low-spin (LS, S ) 0).1 For an assembly of spin crossover molecules, cooperative effects arising from long-range elastic interactions and/or electron-phonon coupling play an essential role on the form of the ST curves.1,2 The occurrence of hysteresis encountered when cooperativity is strong enough is the prerequisite for bistability and thus for eventual application in display or switching devices.3 A crystallographic phase transition can also modify the features of the ST curves.4 In certain cases, ST and structural phase transition are coupled. For [Fe(ptz)6](BF4)2 (ptz ) 1-propyl-tetrazole), the ST triggers the crystallographic phase transition5 and can exist on its own. In certain cases both phenomena can be considered as independent, as for instance for [Fe(btz)6](BF4)2 with btz ) 1-butyl-tetrazole.6 Application of high pressure can be a powerful tool to investigate the interplay between ST and structural phase transition as pressure usually stabilizes the LS state. The pressure dependence of both phenomena can be followed, and in the most favorable cases a decoupling is observed between them.7 * Corresponding author. † Dedicated to Professor Heinrich Vahrenkamp on the occasion of his 60th birthday.
Figure 1. Part of the crystal structure of [Fe(btr)2(NCS)2]‚H2O at 293 K (from ref 8b).
Our attention was directed to the polymeric compound [Fe(btr)2(NCS)2]‚H2O (hereafter labeled 1) with btr ) 4,4′-1,2,4triazole which displays an abrupt ST at low temperature with hysteresis, this latter behavior being probably due to a crystallographic phase transition.8b,9,10 The compound was synthesized
10.1021/jp0004922 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/06/2000
5046 J. Phys. Chem. B, Vol. 104, No. 21, 2000
Letters
Figure 2. χMT versus T plots for [Fe(btr)2(NCS)2]‚H2O in the temperature range 20-300 K, under different pressures up to 10.5 kbar. In Figure 2f, the numbers 1, 2, 3 refer to t ) 0; t ) 36 h; t ) 175 h, respectively.
according to a procedure given elsewhere.9 Its structure, shown in Figure 1, consists of iron(II) ions linked together by btr ligands through the nitrogen atoms occupying the 1 and 1′ position. The thiocyanate anions are coordinated in trans position, whereas the noncoordinated water molecules are linked by hydrogen bonding to the peripheral nitrogen atoms. The layers are connected by means of van der Waals forces and weak hydrogen bond bridges involving the water molecule.8b Figure 2 shows the temperature dependence of the χMT product, χM being the molar magnetic susceptibility per iron(II) ion, at various pressures. They were measured on polycrystalline samples in the temperature range 5-300 K with a PAR 151 Foner type magnetometer. The hydrostatic highpressure cell with silicon oil as the pressure transmitting medium has been described elsewhere.11 The pressure measurement was achieved using the known pressure dependence of a superconducting transition of an inner tin manometer. Data were corrected for magnetization of the sample holder and for diamagnetic contributions. Hydrostaticity was established in our earlier studies.12-14 At ambient pressure, a hysteresis loop associated with an extremely abrupt ST (T1/2 v ) 144 K and T1/2 V ) 121 K) is observed, matching previous reports (Figure 2a).8 At 0.8 kbar, the hysteresis loop broadens and becomes
asymmetric; the transition temperature in the cooling mode is not modified, whereas the transition temperature in the heating mode is slightly shifted to higher temperature (Figure 2b). At 3 kbar, the hysteresis loop moves upward (T1/2 v ) 190 K and T1/2 V ) 164 K) and its width decreases (Figure 2c). The ST becomes less abrupt and a noticeable fraction of iron(II) ions remaining in the HS state on the whole temperature range (∼ 8%) is detected. At 6.7 kbar, the hysteresis width diminishes further and is now shifted to around 216 K (T1/2 v ) 227 K and T1/2 V ) 208 K) (Figure 2d). A dramatic increase of the residual HS sites is observed with around 50% of the molecules being in the HS state at low temperatures. When the pressure is further increased, the ST becomes increasingly gradual and incomplete. At 9.6 kbar, the hysteresis width can still be observed (see insert of Figure 2e) but no longer exists at higher pressures. At 10.5 kbar, a totally HS curve is detected (Figure 2e). It should be mentioned that, for all pressures investigated, the ST curves were not altered by thermal cycling. After releasing the pressure, the ST is found to be shifted by 7 K to lower temperatures (T1/2 v ) 137 K and T1/2 V ) 105 K) and the hysteresis becomes larger (32 K) than that obtained (23 K) at atmospheric pressure. Surprisingly only approximately half of the iron(II) ions are now involved in the spin crossover behavior (Figure 2f). Several
