- Email: [email protected]
Preparation, structure, and physical properties of an organic conductor, (BEDT-TTF)3Br2(H2O)2
Synthetic Metals, 2 7 (1988) A401 -A406
A401
~REPARATION, STRUCTURE, AND PHYSICAL PROPERTIES OF AN ORGANIC CONDUCTOR,
(BEDT-
7TF)3~2~2~2 H. URAYAMA, G. SAITO, T. SUGANO, and M. KINOSHITA ISSP, University of Tokyo, Tokyo (Japan) A. KAWAMOT0 and J. TANAKA University of Nagoya, Nagoya (Japan)
ABSTRACT The BEDT-TTF complex with a small anion, Br , has been synthesized with the accidental inclusion of H20 which may stabilize the crystallization. black plate crystal, TTF)3(CIO4) 2.
(BEDT-TTF)3Br2(H20)2,
The given
is isostructual to (BEDT-
Two bromide anions and two water molecules, Br-2(H20)2, form a
planar square cluster with hydrogen bonds.
This cluster constructs a insulating
sheet which is sandwiched by two-dimensional BEDT-TTF sheets.
The electrical
resistivity shows the metallic behavior down to 185 K and transforms to semiconducting system, which is also confirmed by the ESR measurements.
INTRODUCTION BEDT-TTF is a talented donor to give a variety of charge transfer complexes. Especially, more than ten kinds of BEDT-TTF salts exhibit superconductivity. Among them (BEDT-TTF)2Cu(NCS) 2 recorded the highest critical temperature, IO.L K, under an ambient pressure [I].
Though BEDT-TTF salts which contain symmetric
linear anions (13, IBr2, and Aul 2 [2]) and clusters (Hg3Br8, Hg3CI 8 [3], Cu(NCS) 2 [I]) have been found to be superconductors, a small compact anion salt had not been explored out except the CI- salt [4a].
In this paper the crystal
structure and the physical properties of a Br- complex [5] is presented.
EXPERIMENTAL Black plate-crystals were grown by electrochemical crystallization of BEDTTTF in chlorobenzene and tetrahydrofuran by using tetra-n-butylammonium bromide as a supporting electrolyte during I-2 months.
The lattice parameters of (BEDT-
TTF)3Br2(H20) 2 are; triclinic, PT, a=16.167(2), b=18.125(3), c=7.718(I) ~,
0379-6779/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
A402 ~=91.O5(2), ~=94.09(2), y=148.18(1) °, V=1176.0(4) ~3 and Z=I.
The crystal
structure was solved by the heavy atom method and refined by the block-diagonal least-squares method by using 2574 independent reflections (Cu Ks, 2e<120 °, IFoI >3~(IFoI)).
The final R is 0.082.
The temperature dependence of electrical
resistivity was measured by a standard d.c. four-probe technique with a gold paint as an electrode.
ESR studies were performed by utilizing a JEOL JES-FEIXG
(9.2 GHz) spectrometer over 300-10 K.
The sample crystal was placed at the
center of cylindrical cavity in the TE011 mode.
RESULTS and DISCUSSION Crystal structure The crystal structure of (BEDT-TTF)3Br2(H20) 2 is illustrated in Fig. I. unit cell is composed of three H20 molecules.
The
BEDT-TTF molecules, two bromide anions, and two
The inclusion of H20 was confirmed by the elemental analysis;
Found: C, 26.69; H, 2.09 %. Calcd for Br2S2402C30H28 : C, 26.40; H, 1.97 %.
As
the starting materials were purified with the exclusion of H20 , air humidity had been included into this complex to form the stable single crystals during the course of electroerystallization for I-2 months. (BEDT-TTF)3Br2(H20) 2 has a layered structure: the organic conducting sheets and the inorganic insulator ones stack alternatingly along the [IT0] axis.
In
the latter sheets, Br, Br', 0, and O' of Br2(H20) 2 form a rectangular unit (Fig. o
la).
The distances of Br..O and O''Br' (3.36 and 3.26 A, respectively) are a
little bit longer than the van der Waals contact (3.22 ~) and the angle of Br-OBr' (111°) is close to that of H-O-H (104°), which suggests the hydrogen contact of Br-. H-O-H around an inversion center. TTF)3CI2(H20) 2 [4b].
This part nearly resembles to (BEDT-
A little different point between them is that C14(H20) 4 is
one unit which is composed of a rectangular C12(H20) 2 and the additional hydrogen bonded 2(CI(H20)).
Therefore the unit cell volume of the bromide
complex is a half as large as that of the chloride one.
r~-%~ ~-o~p
o.
~
~ .a ~
b
o k ~
a} Fig. 1. Crystal structure of (BEDT-TTF)3Br2(H20) 2 projected on the a) bc and b) ab plane.
