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complex under high pressure, we present the phase diagram of the N-I transition down to the quantum limit. We discuss the symmetry breaking associated...
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J. Phys. Chem. B 2007, 111, 6167-6172

6167

Down to the Quantum Limit at the Neutral-to-Ionic Phase Transition of (BEDT-TTF)-(ClMe-TCNQ): Symmetry Analysis and Phase Diagram M. H. Leme´ e-Cailleau,*,† E. Collet,*,‡,§ M. Buron-Le Cointe,*,‡ N. Grach,‡ B. Ouladdiaf,† F. Moussa,§ T. Hasegawa,*,| Y. Takahashi,| T. Roisnel,⊥ and H. Cailleau*,‡ Institut Max Von Laue - Paul LangeVin, BP 156, 38042 Grenoble Cedex 9, France, Groupe Matie` re Condense´ e et Mate´ riaux and Centre de Diffractome´ trie X, CNRS - UniVersite´ de Rennes 1, 35042 Rennes Cedex, France, Laboratoire Le´ on Brillouin (CEA-CNRS), CEN-Saclay, 91191 Gif sur YVette Cedex, France, and Correlated Electron Research Centre, AIST, Tsukuba, Ibaraki, 305-8562 Japan ReceiVed: October 30, 2006; In Final Form: March 3, 2007

We report on the neutral-to-ionic (N-I) phase transition in the one-dimensional organic complex (BEDTTTF)-(ClMeTCNQ). The X-ray studies at room temperature show that the neutral phase of (BEDT-TTF)(ClMeTCNQ) is already characterized by a polar long-range ordering, at variance with other charge-transfer compounds comprising noncentrosymmetric molecules. From a detailed neutron diffraction study of this complex under high pressure, we present the phase diagram of the N-I transition down to the quantum limit. We discuss the symmetry breaking associated with the transition and the evolution of its first-order character under pressure.

1. Introduction From a general point of view, charge transfer (CT) is of fundamental interest because it plays a key role in many chemical and biological reactions in nature. In the case of solids, it may simultaneously involve a large number of electrons and atoms, because of the cooperative effects intrinsic to condensed matter with strongly correlated electronic and structural degrees of freedom. Thus, specific electronic-structural excitations, such as stripes,1 multi-polarons,2 and CT exciton strings,3 have been introduced to explain different original physical properties such as colossal magnetoresistance,4 ferroelectricity of organic (TMTTF)2X conductors,5 or ultrarapid photoinduced phase transformations of low-dimensional organic compounds.6 Among this wide field of “electronic crystals”, the family of mixed-stack organic charge-transfer complexes holds a special position because of the apparent simplicity of its nonconventional electronic-structural instabilities. Discovered in the 1980s by Torrance and co-workers,7 quasi-one-dimensional (1D) molecular solids, built of stacks of alternating electron donor (D) and electron acceptor (A) molecules, may present the neutral-ionic (N-I) phase transition under pressure and/or at low temperature. According to the first very simple model, the electron transfer from one D molecule to the next A molecule can occur when the energy cost of the transfer can be counterbalanced by the Madelung energy. A dimerization is always concurrent with the electron transfer and contributes to the stabilization of the ionic phase in the form of polar (D+A-) diamagnetic dimers, as observed from optical8 and electronic9 spectroscopies, as well as from structural investigations.10-12 It has been shown that the N-I phase transition on new complexes * [email protected], [email protected], [email protected], [email protected]. † Institut Max von Laue - Paul Langevin. ‡ Groupe Matie ` re Condense´e et Mate´riaux, CNRS. § Laboratoire Le ´ on Brillouin (CEA-CNRS). | Correlated Electron Research Centre, AIST. ⊥ Centre de Diffractome ´ trie X, CNRS.

