Spectroscopic Studies of the Phase Transition from the Mott

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Spectroscopic Studies of the Phase Transition from the Mott Insulator State to the Charge-Ordering State of κ‑(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII and FeIII) Salts † ́ Andrzej Łapiński,*,† Roman Swietlik, Lahcène Ouahab,‡ and Stéphane Golhen‡ †

Institute of Molecular Physics, Polish Academy of Sciences, ul. Mariana Smoluchowskiego 17, 60-179 Poznań, Poland Organométalliques: Matériaux et Catalyse UMR 6226 CNRS-UR1 Institut des Sciences Chimiques de Rennes, Université de Rennes 1, 35042, Rennes Cedex, France

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ABSTRACT: Polarized reflectivity spectra versus temperature of two isostructural charge-transfer salts κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII and FeIII) (ET = bis(ethylenedithio)tetrathiafulvalene) were studied. The electronic and vibrational spectra exhibit a drastic change at around 150 K. On the basis of the spectral analysis, we deduced the nature of the phase transition. The phase transition at 150 K is due to a charge ordering; above this temperature, strong charge fluctuations are observed.

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INTRODUCTION There is a great interest in the design of organic conductors formed by the organic donors derived from tertrathiafulvalene (TTF) molecule, especially the bis(ethylenedithio)tetrathiafulvalene (ET), and various anions with permanent magnetic moments.1−4

role; they have a considerable influence on conducting, optical, and magnetic properties.7−9 The long-range Coulomb interactions are responsible for charge-ordering phenomena that are observed in many TTF based one- and two-dimensional organic conductors. In the field of charge-transfer salts, which are quasi-one- or quasi-two-dimensional organic salts, there has always been a lot of interest in the charge-ordering (CO) phenomena.10−13 To understand the mechanism of the charge localization, Seo and Fukuyama have carried out mean-field calculations.14,15 They have proposed that a stripe-patterned charge ordering is stabilized in the insulating phase owing to intermolecular Coulomb repulsive forces. Moreover, Tajima et al.16 have adopted the spectral analysis and the mean-field calculations to estimate the charge-ordering patterns in ET salts. Within the large family of two-dimensional ET conductors, the κ-phase salts, in which conducting layers are formed by ET dimers perpendicular to each other, are especially interesting. In the presence of strong on-site Coulomb repulsive interaction, U

Scheme 1. ET molecule

In such magnetic molecular conductors, the conducting and magnetic properties are associated with two individual networks; these networks either can exist independently or can interact mutually, leading to novel physical properties of the material. The most important is a possibility of the interaction between π electrons in conducting organic layers and localized magnetic d electrons in the counterions (π−d interaction), which can be responsible for interesting physical properties.4−6 It is well-established that, in organic conductors, Coulomb interactions between electrons play an important © XXXX American Chemical Society

Received: February 27, 2013 Revised: May 23, 2013

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dx.doi.org/10.1021/jp402027x | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Polarized reflectance spectra of κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII and FeIII) salts obtained from the (001) crystal face at T = 280 and 20 K, for the electrical vector of the incident beam parallel and perpendicular to the b axis. Note the logarithmic wavenumber scale.

intradimer interactions between ET molecules in (1) and (2) are weaker than in other typical κ-ET salts. Moreover, in (1) and (2), interdimer interactions are stronger than intradimer, which is also opposite to the typical κ-ET salts. Recently, comprehensive IR studies of several κ-phase ET salts have shown the absence of charge ordering.34 Here, we report on detailed IR investigations of this scarce phenomenon in κ-ET salts (1) and (2). The spectroscopy methods, especially within the infrared region, are very useful for investigations of crystalline organic conductors, providing information about the electronic structure and electron−electron and electron−phonon interactions.35 Among various methods applicable to the research of the charge ordering,36,37 the vibrational spectroscopy is one of the most powerful methods.38 The charge disproportionation (CD) ratio can be easily estimated since the frequencies of the CC stretching modes of ET sensitively shift depending on the charge density on the molecule. Some preliminary spectroscopic investigations of (1) and (2) have been carried out by us and reported in refs 31 and 32. In the present paper, polarized infrared spectra of (1) and (2) are measured to study the CD effect in the insulating phase. On the basis of these results, from the viewpoint of the charge localization caused by Coulomb interactions, we discuss the nature of phase transition observed at around 150 K.

(compared with the kinetic energy represented by the bandwidth, W), the κ-phase ET salts can be regarded as a half-filled system and they can be treated as a very good model system for studying the Mott insulator state. The possibility of charge disproportionation in ET systems with a half-filled band is of particular interest and has been intensely investigated,17−24 with particular emphasis on the κ-phase salts.25−33 It should be stressed here that the charge-ordering phenomena in the κphase salts with an effectively half-filled band are very rarely observed; only a few salts are reported, such as κ-(ET)4PtCl6· C6H5CN, the triclinic κ-(ET)4[M(CN)6][N(C2H5)4]·3H2O, and the monoclinic κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (with M = CoIII, FeIII, and CrIII) salts. All of these compounds are two-dimensional electron systems with considerably strong electronic correlations. Recently, a new phase of κ-ET salts with Co(CN)63− and Fe(CN)63− anions, containing also N(C2H5)4+cations and H2O molecules, was synthesized, namely, the isostructural monoclinic salts κ-(ET)4[Co(CN)6][N(C2H5)4]·2H2O (1) and κ(ET)4[Fe(CN)6][N(C2H5)4]·2H2O (2). The crystallographic studies have been performed at different temperatures at about 100, 200, and 293 K.32 Along the c axis, conducting layers (ET molecules) alternate with insulating layers, which consist of M(CN)63− anions, N(C2H5)4+ cations, and H2O molecules. Considerable interactions among anions exist along the b axis, through short intermolecular atomic contacts. In the conducting ab plane, two crystallographically independent ET molecules, noted “ETA” and “ETB”, are arranged in two centrosymmetric dimers, ETA-ETA and ETB-ETB, which are mutually perpendicular, creating the κ-type packing pattern. For the crystallographically independent ET molecules, different environments are observed. The detailed crystallographic data are given in ref 32. At ambient temperature, both salts exhibit rather poor conductivity and large paramagnetic susceptibility. Both are in the Mott insulating state and undergo very similar charge-ordering phase transitions at TCO = 150 K.32 Above 150 K, the charge is distributed uniformly among ET molecules (ETA+0.5ETA+0.5, ETB+0.5ETB+0.5), and below 150 K, a charge pattern (ETA+1ETA+1, ETB0ETB0) is observed. A characteristic feature is that, above 150 K, strong fluctuations of charge density distribution are seen. From crystallographic studies, the

