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Structural study of neutral-ionic transition in DMTTF-CA
SYnTI,"IATIIC mUT LS ELSEVIER
Synthetic Metals 70 (1995) 1219--1220
Structural Study of Neutral-Ionic Transition in DMTTF:-CA Y. Nogami a , M. Taoda a , K. Oshima a , S. Aoki b , T. Nakayama b and A. Miura b a Department of Physics, Okayama University, Okayama 700, Japan b Toshiba Research and Development Center, Kawasaki 210, Japan Abstract Dimethyltetrathiafulvalene-p-chloranil (DMTTF-CA), showing a particular neutral(N)-ionic(I) transition from a N phase into a N-I coexisting phase by cooling, was structural analyzed by X-ray at 29K, 48K, 109K and 293K Below Tc of 65K, changes observed in unit cell parameters, bond lengths and anisotropic temperature factors indicate 1) a increase in degree of charge transfer(CI') from a DMTTF donor to a CA acceptor and 2) a formation of a molecular dimerization, in I part of the coexisting phase. A possible staging superlattice was not observed. I. I N T R O D U C T I O N Organic C-q"compounds with mixed column structure where donors(D) and acceptors(A) alternate along the stack can be divided into either a neutral (N) phase and a ionic (I) phase. In
associated with TINIT, we measured X-ray diffuse scattering intensity to detect the possible staging superlattice and solved low-temperature structures to evaluate the direction and amplitude of the molecular dimerization in DMTTF-CA.
these compounds, degree of CI" (p) is mainly determined by the competition among the donor ionization potential, the acceptor electron affinity and the Madelung energy. Torrance et al. has reported the N-I transition from N to I by pressurization in several mixed column compounds[l]. However a temperature induced neutral-ionic transition (TINIT) had been observed only in tetrathia-fulvalene-p-chloranil (q"TF-CA)[2,3]. Recently, two CT compounds showing TINIT, (3,3',5,5') tetramethylbenzidine -tetracyanoquinodimetane (TMB-TCNQ) and DMTTF-CA, have been developed[4,5]. As a result of CT, electronic structure of each molecule is a open shell type with 1/2 spin in the I state, whereas a closed shell in the N state. This quasi-one dimensional magnetic system has dimeric distortion instability just as a spin-Peiels system. In fact, molecular dimerizations in the I state have been observed through appearance of total symmetric ag modes which are forbidden in the regularly stacked N state[3-5]. In TMB-TCNQ and DMTTF-CA, a coexistence of the N state and the I state has been observed below Tc[4,5]. Taking account of a frustration in the Coulomb interaction between favorable intracolumn D + A- contacts and unfavorable intercolumn D + D+ and A-A-contacts, the coexistence has been interpreted in terms of a possible staging superstructure[6]; neutral layers are inserted between ionic layers to reduce the Coulomb repulsion ( e.g. D ÷ D ÷ D+---D+ D O D +) Physical properties of NIT in CT crystals have been investigated using optical[3-5], magnetic[4,7], electronic[8] measurements. However structural information is limited [2,9] except the indication of molecular dimerization by IR as mentioned above. Interested in the structural changes 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02826-K
2. E X P E R I M E N T A L Sample preparation of DMTTF-CA crystals has been reported elsewhere[5]. X-ray diffuse scattering intensity was measured by "monochromatic Laue" camera with cryostat down to 10K at Kagoshima Lab. A CuKcx X-ray generated under 40kV and 140mA was monochromatized and focussed by bent HOPG. X-ray structural analysis was performed with use of Huber four-circle diffractometer. A graphite monochromatized M o K a X-ray(50kV,200mA), and a helium cryocooler down to 15K were used. In both experiments, sample chambers were evacuated to stabilize temperature. O t
Fig. 1 Monochromatic Laue X-ray pattern at 19K
1220
Y. Nogami et al. / Synthetic Metals 70 (1995) 1219-1220
0
i
-1
b
'
/
,=
£8
1 /i i
-2[
1
!