based on TTF or dimethyl-TTF as electron donors can be strongly affected by soft chemical substitution on the acceptor molecules.12 In these complexes, the acceptor molecules are no longer centrosymmetric but are orientationally disordered in the solid, thus restoring the global centrosymmetry of the structure.13 This phenomenon creates a random electric-field distribution which destroys the low-temperature polar ordering, which can only be restored under pressure. A similar phenomenon can also be observed in solid solutions by changing the concentration of either the donor, such as adding tetraselenafulvalene, or the acceptor, such as adding trichloro-p-benzoquinone.14 In a parallel way, interest has also been focused on the mixedstack CT complex (BEDT-TTF)-(ClMeTCNQ) (bis(ethylenedithio)tetrathiafulvalene)-(2-chloro-5-methyltetracyanoquinodimethane) (Figure 1), to look for new electronic and magnetic properties associated with the strong localization effect on the TCNQ molecule and the two-dimensional nature of the intermolecular interactions.15 Optical spectroscopy showed that an N-I phase transition takes place under pressure in this material and is associated with a large electrical conductivity anomaly.16 A complementary ESR study17 at low temperature and under high pressure, giving a local view of this N-I transition, indicates that it can be seen as a quasi-crossover transition, sketched by a double-well potential with a barrier between N and I states of the same order of magnitude as the thermal energy at the transition temperature. The aim of the structural studies presented hereafter is the investigation at long range of the behavior of this system across the transition and the analysis of its evolution when approaching the absolute zero in pressure. The paper is organized as follows: first, we describe the experimental techniques; then, we present a novel approach to the symmetry of (BEDT-TTF)(ClMeTCNQ) under ambient conditions, based on complementary X-ray and neutron diffraction analysis, and the structural signatures characteristic of the N-I phase transition from neutron diffraction investigations under pressure and at low temperature; and finally, we discuss

10.1021/jp0671291 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

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Figure 1. On the left side, BEDT-TTF and ClMeTCNQ molecules. The arrow indicates the orientation of the dipole moment of the acceptor molecule. On the right, a view of sample I, with its bright reflectivity, used for the neutron scattering experiment.

the phase diagram and the evolution of the order of the phase transition when approaching the quantum limit. 2. Experimental Section The starting materials were obtained as follows: ClMeTCNQ was synthesized according the literature,18 while BEDT-TTF was directly purchased from the Tokyo Kasei Corporation. After careful purification (repeated recrystallization for both compounds and additional sublimation for ClMeTCNQ), the complex was prepared from D and A molecules dissolved at equimolar concentration in hot chlorobenzene. This solution was slowly cooled and kept at 50 °C for several days to give rise to prismatic single crystals elongated along the stacking axis a, all having bright metallic reflecting faces (Figure 1). Conventional X-ray diffraction was performed on an Enraf-Nonius 4-circle diffractometer in κ geometry using an incident X-ray wavelength of 0.71 Å and a CCD camera as detector. The data were collected at room temperature from a single crystal of volume ∼1.0 × 0.3 × 0.3 mm.3 Neutron scattering experiments, requested for the investigations at low temperature and high pressure, were carried out on the cold-neutron triple-axis spectrometer 4F1 at the Laboratoire Le´on Brillouin (LLB) and on the thermal-neutron 4-circle diffractometer D10 at the Institut Laue-Langevin (ILL). Both instruments give a very good signalto-background ratio with a good reciprocal space resolution. Harmonic wavelengths are rejected either by a cooled beryllium filter installed on the incident neutron beam for the former or by the guide cutoff for hot neutrons for the latter instrument. Aluminum alloy pressure cells were used with helium as pressure transmitters. These setups, working up to 500 MPa at the ILL or to 650 MPa in an upgraded version at the LLB, offer simultaneously good neutron transparency, perfect hydrostaticity, and control of the pressure down to very low temperature. Indeed, until helium remains fluid, absolutely no pressure gradient is applied to the sample, and the precision in pressure is (2 MPa. At very low temperature, below the solidification line of helium, it has been demonstrated that the applied pressure is still maintained to within (10 MPa with no gradient if isobaric solidification of helium is carefully achieved.19,20 These high-pressure setups were installed in helium cryostats to allow investigations down to 1.5 K with a temperature stability better than 0.1 K. Two large single crystals, issued from two distinct synthesis and crystal-growth batches, were used for the neutron scattering experiments: sample 1 with dimensions of ∼10 × 2 × 2 mm3 and sample 2 with dimensions of ∼4 × 2 × 2 mm3. 3. Results and Discussion 3.1. The Noncentrosymmetric Order of the Neutral Phase. Before discussing the effect of the N-I transition on the structure of (BEDT-TTF)(ClMeTCNQ), it is worthwhile to reexamine the N-phase symmetry, which was initially described