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EXPERIMENTAL SECTION Single crystals of the isostructural title salts were obtained by an electrochemical technique. The crystals were grown on a Pt electrode (1 mm in diameter) in a U-shaped cell with a sintered-glass filter by oxidation of ET in the presence of [N(C2H5)4]3[M(CN)6] (M = CoIII or FeIII) in a mixture of acetonitrile and dichloromethane (1:4) as electrolyte.32 The reflectance spectra were recorded from 650 to 12 000 cm−1 for single crystals of κ-(ET)4[M(CN)6][N(C2H5)4]·H2O (M = CoIII, FeIII) salts, which had a smooth, shiny, and flat reflecting surface with dimensions of approximately 0.7 × 0.3 × 0.02 mm. The best-developed crystal face corresponds to the crystallographic ab plane. For samples of this size, we were able to obtain polarized reflectance spectra of a good quality. The B



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Figure 2. Temperature dependence of the conductivity spectra of κ-(ET)4[CoIII(CN)6][N(C2H5)4]·2H2O. Dotted lines show the decomposition of the charge-transfer band into two components. Note the logarithmic wavenumber scale.

Figure 3. Temperature dependence of the conductivity spectra of κ-(ET)4[FeIII(CN)6][N(C2H5)4]·2H2O.

mid-infrared spectra (650−7000 cm−1) were measured by use of a 77 K MCT detector and a KBr beamsplitter, whereas the near-infrared spectra (4000−12000 cm−1) were measured by use of an InSb detector working at liquid nitrogen temperature (77 K) and a Quartz beamsplitter. The experimental data were collected using a Bruker Equinox 55 FT-IR spectrometer equipped with a Hyperion 1000 microscope with the spectral resolution of 2 cm−1 at temperatures between 10 and 290 K. The reflectance spectra were measured from the (001) crystal face in two perpendicular polarizations: the electrical vector of the polarized IR beam was either parallel or perpendicular to the b crystallographic axis. These two directions correspond to the maximum (E//b) and the minimum (E⊥b) of the reflected energy, respectively. The samples were cooled using a continuous-flow helium cryostat manufactured at Oxford Instruments. The cooling rate was less than 1 K min−1. The absolute values of reflectivity were obtained by comparison with

an aluminum mirror. By comparing the mid-infrared and nearinfrared spectra in the region of overlap, we have found errors as high as 2%. Optical conductivity σ(ω) spectra were obtained by Kramers−Kronig analysis of reflectance data. The lowfrequency data were extrapolated to zero frequency, assuming a constant value, the typical extrapolation for semiconducting and isolating materials. Above the highest frequency point, the reflectance data were extended up to 28 000 cm−1 on the basis of spectra of the salt κ-(ET)2[Cu(NCS)2],39 then as R ∼ ω−2 and above 106 cm−1 as R ∼ ω−4. A standard PEAKFIT computer program was used for spectral analysis of vibrational and electronic features.

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RESULTS AND DISCUSSION Polarized reflectance spectra of the studied isostructural salts at T = 280 and 10 K for polarizations parallel and perpendicular to the b axis are presented in Figure 1. The spectra of both salts C



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Figure 4. Temperature dependence of the wavenumber and intensity of the band B observed in the spectra of κ-(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII and FeIII) salts (see Figures 2 and 3).

transfer transition can be coupled with molecular vibrations, whereas the interdimer charge-transfer transitions produce electronic transport and dc conductivity. Analogously, Visentini et al.47 assigned the band B to the intradimer charge-transfer transition between molecules with charge +1; they have shown that the band at about 7000 cm−1 maximizes for the direction perpendicular to the ET molecular planes. For (1) salt, Ota et al.32 have calculated the electronic band structure using the tight-binding model based on the extended Hückel method excluding electron correlation. They have shown that HOMO bands substantially split because of the dimerized structure in the ET layers, and an average charge of +0.5 per ET leads to a half-filled upper HOMO band, whereas the Fermi surface has a 2D feature with hole-pockets centered at X and electron-pockets centered at C(V). Nevertheless, (1) and (2) behave as Mott insulators; that is, the electron correlation should be taken into account. For the salts in the CO state, if we do not neglect the electron correlations, we are not able to get the Fermi surface. Recently, the study of the physical properties of highly correlated electron systems using mean-field theory has been carried out.48−50 To understand the properties of κ-phase ET salts, Kino and Fukuyama51−53 considered a triangular lattice of ET dimers. The carriers, which move between the dimers’ “lattice sites”, define the two-dimensional metallic properties. However, in the Mott-insulating state, the strong electronic repulsion immobilizes and eventually confines them. The other types of carriers are localized on the dimer “lattice sites”. The interdimer overlap integrals define the hopping between the sites, whereas the intradimer overlap is related to the effective on-site Coulomb interaction Ueff. We propose that the observed electronic feature A response for (1) and (2) is explained by two contributions: charge transfer inside the dimer “lattice sites” (at about 3500 cm−1) and interdimer charge transfer by correlated charge carriers (at about 2500 cm−1). Rice,54 and Yartsev and co-workers55−58 have shown that the electronic band with a maximum at around