measured. Average structure of the coexisting phase is nearly identical with that of the N phase except three points. 1)bond lengths. Table 1 shows double bond lengths in TTF and DMTTF. Below Tc, double bond lengths in DMTTF show similar increases when TTF is fully ionized[11], although the increments in DMTTF are smaller. Note that bond lengths in the coexisting phase are average values between the I state and the N state. Taking account of the average, double bond length variations correspond to the reported change in p(0.6-0.7 for
tions, since they are not modified systematically above Tc. We could not confirm the presence of satellite spots from a superlattice. Probably, N and I parts are disordered. However there remains a possibility that a superlattce with a smaller amplitude (<1% of unit cell) or with a smaller domain length (<100k) exists. Weak satellite spots from a cell-doubling
I and 0.3-0.4 for N)[5]. Below Tc , similar double bond increments in CA were observed. 2) tilting angles. Tilting angles of both molecules against the ab plane increases about 0.2-0.3 degree below Tc. 3) anisotropic temperature factors. '&re found peculiar increases in anisotropic temperature factors along slacking c direction below Tc. For example, U33 for center C atom in DMTTF, which describes a average of a square distortion along the c axis, is 0.032 (293K), 0.015 (109K), 0.019(48K) and 0.018 (29K). On the other hand, other components of anisotropic temperature factors show normal temperature dependencies, e.g. 1_;22 for the center C(b axis) , is 0.030(293K), 0.012(109K), 0.005 (48K) and 0.005 (29K). Since Tc is low, the increments in U33 is not due to a dynamical motion but mainly due to a static disorder along c axis between N and I parts. Thereby we can estimate the direction and the amplitude of the molecular dimerization from Uij; the direction is almost parallel to the c axis and the amplitude is about 0.5% of D-A distance in I part, whereas no
superlattice can be masked by K/2 contaminations[10].
dimeric distortion in N part. The increases in a , 18 and
Figure 2 shows temperature dependencies of unit cell parameters. From this variation, Tc is estimated 65 _+0.5 K. Above Tc, a large shrinkage in c length (stacking direction) was observed. Since D and A alternate along c, this shrinkage enlarges the Madelung energy gain. A similar result has been observed in TFF-CA[2]. Below Tc , every unit cell parameter
angles are associated with this structural change.
i
0
I00 200 300 Temperature [K]
0
lOO 200 Temperature [K]
Fig. 2 Temperature dependencies of lattice parameters
3. R E S U L T S
AND D I S C U S S I O N
Figure 1 shows an X-ray pattern from the DMTTF-CA at about 19K with a three-hour exposure. All the diffraction spots observed are the Bragg reflections or
XJ2 contamina-
decreases abruptly except a increase in c~ and [3. Next we present low-temperature structures. Space group is Pl and R factor is smaller than 3% at every temperature Table 1 Double bond lengths in TTF and DIVITTF of DMTTF-CA TTF 0 TTF " 293K 293K
293K
29K
a [A]
1.349
1.393
1.349
1.358
1.365
1.372
b [,~]
1.314
1.320
1.323
1.332
1,335
1.335
S
S
(H3C) (DMITTF
(CH3)
ACKNOWLEDGMENTS This work was supported by a Grants-in-Aid from the Ministry of Education, Science and Culture and by a grant from Okayama Foundation for Science and Technology. The authors thank the committee of X-ray Laboratory of Okayama University for the use of Huber 4-circle diffractometer and Prof. S. Kagoshima for the use of X-ray camera. REFERENCES
DMTTF-CA 109K 48K
tilting
1. J. B. Torrance et al. Phys. Rev. I,ett. 46(1981)253. 2. J. B. Torrance et at. Phys. Rev. Lett. 47(1981)1747. 3. Y. Tokura et al. Mol. Cryst Liq. Cryst. 125(1985)71. 4. Y. Iwasa et al. Phys. Rev., B42(1990)2374. 5. S. Aoki el at. Phys. Rev. B48(1993) 626. 6. J. Hubbard et al. Phys. R e v Lett., 47 (1981) 1750. 7. T. Mitani et al. Phys. Rev. Lett., 20(1984)842. 8. Y. Tokura et al. Phys. Rev. B38(1988)2215. 9. Y. Kanai et al. Synth. Met. 10(1984)157. 10. Y. Nogami et al. Solid State Commun. 89(1994) 113. 11. H. Okamoto, thesis, the University of Tokyo, 1988.