Figure 2. X-ray diffraction pattern reconstructed in the (b*, c*) and (a*, c*) reciprocal planes which show directly the (0k0):k ) 2n + 1 systematic absence (zero intensity in white circles) and the loss of the (h0l):h + l ) 2n + 1 systematic absence condition (nonzero intensity in black circles).

as isomorphous to that of TTF-CA with space group P21/n and two DA pairs per unit cell. Because of the substitution on the TCNQ molecule, complete disorder of the polar Cl-CH3 orientation was introduced in the refinement in order to restore the centrosymmetry of the unit cell.15 A new precise examination of the X-ray diffraction pattern under ambient conditions has confirmed that the (0k0): k ) 2n + 1 reflections are systematically absent but has also revealed that the diffraction condition related to the existence of a glide plane was not satisfied: many of the (h0l): h + l ) 2n + 1 reflections are indeed weakly diffracting (Figure 2). Parallel tests performed by neutron diffraction at various wavelengths and the large number of such observed “forbidden” reflections discount any effect of multiple scattering or harmonic contamination. Moreover, these (h0l): h + l ) 2n + 1 peaks always have a full width at half-maximum characteristic of long-range order. This clearly establishes that the neutral phase has a noncentrosymmetric space group P21 with two (BEDT-TTF)(ClMeTCNQ) formula units per unit cell. The major consequence of this symmetry is that, in the N phase of (BEDT-TTF)(ClMeTCNQ), there are long-range-ordered polar zones where the Cl-CH3 axes of the acceptor molecules have a preferential orientation (Figure 3). Details of the structural analysis, based on both X-ray and neutron diffraction in order to analyze properly this chlorine-methyl organization, will be published elsewhere.21 This feature is observed for the first time in mixed-stack charge-transfer crystals based on noncentrosymmetric molecules. Up to now, such complexes always presented a centrosymmetric packing, with a static orientational disorder of A molecules.13,14b In contrast to those crystals grown by cosublimation or in an acetonitrile solution, the crystallization of (BEDT-TTF)(ClMeTCNQ) has been done here in chlorobenzene, a solvent with a large polarizability. On (ClMeTCNQ), the molecular

N-I Phase Transition of (BEDT-TTF)(ClMeTCNQ)

Figure 3. Schematic packings above and below the N-I transition in the case of (a) centrosymmetric materials like TTF-CA, and (b) noncentrosymmetric materials like (BEDT-TTF)(ClMeTCNQ). The donor is symbolized by an ellipse, while the acceptor is represented by a simple line for a centrosymmetric molecule or by a small arrow in the case of ClMeTCNQ. On the left side are displayed the highsymmetry phases, while on the right side are displayed in each case one of the domains resulting from the symmetry breaking associated with the N-I transition (the gray arrow shows the dipole moment created by the dimerization along the chain).