are very similar. They show a broad electronic dispersion and a series of vibrational features, qualitatively similar to spectra of other ET-based organic conductors, especially κ-phase ET.28,29,34,39−44 The small spectral differences for two polarizations indicate small electronic anisotropy within the ET conducting layers, as usually observed for the κ-phase ET salts. Both electronic and vibrational parts of the spectra undergo considerable changes due to the charge-ordering transition; these changes are more pronounced for (1). Electronic Spectra. Figures 2 and 3 show the optical conductivity spectra obtained by a Kramers−Kronig transformation of the reflectance data for (1) and (2). The spectra suggest the existence of an optical energy gap; this is in agreement with electrical conductivity measurements.32 Below 2000 cm−1, one observes vibrational features superimposed on the electronic absorption band centered at about 3000 cm−1 (band A). In the optical conductivity spectra, apart from the electronic band A, which consists of at least two components, 2500, 3390 cm−1 (for 1) and 2500, 3540 cm−1 (for 2), another one (band B) is also found for (1) at about 7200 cm−1 and for (2) at 6050 cm−1. One can attribute these bands to intermolecular electronic transitions between neighboring ET molecules; the band A corresponds to the charge-transfer process ET0 + ET+ → ET+ + ET0 (CT2 transition), whereas the band B is attributed to the chargetransfer process ET+ + ET+ → ET2+ + ET0 (CT1 transition). Such interpretation of these bands can be performed using the model proposed by Hubbard.45 Within this model, the position of band A depends on the Coulomb repulsion energy between two electrons on the adjacent molecules (V), and the hopping integrals, whereas the position of band B can be approximately described by the value of the effective on-site Coulomb interaction (U). For κ-phase organic conductors, the broad electronic feature in the mid-infrared (band A), can be associated with two types of excitations: within and between the dimers observed at about 3450 and at 2000 cm−1, respectively.46 The intradimer chargeD



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Figure 5. Temperature dependence of the conductivity spectra of κ-(ET)4[CoIII(CN)6][N(C2H5)4]·2H2O within the region of strong vibrational features.

3500 cm−1 is due to the charge transfer within a dimer coupled to Ag vibrations of the ET molecule, whereas the transition between the dimers observed at about 2500 cm−1 is due to transitions between the Hubbard bands formed by the correlated conduction electrons.59−62 The analysis that permits one to disentangle the intradimer and interdimer carrier contributions is reported in refs 44, 46, and 63. Moreover, we have analyzed the temperature evolution of the position and integral intensity of the band B (see Figure 4), which is a consequence of the charge-ordering phenomenon. Below 150 K, the intensity of the band at about 7000 cm−1 strongly grows on cooling down, though, for E⊥b, it is clearly seen that, even above 150 K, this band still exists. It means that, even at room temperature, charge density fluctuations are so strong that some ET dimers with a total charge of +1 are presentthese are relatively short-lived dimers ETA+ETA+. In (2), the charge fluctuations are expected to be stronger than in (1) since the band B is much broader. Imada and collaborators64 have performed an ab initio study of κ-phase materials applying a single-band extended Hubbard model, and they showed that charge fluctuations can be enhanced by the interdimer Coulomb interaction. Besides A and B bands in the electronic spectra of (1) and (2), one can also find a band observed at about 10 000 cm−1 (band C). It is known that, for TTF derivatives, the electronic transitions of a typical electron donor−acceptor complex fall into two classes: those at high energies generally result from excitations of single molecules and are called localized excitations, and the lower-energy bands are called chargetransfer bands. We attribute the band C to the transition from next-highest occupied molecular orbital (HOMO) to HOMO intramolecular (excitonic) of ET.65 Vibrational Spectra. The ET molecule has 26 atoms, hence 72 vibrational degrees of freedom. For the classification of its normal modes, a planar D2h molecular symmetry has been adopted by Kozlov et al.66,67 and Eldridge et al.68 In the solid state, ET molecules are never flat and their symmetry is lowered due to deformation of the CH2-CH2 end groups. An ionized ET can be either staggered (D2 molecular symmetry) or eclipsed (C2h symmetry).69 The equilibrium geometry of a

neutral ET has a boat conformation (C 2 symmetry). Correlations between the spectral predictions based on D2h and on D2 symmetry can be easily made, the difference being associated with the lack of inversion center (Ag and Au become A, B1g and B1u become B1, B2g and B2u become B2, B3g and B3u become B3. Moreover, A and B1 modes in D2 correlate with A in C2, and B2 and B3 in D2 with B in C2). It is well-known that totally symmetric modes of ET molecules can couple with the CT transition through the energy modulation of the frontier MOs.56−58,70 Because of the coupling, the totally symmetric modes, normally inactive in IR, borrow intensity from the nearby CT electronic transition and occur at a frequency lower than the corresponding Ramanactive modes. Therefore, the IR spectra of the ET salts are characterized by the occurrence of very strong bands, polarized perpendicularly to the molecular planes, like the CT transition. Assuming a planar (D2h) point group symmetry, 12 vibrational modes of Ag symmetry can be activated by the electron− molecular vibration interaction;66 when one takes into account the D2 symmetry, it will be 19 totally symmetric modes.71 The unequal charge distribution between the ET molecular units can be conveniently characterized through vibrational spectroscopy, providing a proper identification and calibration of the charge sensitive normal modes.72−75 The degree of ionicity, or average charge per molecule (ρ), is one of the fundamental parameters characterizing the physical properties of CT salts.76 When Coulomb interaction prevails, ET salts may undergo a charge order (CO) instability, and vibrational spectroscopy is one of the most convenient methods to investigate such phase transition. The three CC stretching modes Ag ν2, Ag ν3, and B1u ν27 in the D2h molecular symmetry (in D2 symmetry, these modes correlate with A ν3, A ν4, and B1 ν22, respectively) exhibit the largest ionization frequency shift (120−130 cm−1).34,74,75 They have been the first ones to be proposed for the determination of ρ in ET,74 and the linear dependence of Ag ν2 and B1u ν27 modes against charge density has been experimentally verified. However, totally symmetric modes are coupled to the electronic system, and hence their positions in IR spectra are usually shifted toward lower frequencies;75 due to this coupling, E