Synthetic Metals 70 (1995) 1219--1220
Structural Study of Neutral-Ionic Transition in DMTTF:-CA Y. Nogami a , M. Taoda a , K. Oshima a , S. Aoki b , T. Nakayama b and A. Miura b a Department of Physics, Okayama University, Okayama 700, Japan b Toshiba Research and Development Center, Kawasaki 210, Japan Abstract Dimethyltetrathiafulvalene-p-chloranil (DMTTF-CA), showing a particular neutral(N)-ionic(I) transition from a N phase into a N-I coexisting phase by cooling, was structural analyzed by X-ray at 29K, 48K, 109K and 293K Below Tc of 65K, changes observed in unit cell parameters, bond lengths and anisotropic temperature factors indicate 1) a increase in degree of charge transfer(CI') from a DMTTF donor to a CA acceptor and 2) a formation of a molecular dimerization, in I part of the coexisting phase. A possible staging superlattice was not observed. I. I N T R O D U C T I O N Organic C-q"compounds with mixed column structure where donors(D) and acceptors(A) alternate along the stack can be divided into either a neutral (N) phase and a ionic (I) phase. In
associated with TINIT, we measured X-ray diffuse scattering intensity to detect the possible staging superlattice and solved low-temperature structures to evaluate the direction and amplitude of the molecular dimerization in DMTTF-CA.
these compounds, degree of CI" (p) is mainly determined by the competition among the donor ionization potential, the acceptor electron affinity and the Madelung energy. Torrance et al. has reported the N-I transition from N to I by pressurization in several mixed column compounds[l]. However a temperature induced neutral-ionic transition (TINIT) had been observed only in tetrathia-fulvalene-p-chloranil (q"TF-CA)[2,3]. Recently, two CT compounds showing TINIT, (3,3',5,5') tetramethylbenzidine -tetracyanoquinodimetane (TMB-TCNQ) and DMTTF-CA, have been developed[4,5]. As a result of CT, electronic structure of each molecule is a open shell type with 1/2 spin in the I state, whereas a closed shell in the N state. This quasi-one dimensional magnetic system has dimeric distortion instability just as a spin-Peiels system. In fact, molecular dimerizations in the I state have been observed through appearance of total symmetric ag modes which are forbidden in the regularly stacked N state[3-5]. In TMB-TCNQ and DMTTF-CA, a coexistence of the N state and the I state has been observed below Tc[4,5]. Taking account of a frustration in the Coulomb interaction between favorable intracolumn D + A- contacts and unfavorable intercolumn D + D+ and A-A-contacts, the coexistence has been interpreted in terms of a possible staging superstructure[6]; neutral layers are inserted between ionic layers to reduce the Coulomb repulsion ( e.g. D ÷ D ÷ D+---D+ D O D +) Physical properties of NIT in CT crystals have been investigated using optical[3-5], magnetic[4,7], electronic[8] measurements. However structural information is limited [2,9] except the indication of molecular dimerization by IR as mentioned above. Interested in the structural changes 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02826-K
2. E X P E R I M E N T A L Sample preparation of DMTTF-CA crystals has been reported elsewhere[5]. X-ray diffuse scattering intensity was measured by "monochromatic Laue" camera with cryostat down to 10K at Kagoshima Lab. A CuKcx X-ray generated under 40kV and 140mA was monochromatized and focussed by bent HOPG. X-ray structural analysis was performed with use of Huber four-circle diffractometer. A graphite monochromatized M o K a X-ray(50kV,200mA), and a helium cryocooler down to 15K were used. In both experiments, sample chambers were evacuated to stabilize temperature. O t
Fig. 1 Monochromatic Laue X-ray pattern at 19K
1220
Y. Nogami et al. / Synthetic Metals 70 (1995) 1219-1220
0
i
-1
b
'
/
,=
£8
1 /i i
-2[
1
!