A403
[112]
[221 ] [11Y1 Fig. 2. Molecular arrangement of (BEDT-TTF)3Br2(H20) 2.
In the donor layers two BEDT-TTF molecules are crystallographically independent.
One(A) is on the center of symmetry and the other(B) is on the
general position as shown in Fig. I.
The similar intramolecular bond lengths
and angles of two independent donors indicate no charge separation which leads the donor to be BEDT-TTF 2/3+.
As shown in Fig. 2, there are three interacting
chains of donors: a stacking along the [11~] axis, two side-by-side ones along the [112] and [221] axes.
Among them short S''S contacts (<3.6 ~) are only
found in the [221] direction.
However, this does not mean that the strongest
interaction is observed along [221] direction since an extended H~ckel calculation and reflectance spectra in the (BEDT-TTF)3(C10&)2, which is isostructural to the bromide complex show that the strongest interaction
[6] and
dispersion [7] are detected along another side-by-side direction [112] but not along the direction [221] with short contacts.
The inclusion of H20 made the
complex of a small bromide anion belong to structurally a tetrahedral anion system in BEDT-TTF salts.
Electrical properties The electrical resistivity versus temperature on the (110) plane of a typical crystal is plotted in Fig. 3.
Above 185 K the salt is weakly metallic with a
room-temperature resistivity of 0.O13 ~ cm. to a semiconducting system below 185 K. still below (0.05 ~cm). 0.021 eV.
The material undergoes a transition
The resistivity at this temperature is
The activation energy just below 185 K is obtained as
In order to confirm the metallic state and observe the transition
property, ESR measurements were performed.
A404
OD-=
E u r o
%
%
,L
i
I
100
200
300
TIK
Fig. 3. Electrical resistivity of (BEDT-TTF)3Br2(H20) 2.
5C
<
@
@
@
@ @
@
@
4C o
@
@
@
@
[]
~: 3c
2.014
[] m
2C
[]
,
[]
10
2,002
c' '
'
a~'
25
'
'
2.010 En 2.006
~ ' ' e / degree
;~
'
;eo
Fig. 4. Angular dependence of g-values and linewidths(AH) of (BEDT-TTF) 3 Br2(H20)2 •
Electron spin resonance studies As shown in Fig. Ib, the two-dimensional sheets parallel to the (ITO) plane are sandwiched by the anion sheets (2(H20)"2Br2).
The Lorentzian ESR spectra
were recorded with the applied magnetic field normal to the [110] axis. shows the angular dependence of g-values and peak-to-peak linewidths.
Figure When
the magnetic field is applied along the a' axis (~[110]) which is closely parallel to the long axis of BEDT-TTF molecules, ga,(=2.014) shows the largest g value.
On the other hand, with the applied field along the c' axis (±[110])
which is nearly parallel to the donor's stacking axis the smallest value (gc,=2.003)
was obtained.
These g-values indicate that these signals are
ascribed to the BEDT-TTF cation radicals [8]. The temperature dependence of the g-values, the peak-to-peak linewidths, and the spin susceptibility
from 300 K to 10 K are depicted in Figs. 5a) and b) when
the magnetic field is applied along the a' axis.
Over the entire temperature
A405 2.014 ~2.012 •
0000
O0000
O0
O0
0 O0
O oo
• 000~0
O0
1£
OO
~ OO
O
00000
O
5O
°O °
°
~0.8
000@000
0 O 0
O0 0
4C
0 0
O°
o
O
0
~3c
°
~0.4 x
0
2C
0 0
o
02 o ~ °
0
1£
~b
o
0
00
IlK
e
0
~0
T/K
~0 a)
300
b)
Fig. 5. Temperature dependence of a) g-values, linewidths(AH), and b) ESR magnetic susceptibility (X) of (BEDT-TTF)3Br2(H20) 2.
range the g-value (2.O013) does not change significantly.
In the high
temperature region above around 200 K no change of the linewidth (50 G) is observed.
Below it the linewidth gradually narrows with decreasing temperature
to 3.5 G at 12 K. x 10 -4 emu/mol)
The spin susceptibility above 200 K is relatively large (9.7
and independent of temperature, which is consistent with the
Pauli-paramagnetism expected for a metal.
This behavior is agreeable to the
metallic behavior of electrical resistivity above 185 K.
Assuming the one-
dimensional tight-binding model, magnetic susceptibility can be described as 2 X=PB NA/ztsin(wN /2) !
where
N A is Avogadoro's
transfer
per donor
is estimated spin
molecule.
as 0.038
susceptibility
number,
eV.
U B is Bohr From
magneton,
the above
equation,
In the low temperature
decreases
with
exponential
N
region
equation
is the amount the transfer below
of charge integral
around
t
200 K, the
of the form,
x=O/T'exp(-J/kBT) in which X is the spin susceptibility, C is the Curie constant, J is the magnetic activation energy, and k B is the Boltzman constant. fitting gives J of 0.017 eV.