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Figure 6. Temperature dependence of an expanded portion of the conductivity spectra of κ-(ET)4[FeIII(CN)6][N(C2H5)4]·2H2O within the region of strong vibrational features.

it, one can find at 1167 cm−1 the band related to the B3u ν67 mode. The phase transition at 150 K induces considerable modifications of the spectral feature related to the Ag ν3 mode, which becomes weaker and shifts toward the higher frequencies. Additionally, due to the charge-ordering phase transition, some new bands related to the fully ionized and neutral ET molecules appear in the IR spectra. One of the most important spectral changes is the appearance of the new bands at 1347 and 1289, 1297 cm−1 related to the Ag ν3 and Ag ν5 modes of the ET+1 cation, which are activated by coupling with the charge-transfer transition CT1. For the salt containing (ET+)2 dimers, these modes have been found: Ag ν3 = 1343 cm−1,47 Ag ν5 = 1284 cm−1,47 and 1294 cm−1.67 The Ag ν3 mode of ET+ is not observed above 150 K since the time of life of (ET+)2 dimers is too short to activate this mode by the e− mv coupling process; it appears only when these dimers are stable. Therefore, this band at 1347 cm−1 is a good indicator of the charge-ordering transition. Other modes useful for the determination of ρ in ET salts are B1 ν29 and B2 ν44 (for D2 molecular symmetry).71 These modes are related to the breathing of the penta-atomic tetrathiafulvalene rings, and they exhibit an ionization frequency shift of about 40 cm−1. For the investigated salts, the B1 ν29 mode is difficult to identify because its oscillating transition dipole moment is directed along the ET molecule; this direction is perpendicular to the most developed crystal face. The B2 ν44 mode can be assigned to weak bands observed in the experimental spectra of (1) and (2) salts at 863 cm−1 (for ET0), at 875 cm−1 (for ET0.5+), and at 900 cm−1 (for ET+). The position of these bands are in good agreement with calculated values of 864 (for ET0), 876 (for ET0.5+), and 903 cm−1 (for ET+) given by Girlando in ref 71. The 890 cm−1 mode, assigned as B3g ν60 in the D2h symmetry68 and as A ν10 in the D2 symmetry,71 has also attracted great attention as a spectral feature that is so sensitive to the charge disproportion and influence of the anion effect in ET salts.69 Moreover, the 890 cm−1 “ring breathing mode” can be coupled to the electronic system when we consider this mode as a totally symmetric mode of a distorted ET molecule, not a perfectly flat one (D2h).71 This mode appears in the

the frequencies of molecular modes may exhibit a nonlinear dependence on ρ.73,75 The e−mv coupling plays also a role in the CO instabilities, as the modulation of the frontier MO, pushing charges back and forth, will ultimately provoke their localization on the molecular sites.71 Figures 5 and 6 show the temperature dependence of the spectra of (1) and (2) recorded parallel and perpendicular to the b axis. These spectra change strongly between 130 and 160 K. This observation indicates the phase transition at about 150 K. The spectra of (1) and (2) are very similar to the results obtained for other κ-type ET salts.28,29,40−42 The most intensive feature in room-temperature spectra of protonated κ-ET salts is the broad resonance between 1100 and 1300 cm−1 (for E//c) and between 1200 and 1350 cm−1 (for E//b).40−43 The position of this feature is related to a totally symmetric ET vibration involving the central CC bond (Ag ν3 mode), and it is obscured by the Fano-type antiresonances with other intramolecular modes. For example, for κ-(ET)2[Cu(SCN)2], for E//c, the dip at 1272 cm−1 becomes four strong antiresonances at 1261, 1276, 1284, and 1294 cm−1, whereas for E//b, the same unresolved dip at 1277 and 1285 cm−1 becomes resonances at 1259, 1274, 1282, and 1292 cm−1.40 In the case of investigated salts, one can find a broad and strong band related to the Ag ν3 mode in the spectral region from 1150 to 1350 cm−1 (for E//b) and 1200−1350 cm−1 (for E⊥b). For this mode, due to the electron molecular vibration coupling, a large low-frequency shift from the calculated frequency of 1427 cm−167 is observed. This shift is larger for E//b (∼200 cm−1 at 280 K) than for E⊥b (∼150 cm−1 at 280 K) because, for E//b, the electronic A band lies closer to corresponding Raman-active modes. The shift and the intensity of the emv-coupled features depend on the size of coupling constant gα and the position of the respective charge-transfer band. In room-temperature spectra, antiresonance dips are observed at 1279 and 1277 cm−1 for (1) and 1280 and 1284 cm−1 for (2) related to antiresonance with the vibration of ethylene groups (mode Ag ν5). At low temperature, spectra instead of these dips the bands at 1280, 1289, and 1297 cm−1 are appeared. Additionally, another dip is observed in both compounds at about 1172 cm−1. At low temperature, instead of F



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experimental spectra for the ET0.5+ at 878 cm−1,44 and for ET+ at 899 cm−1.47 In the optical conductivity spectra of investigated salts, one can find this mode at helium temperature at 868 cm−1 (for ET0), at 883 cm−1 (for ET0.5+), and at 894 cm−1 (for ET+). The frequency variation of the A ν10 mode versus temperature is presented in Figure 7. The intensity of this band strongly reduces, confirming an interaction of this mode with the CT2.