dipole moment lies in the π molecular plane, from the chlorine atom to the methyl group, but the global unit-cell polarization, coming from the three-dimensional combination of these intramolecular dipole moments, is strictly constrained to be along the b axis because of the P21 symmetry. The weakness of these additional Bragg peaks indicates that the deviation from the centrosymmetry is weak, which can explain why no additional absorption band was observed in the earliest powder infrared spectra.15b 3.2. Pressure-Temperature Evolution. From the structural point of view, the investigation of the (P, T) phase diagram of (BEDT-TTF)(ClMeTCNQ) has shown two different signatures associated with the N-I transition: on one hand, the clear discontinuity of the cell parameters (Figure 4), and on the other hand, the appearance of new Bragg peaks (Figure 5). In contrast to the other unit-cell parameters which do not significantly change with temperature in the N phase, the a cell parameter decreases on approaching the phase transition from above. Such a thermal contraction occurring predominantly along the stacking axis in the N phase has also been observed in other mixedstack CT complexes.10,14a As it directly plays on the Madelung energy, it is in favor of an increase of the concentration of I molecules in agreement with ESR measurements.17 At the transition, a weak cell-volume contraction (∆V/V ≈ 0.3%) is observed, resulting from the jump of most of the cell parameters (Figure 4), although these do not all decrease. Only the cell parameter a decreases at the transition with a relative variation of ∆a/a ≈ 0.5%, while the parameters c and β increase with less relative amplitude (∆c/c ≈ 0.2% and ∆β/β ≈ 0.4% at 500 MPa), with the parameter b remaining almost constant within the error bars. In agreement with the local spectroscopic information,15,17 these results show that the N-I phase transition in (BEDT-TTF)(ClMeTCNQ) corresponds to the transformation of the almost regular N chains into chains made of short ionic dimers (contraction along a), but with a reduction of the interchain Coulomb interaction by increasing the c cell param-

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Figure 4. Evolution with temperature of the cell parameters at 500 MPa (sample 1, D10 ILL). At each temperature, the cell parameters of sample 1 were obtained by the refinement of the position of 25 Bragg peaks measured, with an incident wavelength of 1.2566(2) Å.

Figure 5. Longitudinal scan through the (030) Bragg reflection above (O at 200 K) and below (9 at 100 K) the N-I phase transition at 650 MPa (sample 2, 4F1 at LLB, λ ) 4.05 Å).

eters and an opening of the monoclinic β angle. On the other hand, the appearance of (0k0): k ) 2n + 1 Bragg peaks (Figure 5) without any other additional Bragg spots demonstrates that the twofold screw axis is lost in the I phase without any modification of the translational symmetry. The phase transition is then ferroelastic, corresponding to a deformation of the unit cell with a change of space group from P21, Z ) 2, in the N phase to P1, Z ) 2, in the I phase. It takes place at the center of the Brillouin zone, meaning that the structural reorganization associated with the CT and the dimerization take place in phase from one unit cell to the others. Because of the loss of the twofold screw axis at the transition, the order parameter has a B symmetry. It is responsible for the creation of a dipole moment lying in the plan (a, c), coming in addition to the one carried by the substituted TCNQ molecule. Although breaking of the ferroelastic symmetry occurs, no splitting of Bragg peaks,

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Figure 6. Evolution at 540 MPa of the peak intensity of the (030) reflection (0) and the quantity c sin β (b for cooling, O for heating) as a function of the temperature (sample 1, 4F1 at LLB, λ ) 4.05 Å). Figure 8. Pressure-temperature phase diagram of (BEDT-TTF)(ClMeTCNQ) down to the quantum limit, obtained by neutron diffraction (D10 at ILL and 4F1 at LLB) on two different single crystals (sample 1, circles; sample 2, squares) under compressed helium. Transition points, corresponding to heating or cooling, are respectively represented by close or open symbols. The black lines are guides for I the eyes. They represent the metastability limits, the upper one, Tlim for the I phase (solid line), not depending on the sample in contrast to N the lower one, Tlim for the N phase. The gray lines correspond to the transition line obtained by conductivity and ESR techniques, while the gray dotted lines give the limits of coexistence of N and I species in the limit 10-90%.17