Letters χMT vs. T curves were recorded after several elapsed time intervals at ambient pressure. The hysteresis loop is not displaced, but the amount of residual HS sites relaxes at room temperature after a characteristic time of typically 175 h to a steady value of ∼30% (Figure 2f). At liquid helium temperatures, the relaxation time appears to be infinite. Attempts to trap the metastable HS state of the original sample by rapid cooling at ambient pressure were unsuccessful. However, after releasing the pressure, it was possible to retain ∼6% of HS iron(II) ions on quenching from 150 to 4.2 K. For this amount of HS ions, thermal relaxation occurs between 40 and 50 K. These findings demonstrate for the first time that it is possible to induce a complete LS f HS conversion by hydrostatic pressure for an iron(II) ST material. This result is unexpected in the sense that pressure is known to stabilize the LS state as a consequence of the smaller volume for the LS iron(II) ion.1 Appearance of the HS state under pressure was earlier observed on LS iron(II) complexes by Mo¨ssbauer spectroscopy,15-17 but the pressure used in those experiments was quasi hydrostatic and the amount of HS species was no more than 40% at pressures up to 190 kbar. According to our previous experiments, sheared deformations and non-hydrostaticity are able to destroy such sensitive molecular compounds.14 The title compound (1) possesses unusual ST characteristics: a wide hysteresis loop with sharp ST occurring at low temperatures. This is in contrast with its ligand field strength value (10 Dq ) 11600 cm-1)8b which is situated at the lower limit of the range where spin crossover is expected.1 We therefore assume that the ST is probably triggered by a crystallographic phase transition, in contrast to [Fe(ptz)6](BF4)2 where the ST triggers the crystallographic phase transition.5 It should be noted that the isostructural compound [Fe(btre)2(NCS)2] with btre ) 1,2-bis(1,2,4triazol-4-yl)ethane remains HS even at 12 kbar.18 Its crystal structure does not reveal any hydrogen bonds in contrast to (1) which might play a decisive role in the appearance of the structural phase transition and thus of the spin crossover behavior. Analyzing the χMT vs. T curves of (1), one can assume that a new crystallographic phase appears under pressure, denoted as phase B. This phase does not show any ST but a reduction of the χMT product with decreasing temperature (Figure 2e). This behavior differs from that obtained for the original phase, denoted as phase A. Phase A reveals quite usual behavior under pressure: the ST is shifted to higher temperatures and becomes more gradual. With increasing pressure, the relative amount of phase B increases at the expense of phase A, and above 10.5 kbar phase A is no longer observed. After releasing the pressure, the initial ST curve is not restored, which is unusual according to our previous pressure experiments.12-14 Rather, phase B is still observed together with a new phase, denoted as phase C, which can be unambiguously distinguished from the original phase A. Its transition curve is shifted to lower temperatures and its hysteresis width is 30% larger. From the related relaxation experiments, we can conclude that phase B transforms to phase C with time. The slow kinetics of this transformation, as well as the fact that phase C can be quenched, supports the idea of a structural transition of first order to take place. Analyses of the forms of the χMT vs. T curves also confirm that the transformations phase A f phase B f phase C are of first order. As outlined above, a pure HS state was induced (phase B) on application of pressure. To clarify the influence of structural phase transformations on the spin crossover behavior, we used another perturbation able to generate the HS state, namely
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Figure 3. 57Fe Mo¨ssbauer spectra of [Fe(btr)2(NCS)2]‚H2O documenting the LIESST effect at 10 K: (top) before irradiation; (middle) after irradiation with λ ) 514.5 nm (LIESST); (bottom) after irradiation with λ ) 820 nm (reverse-LIESST).
light.19 The aim is to compare the evolution of the metastable HS state generated by pressure and light, after thermal relaxation. For these experiments, the polycrystalline sample was placed in a polished PMMA sample holder and irradiated at 10 K with an Ar+ laser (514 nm, 25 mW cm-2) for 2 h in a Mo¨ssbauer cryostat. 57Fe Mo¨ssbauer measurements were performed in transmission geometry using a constant acceleration Wissel-type spectrometer with a 57Co(Rh) source. Figure 3 shows the Mo¨ssbauer spectra recorded at 10 K before and after irradiation with green light. A complete LS f HS transition was observed (LIESST). To the best of our knowledge, LIESST has been observed for [Fe(btr)2(NCS)2]‚H2O for the first time. The sample was subsequently irradiated with red light resulting in a complete HS f LS back conversion (reverse-LIESST) (Figure 3). Relaxation curves at a fixed temperature were recorded as a function of time. Their sigmoidal shape proves the presence of cooperative interactions in this material.20,21 Moreover, it is noteworthy that the relaxation interval (40-46 K) coincides with the interval obtained from quenching experiments (see above). It should be mentioned that a pure LS doublet is obtained after thermal relaxation. This complete transition to the LS state contrasts with the incomplete character of the ST obtained after release of the pressure (Figure 2f) and supports the suggestion that pressure leads to a structural transformation. The appearance of a HS state was theoretically considered by Kambara22 who found that the HS state can be stabilized by increasing the intramolecular coupling strength by pressure, since the molecular displacements with Eg symmetry can couple only with the HS state. The experimentally observed pressure dependence of the HS fraction for (1) cannot be described by this model, probably because the cooperative interactions that are relevant in the present case were not considered in this theory.
