© Copyright 2002 by the American Chemical Society
VOLUME 106, NUMBER 10, MARCH 14, 2002
LETTERS Observation of Spin Transition in an Octahedrally Coordinated Manganese(II) Compound Shin-ichi Ohkoshi,† Hiroko Tokoro,† Masayoshi Utsunomiya,‡ Mikihisa Mizuno,† Masahiko Abe,‡ and Kazuhito Hashimoto*,† Research Center for AdVanced Science and Technology, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8904, Japan, and Department of Industrial Engineering Chemistry, Faculty of Science and Technology, Science UniVersity of Tokyo, 2641 Yamazaki, Noda, Chiba, 278, Japan ReceiVed: August 30, 2001; In Final Form: NoVember 28, 2001
The thermal spin transition between a high-spin state (S ) 5/2) and an intermediate-spin state (S ) 3/2) of the octhahedrally coordinated Mn(II)(3d5) was observed in Rb(I)Mn(II)[Fe(III)(CN)6] with a very wide hysteresis loop over the temperature range of 231 to 304 K. This phenomenon is accompanied by a structural distortion from the face-centered cubic structure to the tetragonal one caused by a cooperative Jahn-Teller effect in the octahedral Fe(III)C6 moiety. To our knowledge, the present spin transition is the first observation in compounds containing a six-coordinated Mn(II).
Spin transitions between low-spin (LS) and high-spin (HS) states of transition metal ions have been extensively studied in solid compounds containing an octahedrally coordinated metal ion.1-3 The cooperativity plays an important role in spin transition phenomena, and may lead to abrupt transitions in magnetic susceptibility along with a thermal hysteresis loop. The origin of the cooperativity is explained by the interaction between a metal ion and lattice strain, e.g., an electron-phonon coupling,4 a Jahn-Teller distortion,5 and an elastic interaction.6 To date, many studies relating to the spin transitions of metal complexes have been reported, but the spin transition of a sixcoordinate Mn(II) complex with a 3d5 electronic configuration has not yet been observed. It is known that the six-coordinate Mn(II) can take the HS, LS, and intermediate-spin (IS) states depending on the symmetry of the ligand coordination and the strength of the ligand field. For example, the octahederally coordinated Mn(II) (Oh symmetry) in weak and strong ligand fields takes the HS state (S ) 5/2); e.g., Mn(II)(H2O)6,7 and * Author to whom correspondence should be addressed. † The University of Tokyo. ‡ Science University of Tokyo.
the LS state (S ) 1/2); e.g., Mn(II)(CN)6,8 respectively. The pseudo-tetragonally octahedral Mn(II) (D4h symmetry) takes the IS (S ) 3/2) state; e.g., Mn(II)phthalocyanine solid compound.9 In the present study, we report that rubidium(I) manganese(II) hexacyanoferrate(III) containing an octahedrally coordinated Mn(II) shows a thermal spin transition between the HS and IS states of Mn(II) with a large hysteresis loop over the temperature range of 231 to 304 K, and discuss the origin of the spin transition. The rubidium(I) manganese(II) hexacyanoferrate(III) was prepared by reacting an aqueous solution (0.1 mol dm-3) of MnCl2 with a mixed aqueous solution of RbCl3 (1 mol dm-3) and K3[Fe(CN)6] (0.1 mol dm-3) to yield a light brown precipitate. Elemental analyses for Rb, Mn, and Fe by inductively coupled plasma-atomic emission spectrometry (ICP-AES) showed that the formula of the obtained precipitate is Rb(I)Mn(II)[Fe(III)(CN)6]: Calcd: Rb, 24.26; Mn, 15.59; Fe, 15.85. Found: Rb, 24.25; Mn, 15.95; Fe, 15.35. Magnetic susceptibility (χM) measurements for the present compound were carried out using a Quantum Design MPMS 7
10.1021/jp0133687 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/16/2002
2424 J. Phys. Chem. B, Vol. 106, No. 10, 2002
Figure 1. The observed χMT-T plots for Rb(I)Mn(II)[Fe(III)(CN)6] with decreasing (0) and increasing temperature (O) and the calculated χMT-T plots (s) with Mn(II)IS (S ) 3/2) and Fe(III)LS (S ) 1/2), J ) +1.1 cm-1, and g ) 2.3.