Figure 7. Evolution at 540 MPa of the quantity c sin β (b) and of the full width at half-maximum (fwhm: 4) of the (004) Bragg reflection when crossing the N-I transition (sample 1, 4F1 at LLB, λ ) 5.7 Å).

due to a possible deviation of the R or γ angles from 90° in the triclinic space group, has thus far been observed even using the best reciprocal-space resolution. Whatever the pressure of our investigations, the N-I transition of (BEDT-TTF)(ClMeTCNQ) always takes place without thermal hysteresis, as can be noticed from the variation, on cooling or heating, of the quantity c sin β, which is directly related to the position of the (004) Bragg peak (Figure 6). The jump of the lattice parameters (Figure 4) is a direct signature of the first-order nature of the phase transition, and the two phases coexist over a narrow temperature range. This phenomenon is clearly demonstrated by a thorough analysis versus temperature of the evolution of the (004) Bragg peak across the transition, working at high resolution (Figure 7). Besides the evolution in position in the reciprocal space, the broadening of the (004) Bragg peak at the transition can then be observed at the crossing of the transition. The observed (004) Bragg peak actually corresponds to the superposition of the (004) reflections issuing from N and I domains coexisting on a macroscopic scale. This indicates the first-order character of the transition. The variation of the (030) Bragg peak intensity (Figure 7) results from both the growth of the I phase into the N one and the saturation of the nonzero-order parameter in the I phase, when moving away from the phase transition. This is the first time that the first-order character of the phase transition in (BEDTTTF)(ClMeTCNQ) is evidence. Indeed, the previous investigations analyzed this transition as an almost continuous one16 or as a crossover.17

3.3. Phase Diagram Down to the Quantum Limit. The (P, T) phase diagram of (BEDT-TTF)(ClMeTCNQ) (Figure 8) reported here complements those determined from conductivity16 and ESR17 measurements, here with data in the low-temperature region. It shows new features radically different from those of any other mixed-stack CT complexes exhibiting the N-I transition, either of TTF-CA type or from solid-solution based on chemically substituted D or A molecules.12 The first thing to notice is that our results do not strictly extrapolate from the previous results, which were obtained from heating measurements.16,17 The main difference concerns the line of transition itself, which differs clearly in the low-pressure part of the phase diagram but tends to join at high pressure. This discrepancy in absolute pressure values at a given temperature can be easily explained by the fact that the helium high-pressure setup used for neutron scattering allows a direct measurement of the applied pressure on the sample, while the clamp techniques need an a posteriori calculation of the pressure to take into account the losses induced by the cooling and solidification of the liquid transmitter. Moreover, the use here of a unique single crystal prevents any pressure-gradient effects. As expected, the effect of pressure stabilizes the ionic phase by playing on the Madelung contribution in the energy balance. The N-I phase transition goes down to absolute zero in temperature for a moderate pressure around 200 MPa with an infinite slope (dTN-I/dP) at 0 K, which is in agreement with the Nernst principle. In the quantum limit, the ferroelectric interactions here are sufficient to overcome the quantum charge fluctuations and give rise to a polar ionic fundamental state at high pressure. The unit-cell parameter jumps diminish until they vanish around 200 MPa (Figure 9), indicating that the transition becomes second-order at 0 K. In the classical regime, i.e., above 300 MPa, the (dTN-I/ dP) positive slope is steeper than in the case of TTF-CA21 and the N phase exists over a much wider (P, T) range. Simple geometrical considerations may help to understand this situation: the HOMO orbital of the BEDT-TTF donor molecule

N-I Phase Transition of (BEDT-TTF)(ClMeTCNQ)