between donors and anions and their contributions to the phase transition. Figure 8 shows the temperature dependence of the modes related to CN triple bond vibrations (∼2100 cm−1) in M(CN)63− (M = CoIII and FeIII) anions and CH stretching in ET molecules (∼3400 cm−1). For these modes, the frequency dependences, associated with the 150 K phase transition, are observed. They reflect the sensitivity of the CN−CH 2 interaction to the phase transition. The interaction between the hydrogen atom and the CN group of the anions leads to a blue shift of the CN stretching frequency with decreasing temperature. The examination of the CH 2 stretching frequencies in ET salts containing different anions leads us to the conclusion that, in (2), the degree of donor−anion interaction is slightly smaller. This is concluded by means of the red shift of the C−H band for (1), which is due to the stronger hydrogen-bonding like interaction of the donor with the anion. These observations are consistent with the results of X-ray investigations performed for these salts; the intermolecular distances between the M(CN)63− anion and CH2 groups are shorter for (1) in comparison to (2).32

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CONCLUSION

In this paper, we have presented our detailed IR investigations of the charge-ordering transition at T = 150 K and charge fluctuations above 150 K in the two isostructural salts κ(ET)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII, FeIII), a very rare phenomenon in the κ-phase ET salts. In these compounds, a transition from the charge pattern (ETA+0.5ETA+0.5, ETB+0.5ETB+0.5) above 150 K to the charge pattern (ETA+1ETA+1, ETB0ETB0) below 150 K is found. In the phase above 150 K, we see a mid-IR electronic dispersion A centered at about 3000 cm−1, which is a superposition of interdimer (across a Mott gap) and intradimer charge-transfer transitions. As a consequence of the charge ordering, both electronic and vibrational spectra are seriously modified. The most important proof of the charge ordering is the appearance of the B electronic band at about 7000 cm−1 attributed to charge transfer in ET+1ET+1 dimers and also the vibrational band at 1347 cm−1 being the result of coupling of the CC mode of ET with this electronic

Figure 7. Temperature variation of the B3g ν60 (D2h) mode (A ν10 for D2 symmetry) (frequency and intensity).

From our previous investigations of (1) and (2) salts,32 it is known that the strong electron−electron correlation of πelectrons and the Coulomb interaction between ET and inorganic layers play an important role. The charge order pattern is parallel to the b axis, and it coincides with the arrangement of anion layers, where the chains of M(CN)63− and N(C2H5)4+ are also parallel to this direction.32 It is an important question about the role of electrostatic interactions

Figure 8. Temperature variation of the CN and C−H modes (frequency and intensity). G



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excitation. The charge density strongly fluctuates in the Mott insulting phase above 150 K; therefore, the band B of weaker intensity is also detected in this phase. Nevertheless, the time of life of ET+1ET+1 dimers is too short to activate the CC mode at 1347 cm−1. The band B in salt (2) is broader in comparison to that in (1), indicating thus stronger charge fluctuations in (2). The charge ordering has a strong influence on several vibrational features, especially those related to the CC stretching modes activated by coupling with electrons, which are seen in the range of 1150−1350 cm−1 above 150 K. These modifications and also the presence of the electronic band A suggest that, below 150 K, the charge is not fully separated between ET molecules, but there exist charge-rich and chargepure molecules. The modifications of bands related to C−H stretching indicate that, apart from the long-range Coulomb interactions between electrons, also the anions can have a significant influence on the formation of the charge ordered state.

