MEM TCNQ

The dimer models were also used or proposed to interpretation of the reflectivity spectra of K-TCNQ [34], methylethylmorpholinium (MEM) (TCNQ)2 [52], decamethylferrocenium (DMeFc)-TCNQ [56], A-methyl-... 

Figure 16 IR reflectivity of the low-temperature (solid line) and high-temperature (dashed line) phases of MEM(TCNQ)2 single crystals. (From Ref. 97.)...

Figure 17 Absorption spectra of MEM(TCNQ)2 at a temperature T minus that at 22 K. Absorption spectra at 4 and 22 K are shown in the inset in (a). Frequency of absorption lines (b) and the line width (c) as a function of temperature. (From Ref. 99.)...

The two materials MEM(TCNQ)2 and TEA(TCNQ)2 (MEM+ = N-methyl-N-ethylmorpholinium, TEA+ = triethylammonium) are among the best... 

MEM(TCNQ)2 and TEA(TCNQ)2 are two 1 2 TCNQ salts, with one MEM or TEA donor molecule per two TCNQ acceptor molecules, in which there is a complete charge transfer from the donor as indicated by the formulas MEM+(TCNQ)2 and TEA+(TCNQ)2. In consequence, the mean... 

For MEM(TCNQ)2 [24,25], there is only one molecular formula per unit cell, or Z = 1. In consequence, the segregated TCNQ chains in this salt are built up from identical A-B dimers according to the simple dimerized sequence... 

Then, each TCNQ chain can be viewed as a sequence of parallel (A-B A-B) dimers in the case of MEM(TCNQ)2, and a sequence of antiparallel (A-B B-A) dimers in the case of TEA(TCNQ)2. This means in particular that according to crystal symmetry, dimerization is intrinsically present in the first salt, and tetramerization is intrinsically present in the second, irrespective of temperature. 

Within a one-electron description (i.e., U = 0, U being the on-site Coulomb repulsion [2,3], regular conducting TCNQ chains with p = electron per molecular site correspond to quarter-filled electronic bands. Consequently, the Fermi wave vector is in this case kF = n/4d, d being the spacing parameter between adjacent sites, and the chains are metallic. This is the case, for instance, for MEM(TCNQ)2 and TEA(TCNQ)2. Note that in these two salts the cations MEM+ and TEA+ are diamagnetic and do not participate in electrical conduction. 

Electrical and Magnetic Properties of MEM(TCNQ)2 Contrary to TEA(TCNQ)2, the chain structure in MEM(TCNQ)2 [24,25] is found to go discontinously from a very weak dimerization above 335 K to a strong dimerization below 335 K. This is a first-order transition which is reminiscent here of a 4kF transition, although there is still no symmetry breaking [28]. An additional tetramerization also starts to develop continuously below 19 K. This last transition, which now involves symmetry breaking, can be identified with a true second-order 2kF transition [17,18,28] (see Section 7). 

The fact that J is T-independent in the case of MEM(TCNQ)2 is very surprising. After Oostra [40], this is a purely accidental feature which may be associated with the relative constancy of the transfer integrals in this case. On heating, o-y in MEM(TCNQ)2 increases discontinuously at 335 K by about three orders of magnitude, with a hysteretic effect characteristic of a first-order transition [17-19], whereas x decreases by only about 7%. In the new phase, above 335 K, cr is almost T independent o"u = 20 S/cm and Eg = 0 [41]. x as a Curie-Weiss-like temperature dependence x = 1 T + 0), with 0 = 70 K. MEM(TCNQ)2 then resembles a highly cor-... 

Figure 12 Temperature dependence, above 250 K, of the electrical conductivity of MEM(TCNQ)2 crystals. Axis c is the chain axis and axes a and b are almost orthogonal to c. (After Ref. 41.)...

Correlation Effects in MEM(TCNQ)2 and TEA(TCNQ)2 This important aspect has been reviewed in a thorough way by Pedersen et al. [36], in particular for the special case of the two salts under concern here. The experimental magnetic susceptibility has a maximum value of... 

The fact that there is apparently only one, 2kF, transition in TEA(TCNQ)2 but two, 2kp and 4/cF, transitions in MEM(TCNQ)2 has been considered as an indication that Coulomb repulsions, although significant in both salts, are relatively weaker in the first [36]. This trend is also supported by the fact that the exchange energy J t2IU [3] is about four times smaller in MEM(TCNQ)2 (/ = 53 K) than in TEA(TCNQ)2 (Jl = 230 K at T = 200 K), whereas the (effective) transfer integral t is expected to be about the same (see the discussion above) [36]. 

After Pedersen and Carneiro, it is mostly the magnitude of the nearest-neighbor Coulomb repulsion energy, V, which is responsible for the difference between TEA(TCNQ)2 and MEM(TCNQ)2. The magnitude of V is determined largely by the shape and polarizability of the cations, and the cation TEA+ is believed to screen more efficiently than the cation MEM+, the intermolecular Coulomb repulsion represented by V [36]. 

However, it will be seen below that the transitions observed in both MEM(TCNQ)2 and TEA(TCNQ)2, although bearing some strong characteristics of Peierls transitions, also present several additional features clearly attributable to sterical effects, which make them somewhat more complicated in nature. 