J. Phys. Chem. B, Vol. 111, No. 22, 2007 6171 hysteresis phenomena of first-order phase transitions. It can be understood as follows: the polar order of the N phase, due the dipole moment on the ClMeTCNQ molecules, tends to stabilize this neutral phase at low temperature. Such a phenomenon will be related to the size of the polar-ordered zones in the N phase and therefore will directly depend on the sample, the size of the ordered zones depending on the crystal growth conditions. On the contrary, the fact that the temperature of transition from the I phase to the N phase does not depend on the sample gives an indication that the A dipole ordering weakly modifies the total energy of the I phase. Then, the transition from N to I remains dominated by the mechanism of the classical neutralionic transition. 4. Conclusions

Figure 9. Temperature evolution of the quantity c sin β at different pressures: 9, 200 MPa; O, 300 MPa; b, 400 MPa (sample 1, 4F1 at LLB, λ ) 4.05 Å).

extends out to the most external sulfur atoms. It overlaps in a much less favorable way the LUMO orbital of the ClMeTCNQ acceptor molecule than in the case of TTF-CA, where donor and acceptor molecules are much smaller. For the charge transfer to take place requires a sufficient overlap, which can be achieved here only by a much closer packed geometry, as can be seen by comparing the values of the cell parameter a at the transition. Although the complex DMTTF-QBr412 has a very similar phase diagram to that of (BEDT-TTF)(ClMeTCNQ), their comparison is not straightforward, since the packing and therefore the interstack interactions are radically different. On the other hand, the evolution of the cell parameter c jump at the transition does not indicate any tendency toward any critical point at very high pressure, in contrast to the case of TTF-CA. Indeed, above 60 K, the (004) Bragg peak is always broadened (Figure 6), characteristic of the coexistence at a macroscopic scale of the N and I phases, and is accompanied by an increase of ∼0.023 Å in the c sin β parameter, whatever the pressure between 300 and 650 MPa. This phenomenon, also observed for the cell parameter b, is independent of the crystal and defines a highpressure region where the modification of the interchain interactions at the transition are weakly pressure dependent. A similar study versus pressure and temperature on the cell parameter a would be essential to analyze in a similar way the evolution of the 1D intrachain interactions across the transition, but it requires that new large single crystals would be available for further neutron scattering experiments. In this high-pressure range, the N-I phase coexistence, over length scales characteristic of long-range order, extends over a few degrees (Figures 7 and 8). This discrepancy with the conductivity16 and ESR results,17 which evidence the coexistence of molecules in N and I state over a wider range of temperature, can be explained by the fact that these techniques are sensitive to the modification of the electronic state at the molecular scale, while the diffraction techniques see phenomena at the scale of several unit cells. On the other hand, the metastability limit of the N and I phases, respectively, TNlim and TIlim, have very different behavior versus pressure and depend strongly on the sample. Both increase with pressure, but if TIlim is sample-independent, TNlim strongly depends on the sample: for sample 1, it is only few degrees lower than TIlim, and this difference is almost pressure-independent, while for sample 2, the difference between TIlim and TNlim drastically increases with pressure, leading to an overstabilization of the N phase under pressure. This phenomenon is intriguing and differs from the conventional behavior of