superconducting quantum interference magnetometer. The temperature dependence of the χMT values (χMT-T plots) for this compound is shown in Figure 1. The χMT value at 320 K was 4.65 cm3 K mol-1 [high-temperature (HT) phase]. As the sample was cooled, the χMT value started sharply decreasing around 245 K and reached 3.16 cm3 K mol-1 at T ) 200 K [low-temperature (LT) phase]. Conversely, as the sample in the LT phase was warmed, the χMT value suddenly increased around 285 K and reached the value in the HT phase at 320 K. The transition temperatures of HT f LT (T1/2V) and LT f HT (T1/2v) were 231 and 304 K, respectively, and the width of the thermal hysteresis loop (∆T ) T1/2v - T1/2V) was 73 K. As the LT phase was cooled to the very low temperature region, it exhibited a spontaneous magnetization with a magnetic ordering temperature (Tc) of 12 K. The saturation magnetization (Ms) and coercive field (Hc) values at 2 K were 3.6 µB and 650 G, respectively. Moreover, from the temperature dependence of the χ-1 plots, the Weiss temperature value was estimated to be +15 K. The UV-Vis, IR, and X-ray powder diffraction (XRD) patterns spectra showed that the spin transition is accompanied by a geometrical structure change. For example, the color of the compound in the HT and LT phases are light brown and dark brown, respectively. The UV-Vis absorption spectrum in the HT phase consisted of three absorption peaks at 406, 504, and
Letters 614 nm. Conversely, that in the LT phase consisted of one broad absorption around 615 nm. The IR spectrum also changed according to the spin transition. The sharp IR peak due to the CN stretching mode in the HT phase was observed at 2152 cm-1 (line width ) 9 cm-1), but that appeared as a broad peak at 2095 cm-1 (line width ) 65 cm-1) in the LT phase. Moreover, the XRD patterns provide direct evidence of the structural change between the HT and LT phases. Figure 2 shows the XRD patterns at different temperatures in the spin transition range from the HT to LT phases. The XRD pattern in the HT phase was consistent with that of the face-centered cubic (F432) structure with a lattice constant of 10.533 Å (at 300 K). As the sample was cooled, the XRD pattern peaks in the HT phase gradually disappeared and then a different pattern of peaks appeared. The observed XRD pattern in the LT phase showed a tetragonal structure (I4222; a ) b ) 10.026 Å, c ) 10.520 Å at 160 K). The unit cell volume of 1169 Å3 in the HT phase was reduced by about 10% to 1057 Å3 in the LT phase.10 This tetragonal structure returned to the original cubic one by warming, indicating that the present structural change is reversible. The spin states of the metal ions in the HT and LT phases are now considered. The observed χMT values in the HT phase are close to the theoretical spin-only moment value of 4.75 cm3 K mol-1 for the sum of the Mn(II)HS (t2g3eg2, S ) 5/2) and Fe(III)LS (t2g5, S ) 1/2) states,11 and hence, it is reasonably considered that the electronic state of Mn(II)HS-NC-Fe(III)LS is the ground state in the HT phase. Next, we determined the spin state in the LT phase on the basis of the magnetic data in the ferromagnetic and paramagnetic regions. The positive Weiss temperature value indicates that the magnetic interaction among the unpaired electrons of the metal ions in the LT phase is ferromagnetic. Its superexchange interaction (J) value among the metal ions is estimated to be +1.1 cm-1 from the Tc value of the LT phase. This ferromagnetic interaction and the observed Ms value suggest that the manganese and iron ions in the LS phase take the Mn(II)IS (S ) 3/2) and Fe(III)LS (S ) 1/2) states, respectively. Using these spin states and the J value, the theoretical χMT values can be calculated. The calculated χMT vs T plots reproduced the observed values very well, assuming the g-factor of 2.3 (Figure 1, solid line).12 Hence, we concluded that the present spin transition is due to the change in the spin
Figure 2. (a) Temperature dependences of XRD spectra for Rb(I)Mn(II)[Fe(III)(CN)6]. (* is Cu of the sample holder.) (b) Schematic illustration of the crystal structures in the high-temperature and low-temperature phases. Large gray circle is Rb(I) ion, middle black circle is Fe(III) ion, middle white circle is Mn(II) ion, small black circle is N atom, and small white circle is C atom.
Letters
J. Phys. Chem. B, Vol. 106, No. 10, 2002 2425 knowledge, the present spin transition is the first such observation in compounds containing a six-coordinate Mn(II). The lightinduced excited spin state trapping (LIESST) effect in the compound containing Mn(II) has not yet been reported, but the present compound could show such a photo effect because this compound has a bistability in the electronic state and a strong cooperativity. Work along this line is currently under way. To date, we have proposed various new magnetic functionalities such as photomagnetism,13,14 a magnet exhibiting two compensation temperatures,15 and colored magnetic films,16 using various metal polycyanides. The present results also show that metal polycyanides17-20 are superior compounds for designing novel functionalized magnetic materials. References and Notes
Figure 3. Schematic illustration of the mechanism of the spin transition in Rb(I)Mn(II)[Fe(III)(CN)6].