Finally, it may be remarked that the two salts MEM(TCNQ)2 and TEA(TCNQ)2 bear several points of resemblance in their physical properties with the most interesting salts of the (TMTTF)2X series (TMTTF = tetramethyltetrathiafulvalene) [14]. They are all organic conductors with a quasi-one-dimensional character, with p = 5 (or ), and with a dominant electron-electron interaction. They exhibit comparable modest values of the electrical conductivity at high temperature, which indicate that the electrons are not very delocalized in the materials, and in all of them an underlying 4kF dimerization is also present, due to the cations. These common features, which are thought to be at the origin of sizable Umklapp scattering effects in the salts of the (TMTTF)2 series [14], could also be able to produce the same kinds of effects in MEM(TCNQ)2 and TEA(TCNQ)2 (see also Chapter 2). 

Spin-Peierls Transition in MEM(TCNQ)2 and Related Compounds... 

The spin-Peierls transition has been the object of several recent papers. Examples of the first case, with p = 1, are provided by the insulating alkali metal simple salts [47] (see below), and examples of the second case, with p = 3 or I, are given by the salt MEM(TCNQ)2 [17-19,46], by the salts of the (TMTTF)2X series [46,48], or by the salts of the (BCPTTF)2X series (BCPTTF = benzocyclopentyltetrathiafulvalene) [46,49]. 

However, in the case of MEM(TCNQ)2, which is considered as one of the most representative spin-Peierls materials, with TsP = 19 K, the results are quite at variance with the normal behavior described above [46]. For this material, critical fluctuations are also observed correctly, by x-rays, below Tp = 40 K, but they are only of a three-dimensional nature. Moreover, these fluctuations do not produce any detectable effect below TF on the Bonner-Fisher dependence of the magnetic susceptibility. Consequently, this law is perfectly followed, with J = 53 K, down to 19 K [17,18,46]. Some earlier comments on this point have also been given by Schulz [50]. 

It is important to remark here that the periodicity of the cation sublattice in the chain direction z just coincides with 2kF periodicity in the case of TEA(TCNQ)2, and with 4kF periodicity in the case of MEM(TCNQ)2. This fact would suffice by itself to account, in terms of electron-cation interaction and commensurability effect, for the intrinsic chain tetramerization of TEA(TCNQ)2 and for the intrinsic chain dimerization of MEM(TCNQ)2, in particular for the residual dimerization still above 335 K. However, the cation subsystem certainly has a more direct, steric influence than through only the electron-ion interaction discussed above on the overall structural properties of the organic salts. 

Static Disorder and Thermal Motion of Cations Significant modifications of the thermal motion of cations, corresponding to the loss of degrees of freedom on cooling, are observed in both MEM(TCNQ)2 and TEA(TCNQ)2, near 335 K and 210 K, respectively. Such cation rearrangements appear better as the cause rather than as the consequence of the correlative 2k and 4kv electronic transitions. In effect, structural changes are so important that they can hardly be attributed to instabilities of the electronic subsystems. 

In both salts, cations are found, from x-ray studies, to be disordered between two preferential orientations. In MEM(TCNQ)2, the occupancies, x and 1 - x, of these two orientations are found to be T dependent. Below 113 K, x = 1, and from 113 to 243 K, x varies from 1 to 0.5 [24,25]. In TEA(TCNQ)2, the two occupancies are almost identical x = 1 - x = 0.5 [22]. This twofold disorder is definitely static at low temperature. However, due to thermal factors, it becomes more and more difficult to resolve the two oreintations at high temperature and also to conclude whether the disorder is static or dynamic in nature [22,24]. 

A still more singular case than MEM(TCNQ)2 is provided by DMTM(TCNQ)2, a salt of the same family (DMTM = dimethylthimor-pholinium) [19,40]. This salt is also found to undergo a first-order phase transition, at Tc = 272 K but below Tc, the electrical conductivity increases... 

At some critical temperature, a structural distortion to dimerized chains occurs together with the opening of a gap in the magnetic excitations. Such a distortion is attributable to a 2kF spin-Peierls transition, just as in MEM(TCNQ)2 (cf. Section III.A.7). A 2kF transition results into chain tetramerization for p = 5 and into chain dimerization for p = 1, as also shown in Fig. 5. For instance, the spin-Peierls transition takes place at T sp = 395 K in KTCNQ and at TsP = 381 K in RbTCNQ [67] (a second form, called RbTCNQ II, also exists for the Rb salt [69], but it is not considered here). By analogy with the case of polymers, the dimerized phase at low temperature is also called a bond-ordered-wave (BOW) phase [47]. 

Figure 21 Temperature dependence of the dc electrical conductivity and the static magnetic susceptibility of MEM-(TCNQ)2. (Reproduced from S. Huizinga et ah. Quasi One-Dimensional Conductors II ed. S. Barisic et al., (Springer, Berlin, 1979) p. 45. Figs.

MEM(TCNQ)2 by the Groningen group and that shown in Fig. 6b to that observed in PY(TCNQ)2 by the Budapest group. ... 

Pyrene-TCNE TTF-TCNQ (MEM)(TCNQ)2 (TMTSF)2(CI04)... 

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