In this study, we have shown that the (BEDT-TTF)(ClMeTCNQ) is a rather more complex case of the N-I transition than all those already discussed in the literature. Indeed, it is the first example of a mixed-stack CT complex based on a noncentrosymmetric acceptor which already shows in the neutral phase a polar order, based here on the orientational ordering of the dipole moment lying on the chlorine-methyl group axis. In comparison to the case of other N-I complexes, the neutral phase of (BEDT-TTF)(ClMeTCNQ) is stabilized because the important modifications of the crystal structure do not favor the overlap between the HOMO donor orbital with the LUMO acceptor orbital. Therefore, the neutral-ionic phase transition can only take place under pressure, joining the quantum limit at around 200 MPa with second-order character. In the classical regime, at high temperature, the phase transition is always first-order with, on one hand, a well-marked coexistence of N and I phases with long-range order over only a few degrees and, on the other hand, metastability phenomena which can extend over several tens of degrees and are strongly sample dependent. This behavior is due to the polar ordering of the ClMeTCNQ molecules, which stabilizes the N phase, while this effect becomes negligeable at the transition from the I to the N phase. Because of the competing interactions, and in particular the different dipolar ones ((ClMeTCNQ) polar ordering, dipolar interaction associated with the dimerization process), the N-I transition of (BEDT-TTF)(ClMeTCNQ) remains first-order always, even at the highest pressure, contrary to any other N-I transition. These results encourage a complete crystallographic investigation on both sides of the N-I phase transition. Indeed, never considered, the subtle new ordering evidenced in this compound now requires more complete structural investigations with the aim toward understanding in a more detailed way the mechanism of the phase transition. Acknowledgment. The authors are grateful to Louis Me´le´si (ILL) and Franc¸ ois Maignan (LLB) for high-pressure technical support and to G. J. McIntyre for fruitful discussion. This project was partially supported by a CEA grant. References and Notes (1) Emery, V. J.; Kivelson, S. A.; Tranquada, J. M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 8814-8817. (2) Kivelson, S. A.; Bindloss, I. P.; Fradkin, E.; Oganesyan, V.; Tranquada, J. M.; Kapitulnik, H. C. ReV. Mod. Phys. 2003, 75, 12011241. (3) Nagaosa, N.; Takimoto, J. I. J. Phys. Soc. Jpn. 1986, 55 (8), 27452753. (4) (a) Colossal magneto-resistiVe oxides; Tokura, Y., Ed.; Gordon and Breach Science Publishers: Basingstoke, U.K., 2000. (b) Colossal magnetoresistance, charge ordering and related properties of

6172 J. Phys. Chem. B, Vol. 111, No. 22, 2007 manganese oxides; Rao, C. N. R., Raveau, B., Eds.; World Scientific: Singapore, 1998. (5) Monceau, P.; Nad, F. Ya.; Brazovskii, S. Phys. ReV. Lett. 2001, 86 (18), 4080-4083. (6) (a) Koshihara, S.; Tokura, Y.; Takeda, K.; Koda, T. Phys. ReV. Lett. 1992, 68, 1148-1151. (b) Relaxations of excited states and photoinduced phase transitions; Nasu, K., Ed.; Springer Series in Solid State Sciences, Vol. 124; Springer: Berlin, 1997. (7) (a) Torrance, J. B.; Vazquez, J. E.; Mayerle, J. J.; Lee, V. Y. Phys. ReV. Lett. 1981, 46, 253-257. (b) Torrance, J. B.; Girlando, A.; Mayerle, J. J.; Crowley, J. I.; Lee, V. Y.; Batail, P.; LaPlaca, S. J. Phys. ReV. Lett. 1981, 47, 1747-1750. (8) Girlando, A.; Marzola, F.; Pecile, C.; Torrance, J. B. J. Chem. Phys. 1983, 79, 1075-1085. (9) Mitani, T.; Saito, G.; Tokura, Y.; Koda, T. Phys. ReV. Lett. 1984, 53 (8), 842-845. (10) Le Cointe, M.; Leme´e-Cailleau, M. H.; Cailleau, H.; Toudic, B.; Toupet, L.; Heger, G.; Moussa, F.; Schweiss, P.; Kraft, K. H.; Karl, N. Phys. ReV. B 1995, 51, 3374-3386. (11) Collet, E.; Buron-Le Cointe, M.; Leme´e-Cailleau, M. H.; Cailleau, H.; Toupet, L.; Meven, M.; Mattauch, S.; Heger, G.; Karl, N. Phys. ReV. B 2001, 63, 054105. (12) Horiuchi, S.; Okimoto, Y.; Kumai, R.; Tokura, Y. Science 2003, 299, 229-232. (13) Horiuchi, A.; Okimoto, Y.; Kumai, R.; Tokura, Y. J. Am. Chem. Soc. 2001, 123, 665-670.

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