state of Mn(II) between the HS (S ) 5/2) and IS (S ) 3/2) states. We will now discuss the mechanism of the present spin transition. The tetragonal structure in the LT phase suggests that the structural distortion is due to the Jahn-Teller effect of a ferrodistortive pseudo-tetragonally octahedral elongation (B1g oscillator mode). However, it is well-known that a Jahn-Teller distortion does not occur in the octahedrally coordinated Mn(II). In addition, the existence of the large thermal hysteresis loop indicates that a strong cooperativity operates in the present spin transition. These considerations suggest that the driving force of the spin transition is possibly the cooperative JahnTeller distortion in the Fe(III)C6 moiety. Such a structural distortion induces the change of the d-orbital symmetries in both the Fe(III)C6 and Mn(II)N6 moieties from the Oh to D4h symmetries; i.e., the eg orbitals split into 2b1g(x2-y2) and 2a1g(z2), and the t2g orbitals into b2g(xy) and eg(xz, yz). Successively, according to the ligand field stability, the spin conversion from Mn(II)HS to Mn(II)IS (eg3b2g12a1g1, S ) 3/2) will take place. These processes are schematically shown in Figure 3. In summary, we have observed a thermal spin transition between the high-spin and intermediate-spin states of Mn(II) in Rb(I)Mn(II)[Fe(III)(CN)6] with a surprisingly wide hysteresis loop (T1/2V ) 231 K and T1/2v ) 304 K). This spin transition is explained by the coorperative Jahn-Teller effect in the Fe(III)C6 moiety and the ligand field stability of the pseudotetragonally octahedral Mn(II)N6 moiety. In addition, the origin of the large thermal hysteresis is the strong cooperativity occurring in the present compound. This is because all the metal ions are linked by a CN group with 3-D structure. To our
(1) Goodwin, H. A. Coord. Chem. ReV. 1976, 18, 293. (2) Gu¨tlich, P. Struct. Bonding (Berlin) 1981, 44, 83. (3) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (4) Zimmermann, R. J. Phys. Chem. Solids 1983, 44, 151. (5) Kambara, T. J. Phys. Soc. Jpn. 1980, 49, 1806. (6) Ohnishi, S.; Sugano, S. J. Phys. 1981, C14, 39. (7) McCaffery, A. J.; Stephens, P. J.; Schatz, P. N. Inorg. Chem. 1967, 6, 1614. (8) Entley, W. R.; Girolami, G. S. Inorg. Chem. 1994, 33, 5165. (9) Barraclough, C. G.; Martin, R. L.; Mitra, S.; Sherwood, R. C. J. Chem. Phys. 1970, 53, 1638. (10) The positions of the Rb(I) ions in both the HT and LT states are those of a tetragonal structure with the space group of I4222; i.e., a ) b ) 7.448, c ) 10.533 Å in the HT phase, and a ) b ) 7.090, c ) 10.520 Å in the LT phase. In any cases, the positions of Rb(I) correspond to the center of the -CN-Mn(II)-NC-Fe(III)-CN- lattice. (11) The analogue compound of the present system, Mn(II)1.5[Fe(III)(CN)6]‚zH2O (Mn(II); HS, S ) 5/2, Fe(III); LS, S ) 1/2), is known to show ferrimagnetism with a Tc value of 9 K (Hoden, A. N.; Matthias, B. T.; Anderson, P. W.; Luis, H. W. Phys. ReV. 1956, 102, 1463, and Verdaguer, M.; Mallah, T.; Gadet, V.; Castro, I.; He´ary, C.; Thie´aut, S.; Veillet, P. Conf. Coord. Chem. 1993, 14, 19). For this analogue compound, however, the spin transition has not been observed. To understand this difference, we are now studying the physical properties of the intermediate compositional material between Rb(I)Mn(II)[Fe(III)(CN)6] and Mn(II)1.5[Fe(III)(CN)6]‚zH2O. (12) In the present analysis, only the g-factor is used as a fitting parameter. However, there is a possibility of a contribution due to orbital angular momentum. Such an increase in the χMT value was reported in a Mn(II) phthalocyanine solid compound.9 (13) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (14) Ohkoshi, S.; Yorozu, S.; Sato, O.; Iyoda, T.; Fujishima A.; Hashimoto, K. Appl. Phys. Lett. 1997, 70, 1040. (15) Ohkoshi, S.; Abe, Y.; Fujishima, A.; Hashimoto, K. Phys. ReV. Lett. 1999, 82, 1285. (16) Ohkoshi, S.; Fujishima, A.; Hashimoto, K. J. Am. Chem. Soc. 1998, 120, 5349. (17) Ferlay, S.; Mallah, T.; Ouahe´s, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (18) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (19) Hatlevik, Ø.; Bushmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. AdV. Mater. 1999, 11, 914. (20) Miller, J. S. MRS Bull. 2000, 25, 60.