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Studied by Vibrational Spectroscopy. Phys. Rev. B 2002, 65, 085110−085117. (13) Seo, H.; Merino, J.; Yoshioka, H.; Ogata, M. Theoretical Aspects of Charge Ordering in Molecular Conductors. J. Phys. Soc. Jpn. 2006, 75, 051009−051028. (14) Seo, H.; Fukuyama, H. Frustrated Spin System in θ-(BEDTTTF)2RbZn(SCN)4. J. Phys. Soc. Jpn. 1998, 67, 1848−1851. (15) Seo, H. Charge Ordering in Organic ET Compounds. J. Phys. Soc. Jpn. 2000, 69, 805−820. (16) Tajima, H.; Kyoden, S.; Mori, H.; Tanaka, S. Estimation of Charge-Ordering Patterns in θ-ET2MM′(SCN)4 (MM′=RbCo, RbZn, CsZn) by Reflection Spectroscopy. Phys. Rev. B 2000, 62, 9378−9385. (17) Hotta, C. Classification of Quasi-Two Dimensional Organic Conductors Based on a New Minimal Model. J. Phys. Soc. Jpn. 2003, 72, 840−853. (18) Hotta, C. Quantum Electric Dipoles in Spin-Liquid Dimer Mott Insulator κ-ET2Cu2(CN)3. Phys. Rev. B 2010, 82, 241104−241107. (19) Naka, M.; Ishihara, S. Electronic Ferroelectricity in a Dimer Mott Insulator. J. Phys. Soc. Jpn. 2010, 79, 063707−063710. (20) Gomi, H.; Imai, T.; Takahashi, A.; Aihara, M. Purely Electronic Terahertz Polarization in Dimer Mott Insulators. Phys. Rev. B 2010, 82, 035101−035107. (21) Li, H.; Clay, R. T.; Mazumdar, S. The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band. J. Phys.: Condens. Matter 2010, 22, 272201−272207. (22) Dayal, S.; Clay, R. T.; Li, H.; Mazumdar, S. Paired Electron Crystal: Order from Frustration in the Quarter-Filled Band. Phys. Rev. B 2011, 83, 245106−245117. (23) Clay, R. T.; Dayal, S.; Li, H.; Mazumdar, S. Beyond the Quantum Spin Liquid Concept in Frustrated Two Dimensional Organic Superconductors. Phys. Status Solidi B 2012, 249, 991−994. (24) Hotta, C. Theories on Frustrated Electrons in Two-Dimensional Organic Solids. Crystals 2012, 2, 1155−1200. (25) Doublet, M.-L.; Canadell, E.; Shibaeva, R. P. Concerning the First-Order Transition in the κ-Phase (BEDT-TTF)4PtCl6 C6H5CN. J. Phys. I (France) 1994, 4, 1479−1490. (26) Le Maguerès, P.; Ouahab, L.; Conan, N.; Gomez-García, C. J.; Delhaès, P.; Even, J.; Bertault, M. Phase Transitions of New BEDTTTF κ-Phase Salts with Hexacyanometalate Anions [M(CN)63− M=Co(III) and Fe(III)]. Solid State Commun. 1996, 97, 27−32. (27) Ouahab, L.; Golhen, S.; Le Hoerff, T.; Guillevic, J.; Tual, L.; Hélou, N.; Amiell, J.; Delhaès, P.; Binet, L.; Fabre, J. M. Radical Cation Salts with Paramagnetic Hexacyanometallate Anions. Synth. Met. 1999, 102, 1642−1645. ́ (28) Swietlik, R.; Łapiński, A.; Połomska, M.; Ouahab, L.; Guillevic, J. Spectroscopic Evidence of the Charge Ordering in κ-[Et4N](BEDTTTF)4M(CN)6·3H2O (M = CoIII, FeIII, CrIII). Synth. Met. 2003, 133− 134, 273−275. ́ (29) Swietlik, R.; Łapiński, A.; Ouahab, L.; Yakushi, K. Charge Ordering in the κ-Phase BEDT−TTF Salts with Co(CN)6 and Fe(CN)6 Anions Studied by Infrared and Raman Spectroscopies. C. R. Chim. 2003, 6, 395−403. ́ (30) Swietlik, R.; Ouahab, L.; Guillevic, J.; Yakushi, K. Infrared and Raman Studies of the Charge Ordering in the Organic Semiconductor κ-[Et4N][Co(CN)6(ET)4]·3H2O. Macromol. Symp. 2004, 212, 219− 224. ́ (31) Swietlik, R.; Łapiński, A.; Połomska, M.; Ouahab, L.; Ota, A. Infrared and Raman Investigations of the Charge Ordering Phenomena in the Monoclinic Salts κ-(BEDT-TTF)4[M(CN)6][N(C2H5)4]·2H2O (M = CoIII, FeIII). J. Low Temp. Phys. 2006, 142, 653− 656. (32) Ota, A.; Ouahab, L.; Golhen, S.; Yoshida, Y.; Maesato, M.; Saito, ́ G.; Swietlik, R. Phase Transition from Mott Insulating Phase into the Charge Ordering Phase with Molecular Deformation in ChargeTransfer Salts κ-(ET)4[M(CN)6][N(C2H5)4] 2H2O (M = CoIII and FeIII). Chem. Mater. 2007, 19, 2455−2462. (33) Yasin, S.; Rose, E.; Dumm, M.; Drichko, N.; Dressel, M.; Schlueter, J. A.; Zhilyaeva, E. I.; Torunova, S.; Lyubovskaya, R. N.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+48) 61-869-5201. Fax: (+48) 61-868-4524. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Centre (Decision No. DEC-2012/04/M/ST3/00774).

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REFERENCES

(1) Enoki, T.; Yamaura, J.; Miyazaki, A. Molecular Magnets Based on Organic Charge Transfer Complexes. Bull. Chem. Soc. Jpn. 1997, 70, 2005−2023. (2) Mori, T. J. Magnetic Interactions in Insulating BEDT-TTF Salts. Solid State Chem. 2002, 168, 433−437. (3) Coronado, E.; Day, P. Magnetic Molecular Conductors. Chem. Rev. 2004, 104, 5419−5448. (4) Enoki, T.; Miyazaki, A. Magnetic TTF-Based Charge-Transfer Complexes. Chem. Rev. 2004, 104, 5449−5478. (5) Nishijo, J.; Miyazaki, A.; Enoki, T.; Watanabe, R.; Kuwatani, Y.; Iyoda, M. d-Electron-Induced Negative Magnetoresistance of a π−d Interaction System Based on a Brominated-TTF Donor. Inorg. Chem. 2005, 44, 2493−2506. (6) Sugimoto, T.; Fujiwara, H.; Noguchi, S.; Murata, K. New Aspects of π−d Interactions in Magnetic Molecular Conductors. Sci. Technol. Adv. Mater. 2009, 10, 024302−024321. (7) McKenzie, R. H. A Strongly Correlated Electron Model for the Layered Organic Superconductors κ-(BEDT-TTF)2X. Comments Condens. Matter. Phys. 1998, 18, 309−337. (8) Sasaki, T.; Ito, I.; Yoneyama, N.; Kobayashi, N.; Hanasaki, N.; Tajima, H.; Ito, T.; Iwasa, Y. Electronic Correlation in the Infrared Optical Properties of the Quasi-Two-Dimensional κ-Type BEDT-TTF Dimer System. Phys. Rev. B 2004, 69, 064508−064514. (9) Giamarchi, T. Theoretical Framework for Quasi-One Dimensional Systems. Chem. Rev. 2004, 104, 5037−5056. (10) Hiraki, K.; Kanoda, K. Wigner Crystal Type of Charge Ordering in an Organic Conductor with a Quarter-Filled Band: (DIDCNQI)2Ag. Phys. Rev. Lett. 1998, 80, 4737−4740. (11) Chow, D. S.; Zamborszky, F.; Alavi, B.; Tantillo, D. J.; Baur, A.; Merlic, C. A.; Brown, S. E. Charge Ordering in the TMTTF Family of Molecular Conductors. Phys. Rev. Lett. 2000, 85, 1698−1701. (12) Yamamoto, K.; Yakushi, K.; Miyagawa, K.; Kanoda, K.; Kawamoto, A. Charge Ordering in θ-(BEDT-TTF)2RbZn(SCN)4 H