measured. Average structure of the coexisting phase is nearly identical with that of the N phase except three points. 1)bond lengths. Table 1 shows double bond lengths in TTF and DMTTF. Below Tc, double bond lengths in DMTTF show similar increases when TTF is fully ionized[11], although the increments in DMTTF are smaller. Note that bond lengths in the coexisting phase are average values between the I state and the N state. Taking account of the average, double bond length variations correspond to the reported change in p(0.6-0.7 for
tions, since they are not modified systematically above Tc. We could not confirm the presence of satellite spots from a superlattice. Probably, N and I parts are disordered. However there remains a possibility that a superlattce with a smaller amplitude (<1% of unit cell) or with a smaller domain length (<100k) exists. Weak satellite spots from a cell-doubling
I and 0.3-0.4 for N)[5]. Below Tc , similar double bond increments in CA were observed. 2) tilting angles. Tilting angles of both molecules against the ab plane increases about 0.2-0.3 degree below Tc. 3) anisotropic temperature factors. '&re found peculiar increases in anisotropic temperature factors along slacking c direction below Tc. For example, U33 for center C atom in DMTTF, which describes a average of a square distortion along the c axis, is 0.032 (293K), 0.015 (109K), 0.019(48K) and 0.018 (29K). On the other hand, other components of anisotropic temperature factors show normal temperature dependencies, e.g. 1_;22 for the center C(b axis) , is 0.030(293K), 0.012(109K), 0.005 (48K) and 0.005 (29K). Since Tc is low, the increments in U33 is not due to a dynamical motion but mainly due to a static disorder along c axis between N and I parts. Thereby we can estimate the direction and the amplitude of the molecular dimerization from Uij; the direction is almost parallel to the c axis and the amplitude is about 0.5% of D-A distance in I part, whereas no
superlattice can be masked by K/2 contaminations[10].
dimeric distortion in N part. The increases in a , 18 and
Figure 2 shows temperature dependencies of unit cell parameters. From this variation, Tc is estimated 65 _+0.5 K. Above Tc, a large shrinkage in c length (stacking direction) was observed. Since D and A alternate along c, this shrinkage enlarges the Madelung energy gain. A similar result has been observed in TFF-CA[2]. Below Tc , every unit cell parameter
angles are associated with this structural change.
i
0
I00 200 300 Temperature [K]
0
lOO 200 Temperature [K]
Fig. 2 Temperature dependencies of lattice parameters
3. R E S U L T S
AND D I S C U S S I O N
Figure 1 shows an X-ray pattern from the DMTTF-CA at about 19K with a three-hour exposure. All the diffraction spots observed are the Bragg reflections or
XJ2 contamina-
decreases abruptly except a increase in c~ and [3. Next we present low-temperature structures. Space group is Pl and R factor is smaller than 3% at every temperature Table 1 Double bond lengths in TTF and DIVITTF of DMTTF-CA TTF 0 TTF " 293K 293K
293K
29K
a [A]
1.349
1.393
1.349
1.358
1.365
1.372
b [,~]
1.314
1.320
1.323
1.332
1,335
1.335
S
S
(H3C) (DMITTF
(CH3)
ACKNOWLEDGMENTS This work was supported by a Grants-in-Aid from the Ministry of Education, Science and Culture and by a grant from Okayama Foundation for Science and Technology. The authors thank the committee of X-ray Laboratory of Okayama University for the use of Huber 4-circle diffractometer and Prof. S. Kagoshima for the use of X-ray camera. REFERENCES
DMTTF-CA 109K 48K
tilting
1. J. B. Torrance et al. Phys. Rev. I,ett. 46(1981)253. 2. J. B. Torrance et at. Phys. Rev. Lett. 47(1981)1747. 3. Y. Tokura et al. Mol. Cryst Liq. Cryst. 125(1985)71. 4. Y. Iwasa et al. Phys. Rev., B42(1990)2374. 5. S. Aoki el at. Phys. Rev. B48(1993) 626. 6. J. Hubbard et al. Phys. R e v Lett., 47 (1981) 1750. 7. T. Mitani et al. Phys. Rev. Lett., 20(1984)842. 8. Y. Tokura et al. Phys. Rev. B38(1988)2215. 9. Y. Kanai et al. Synth. Met. 10(1984)157. 10. Y. Nogami et al. Solid State Commun. 89(1994) 113. 11. H. Okamoto, thesis, the University of Tokyo, 1988.