5048 J. Phys. Chem. B, Vol. 104, No. 21, 2000 In summary, the role of a structural change is considered to be the main factor in the appearance of a HS state under pressure for [Fe(btr)2(NCS)2]‚H2O as well as in the interplay between ST and structural phase transition. We assume that a structural phase transition probably governs the ST. We aim to clarify this behavior further by X-ray measurements at variable temperatures and pressures and to establish the phase diagram of (1) in order to specify the variety of structural modifications induced by pressure. Acknowledgment. Financial support from the Fonds der Chemischen Industrie and the Materialwissenschafliches Forschungszentrum der Universita¨t Mainz and the TMR Network ERB-FMRX-CT98-0199 entitled “Thermal and Optical Switching of Molecular Spin States (TOSS)” is gratefully acknowledged. G.L. appreciates financial support from the Deutsche Forschungsgemeinschaft. References and Notes (1) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024. (2) (a) Sanner, I.; Meissner, E.; Ko¨ppen, H.; Spiering, H. Chem. Phys. 1984, 86, 227. (b) Willenbacher, N.; Spiering, H. J. Phys. C: Solid State Phys. 1988, 21, 1423. (c) Spiering, H., Willenbacher, N. J. Phys.: Condens. Matter 1989, 1, 10089. (3) Kahn, O.; Launay, J. P. Chemtronics 1988, 3, 140. (4) (a) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85, 219. (b) Wiehl, L.; Spiering, H.; Gu¨tlich, P.; Knorr, K. J. Appl. Crystallogr. 1990, 23, 151.
Letters (5) (a) Jung, J.; Schmitt, G.; Wiehl, L.; Hauser, A.; Knorr, K.; Spiering, H.; Gu¨tlich, P. Z. Phys. B. 1996, 100, 523. (b) Jeftic, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743. (6) Schmitt, G. Ph.D. Thesis, University of Mainz (Germany), 1996. (7) Jeftic, J.; Hauser, A. J. Phys. Chem. B 1997, 101, 10262. (8) (a) Vreugdenhil, W.; Haasnoot, J. G.; Kahn, O.; Thuery, P.; Reedijk, J. J. Am. Chem. Soc. 1987, 109, 5272. (b) Vreugdenhil, W.; van Diemen, J. H.; de Graaff, R. A. G.; Haasnoot, J. G.; Reedijk, J.; van der Kraan, A. M.; Kahn, O.; Zarembowitch, J. Polyhedron 1990, 9, 2971. (9) Ozarowski, A.; Shunzhong, Y.; McGarvey, B. R.; Mislankar, A.; Drake, J. E. Inorg. Chem. 1991, 30, 3167. (10) Martin, J. P.; Zarembowitch, J.; Dworkin, A.; Haasnoot, J. G.; Codjovi, E. C. Inorg. Chem. 1994, 33, 2617. (11) Baran, M.; Levchenko, G.; Dyakonov, V. P.; Shymchak, G. Physica C 1995, 241, 383. (12) Garcia, Y.; van Koningsbruggen, P. J.; Lapouyade, R.; Fourne`s, L.; Rabardel, L.; Kahn, O.; Ksenofontov, V.; Levchenko, G.; Gu¨tlich, P. Chem. Mater. 1998, 10, 2426. (13) Ksenofontov, V.; Levchenko, G.; Spiering, H.; Gu¨tlich, P.; Letas, J. F.; Bouhedja, Y.; Kahn, O. Chem. Phys. Lett. 1998, 294, 545. (14) Ksenofontov, V.; Spiering, H.; Schreiner, A.; Levchenko, G.; Goodwin, H. A.; Gu¨tlich, P. J. Phys. Chem. Solids 1999, 60, 393. (15) Fisher, D. C.; Drickamer, H. G. J. Chem. Phys. 1971, 54, 4825. (16) Slichter, C. P.; Drickamer, H. G. J. Chem. Phys. 1972, 56, 2142. (17) Long, G. J.; Hutchinson, B. B. Inorg. Chem. 1987, 26, 608. (18) Garcia, Y.; Ksenofontov, V.; Bravic, G.; Chasseau, D.; Gu¨tlich, P., unpublished results. (19) Decurtins, S.; Gu¨tlich, P.; Ko¨hler, C. P.; Spiering, H.; Hauser, A. Chem. Phys. Lett. 1984, 105, 1. (20) Hauser, A. Chem. Phys. Lett. 1992, 192, 65. (21) Hauser, A.; Gu¨tlich, P.; Spiering, H. Inorg. Chem. 1986, 25, 4245. (22) Kambara, T. J. Phys. Soc. Jpn. 1981, 50, 2257.