Article

Electronic and Magnetic Studies of κ-(BEDT-TTF)2Hg(SCN)2Cl. Physica B 2012, 407, 1689−1691. (34) Sedlmeier, K.; Elsässer, S.; Neubauer, D.; Beyer, R.; Wu, D.; Ivek, T.; Tomić, S.; Schlueter, J. A.; Dressel, M. Absence of Charge Order in the Dimerized κ-Phase BEDT-TTF Salts. Phys. Rev. B 2012, 86, 245103−245112. (35) Bozio, R.; Pecile, C. In The Physics and Chemistry of Low Dimensional Solids; Alcácer, L., Ed.; Reidel: Dordrecht, The Netherlands, 1980; p 165. (36) Guionneau, P.; Kepert, C. J.; Chasseau, D.; Truter, M. R.; Day, P. Determining the Charge Distribution in BEDT-TTP Salts. Synth. Met. 1997, 86, 1973−1974. (37) Hatsui, T.; Okamoto, K.; Yokoyama, T.; Kitajima, Y.; Tanaka, H.; Ojima, E.; Kobayashi, A.; Ohta, T. Sulfur K-edge X-ray Absorption Spectra for BETS and BEDT-TTF Charge Transfer Salts: A Novel Probe for the Determination of Hole Concentration. Chem. Phys. Lett. 2000, 330, 309−314. (38) Ouyang, J.; Yakushi, K.; Misaki, Y.; Tanaka, T. Raman Spectroscopic Evidence for the Charge Disproportionation in a Quasi-Two-Dimensional Organic Conductor θ -(BDT-TTP)2Cu(NCS)2. Phys. Rev. B 2001, 63, 054301−054306. (39) Sugano, T.; Hayashi, H.; Kinoshita, M.; Nishikida, K. Infrared Conductivity and Electron−molecular−Vibration Coupling in the Organic Superconductor Di[bis(ethylenedithio)tetrathiafulvalene] Bis(isothiocyanato)cuprate(I), κ-(BEDT-TTF)2 [Cu(NCS)2]: Protonated and Deuterated Salts. Phys. Rev. B 1989, 39, 11387−11397. (40) Kornelsen, K.; Eldridge, J. E.; Homes, C. C.; Wang, H. H.; Williams, J. M. Optical Properties of the 10 K Organic Superconductor (BEDT-TTF)2[Cu(SCN)2]. Solid State Commun. 1989, 72, 475−480. (41) Kornelsen, K.; Eldridge, J. E.; Wang, H. H.; Williams, J. M. Infrared Optical Properties of the Deuterated Form of the 10K Organic Superconductor (BEDT-TTF)2[Cu(NCS)2]. Solid State Commun. 1990, 74, 501−507. (42) Kornelsen, K.; Eldridge, J. E.; Wang, H. H.; Williams, J. M. Infrared Optical Properties of the 10-K Organic Superconductor (BEDT-TTF)2[Cu(NCS)2] [Where (BEDT-TTF) is Bis(ethylenedithio)tetrathiafulvalene]. Phys. Rev. B 1991, 44, 5235−5245. (43) Kornelsen, K.; Eldridge, J. E.; Wang, H. H.; Charlier, H. A.; Williams, J. M. Infrared Study of the Metal-Insulator Transition in the Organic Conductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl. Solid State Commun. 1992, 81, 343−349. (44) Faltermeier, D.; Barz, J.; Dumm, M.; Dressel, M.; Drichko, N.; Petrov, B.; Semkin, V.; Vlasova, R.; Mezière, C.; Batail, P. BandwidthControlled Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x: Optical Studies of Localized Charge Excitations. Phys. Rev. B 2007, 76, 165113−165124. (45) Hubbard, J. Generalized Wigner Lattices in One Dimension and Some Applications to Tetracyanoquinodimethane (TCNQ) Salts. Phys. Rev. B 1978, 17, 494−505. (46) Dressel, M.; Faltermeier, D.; Dumm, M.; Drichko, N.; Petrov, B.; Semkin, V.; Vlasova, R.; Méziere, C.; Batail, P. Disentangling the Conductivity Spectra of Two-Dimensional Organic Conductors. Physica B 2009, 404, 541−544. (47) Visentini, G.; Mateo, M.; Bellitto, C.; Girlando, A. Experimental Determination of BEDT-TTF+ Electron-Molecular Vibration Constants through Optical Microreflectance. Phys. Rev. B 1998, 58, 9460− 9467. (48) Georges, A.; Kotliar, G.; Krauth, W.; Rozenberg, M. J. Dynamical Mean-Field Theory of Strongly Correlated Fermion Systems and the Limit of Infinite Dimensions. Rev. Mod. Phys. 1996, 68, 13−125. (49) Kotliar, G.; Vollhardt, D. Strongly Correlated Materials: Insights from Dynamical Mean-Field Theory. Phys. Today 2004, 57, 53−59. (50) Ferber, J.; Foyevtsova, K.; Jeschke, H. O.;Valentí, R. LDA +DMFT for Organic Molecular Crystals: Spectral and Optical Properties of κ-(BEDT-TTF)2Cu[N(CN)2]Cl. cond-mat.str-el 2012, arXiv:1209.4466v1. (51) Kino, H.; Fukuyama, H. Electronic States of Conducting Organic κ-(BEDT-TTF)2X. J. Phys. Soc. Jpn. 1995, 64, 2726−2729.

(52) Kino, H.; Fukuyama, H. Interrelationship among Electronic States of α-(ET)2I3, (ET)2MHg(SCN)4 and κ-(ET)2X. J. Phys. Soc. Jpn. 1995, 64, 4523−4526. (53) Kino, H.; Fukuyama, H. Phase Diagram of Two-Dimensional Organic Conductors: (BEDT-TTF) 2X. J. Phys. Soc. Jpn. 1996, 65, 2158−2169. (54) Rice, M. J. Organic Linear Conductors as Systems for the Study of Electron-Phonon Interactions in the Organic Solid State. Phys. Rev. Lett. 1976, 37, 36−39. (55) Delhaes, P.; Yartsev, V. M. In Advances in Spectroscopy; Clark, R. J. H.; Hester, R. E., Eds.; Wiley and Sons: New York, 1993; Vol. 22, p 199. (56) Yartsev, V. M.; Drozdova, O. O.; Semkin, V. N.; Vlasova, R. M. Modelling κ Phase Organic Conductors. J. Phys. I (France) 1996, 6, 1673−1681. (57) Yartsev, V. M. In Materials and Measurements in Molecular Electronics; Kajimura, K., Kanoda, S., Eds.; Springer: Berlin, 1996; p 189. (58) Yartsev, V. M.; Graja, A. Electron-Molecular Vibrational Coupling and Electronic Structure of κ-(BEDT-TTF)2Cu[N(CN)2Cl0.5Br0.5; Optical Study at Temperatures down to 15 K. Int. J. Mod. Phys. B 1998, 12, 1643−1672. (59) Susaki, T.; Ito, I.; Yoneyama, N.; Kobayashi, N.; Hanasaki, N.; Tajima, H.; Ito, T.; Iwasa, Y. Electronic Correlation in the Infrared Optical Properties of the Quasi-Two-Dimensional κ-type BEDT-TTF Dimer System. Phys. Rev. B 2004, 69, 064508−064514. (60) Rozenberg, M. J.; Kotliar, G.; Kajueter, H.; Thomas, G. A.; Rapkine, D. H.; Honig, J. M.; Metcalf, P. Optical Conductivity in Mott-Hubbard Systems. Phys. Rev. Lett. 1995, 75, 105−108. (61) Rozenberg, M. J.; Kotliar, G.; Kajueter, H. Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems. Phys. Rev. B 1996, 54, 8452−8468. (62) McKenzie, R. H. A Strongly Correlated Electron Model for the Layered Organic Superconductors κ-(BEDT-TTF)2X. Comments Condens. Matter Phys. 1998, 18, 309−337. (63) Elsässer, S.; Wu, D.; Dressel, M.; Schlueter, J. A. Power-Law Dependence of the Optical Conductivity Observed in the Quantum Spin-Liquid Compound κ-(BEDT-TTF)2Cu2(CN)3. Phys. Rev. B 2012, 86, 155150−155157. (64) Shinaoka, H.; Misawa, T.; Nakamura, K.; Imada, M. Mott Transition and Phase Diagram of κ-(BEDT-TTF)2Cu(NCS)2 Studied by Two-Dimensional Model Derived from Ab Initio Method. J. Phys. Soc. Jpn. 2012, 81, 034701−034712. (65) Hasegawa, T.; Kagoshima, S.; Mochida, T.; Sugiera, S.; Iwasa, Y. Electronic States and Anti-Ferromagnetic Order in Mixed-Stack Charge-Transfer Compound (BEDT-TTF)(F2TCNQ). Solid State Commun. 1997, 103, 489−493. (66) Kozlov, M. E.; Pokhodnia, K. I.; Yurchenko, A. A. The Assignment of Fundamental Vibrations of BEDT-TTF and BEDTTTF-d8. Spectrochim. Acta A 1987, 43, 323−329. (67) Kozlov, M. E.; Pokhodnia, K. I.; Yurchenko, A. A. Electron Molecular Vibration Coupling in Vibrational Spectra of BEDT-TTF Based Radical Cation Salts. Spectrochim. Acta A 1989, 45, 437−444. (68) Eldrige, J. E.; Homes, C. C.; Williams, J. M.; Kini, A. M.; Wang, H. H. The Assignment of the Normal Modes of the BEDT-TTF Electron-Donor Molecule Using the Infrared and Raman Spectra of Several Isotopic Analogs. Spectrochim. Acta A 1995, 51, 947−960. ́ (69) Musfeldt, J. L.; Swietlik, R.; Olejniczak, I.; Eldridge, J. E.; Geiser, U. Understanding Electron-Molecular Vibrational Coupling in Organic Molecular Solids: Experimental Evidence for Strong Coupling of the 890 cm−1 Mode in ET-Based Materials. Phys. Rev. B 2005, 72, 014516−014522. (70) Duke, C. B. Electronic Localization and Delocalization in Organic Metals and Semiconductors. Ann. N. Y. Acad. Sci. 1978, 313, 166−178. (71) Girlando, A. Charge Sensitive Vibrations and ElectronMolecular Vibration Coupling in Bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF). J. Phys. Chem. C 2011, 115, 19371−19378. I



Article

(72) Bozio, R.; Pecile, C. In Spectroscopy of Advanced Materials; Clark, R. J. H., Hester, R.E., Eds.; Wiley: Chichester, U.K., 1990; pp 1−86. (73) Pecile, C.; Painelli, A.; Girlando, A. Studies of Organic Semiconductors for 40 YearsV. Mol. Cryst. Liq. Cryst. 1989, 171, 69−87. (74) Wang, H. H.; Ferraro, J. R.; Williams, J. M.; Geiser, U.; Schlueter, J. A. Rapid Raman Spectroscopic Determination of the Stoichiometry of Microscopic Quantities of BEDT−TTF-Based Organic Conductors and Superconductors. J. Chem. Soc., Chem. Commun. 1994, 1893−1894. (75) Yamamoto, T.; Uruichi, M.; Yamamoto, K.; Yakushi, K.; Kawamoto, A.; Taniguchi, H. Examination of the Charge-Sensitive Vibrational Modes in Bis(ethylenedithio)tetrathiafulvalene. J. Phys. Chem. B 2005, 109, 15226−15235. (76) Paineilli, A.; Girlando, A. Electron−Molecular Vibration (e− mv) Coupling in Charge Transfer Compounds and Its Consequences on the Optical Spectra: A Theoretical Framework. J. Chem. Phys. 1986, 84, 5655−5671.

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