The calculated
The decrease in both the spin susceptibility and
the linewidths below around 200 K is consistent with the metal-semiconductor transition of electrical resistivity.
The behavior of the linewidth and the
magnetic susceptibility is similar to that of (BEDT-TTF)3(CI04) 2.
However, the
g-value of the 6104 salt changes clearly at the transition temperature from metal to semiconductor, whereas that of the bromide salt is constant.
Therefore
the further study is necessary to clarify whether the transition is originated from Peierls instability or not.
A406
REFERENCES I
H. Urayama, H. Yamochi, G. Saito, K. Nozawa, T. Sugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto, and J. Tanaka, Chem. Lett., 1988 55.
2
E. B. Yagubskii, I. F° Shchegolev, V. N. Laukhin, P. A. Kononovich, M. V. Karatsovnic, A. V. Zvarykiha, and L. I. Buravov, Pis'ma Zh. Eksp. Theor. Fiz. 39 (1984) 12; H. H. Wang, H. A. Beno, V. Geiser, M. A. Firestone, K. S. Webb, L. Nunez, G. W. Grabtree, K. D. Carlson, J. M. Williams, L. J. Azevedo, J. F. Kwak, J. E. Schirber, Inorg. Chem., 24 (1985) 2466; J. M. Williams, H. H. Wang, M. A. Beno, T. J. Emge, L. M. Sowa, P. T. Copps, F. Behroozi, L. N. Hall, K. D. Carlson, G. W. Crabtree, Inorg. Chem., 23 (1984) 3839. R. N. Lynbovskaya, R. B. Lyubovskii, R. P. Shibaeva, M. Z. Aldoshina, L. M. Gol'denberg, L. P. Rozenber, M. L. Khidekel, and Yu. F. Shul'pyakov, Pis'ma Zh. Eksp. Teor. Fiz., 42 (1985) 380; R. N. Lyubovskaya, E. A. Zhilyaeva, A. V. Zvarykina, V. N. laukhin, R. B. Lyubovskii, and S. I. Pesotskii, ibid. r 45 (1987) 416.
4
a) T. Mori and H. Inokuchi, Solid State Commun., 64 (1987) 335; b) T. Mori and H. Inokuchi, Chem. Lett., 1987 1657; D. Chasseau, D. Watkin, M. Rosseinsky, M. Kurmoo, and P. Day, Synth. Met., 24 (1988)
5
H. Urayama, O. Saito, A. Kawamoto, and J. Tanaka, Chem. Lett., 1987, 1753.
6
H. Kobayashi, R. Kato, T. Mori, A. Kobayashi, Y. Sasaki, G. Saito, T. Enoki,
7
H. Kuroda, K. Yakushi, H. Tajima, and G. Saito, Mol. Cryst. Liq. Cryst. 125
and H. Inokuchi, Chem. Lett., 1984, 179.
(1985) 135. T. 8ugano, G. Saito, and M. Kinoshita, Phy. Rev. B 34 (1986) 117. T. Enoki, K. Imaeda, M. Kobayashi, H. Inokuchi, and G. Saito, Phys. Rev. B 33 (1986) 1553.
A401
~REPARATION, STRUCTURE, AND PHYSICAL PROPERTIES OF AN ORGANIC CONDUCTOR,
(BEDT-
7TF)3~2~2~2 H. URAYAMA, G. SAITO, T. SUGANO, and M. KINOSHITA ISSP, University of Tokyo, Tokyo (Japan) A. KAWAMOT0 and J. TANAKA University of Nagoya, Nagoya (Japan)
ABSTRACT The BEDT-TTF complex with a small anion, Br , has been synthesized with the accidental inclusion of H20 which may stabilize the crystallization. black plate crystal, TTF)3(CIO4) 2.
(BEDT-TTF)3Br2(H20)2,
The given
is isostructual to (BEDT-
Two bromide anions and two water molecules, Br-2(H20)2, form a
planar square cluster with hydrogen bonds.
This cluster constructs a insulating
sheet which is sandwiched by two-dimensional BEDT-TTF sheets.
The electrical
resistivity shows the metallic behavior down to 185 K and transforms to semiconducting system, which is also confirmed by the ESR measurements.
INTRODUCTION BEDT-TTF is a talented donor to give a variety of charge transfer complexes. Especially, more than ten kinds of BEDT-TTF salts exhibit superconductivity. Among them (BEDT-TTF)2Cu(NCS) 2 recorded the highest critical temperature, IO.L K, under an ambient pressure [I].
Though BEDT-TTF salts which contain symmetric
linear anions (13, IBr2, and Aul 2 [2]) and clusters (Hg3Br8, Hg3CI 8 [3], Cu(NCS) 2 [I]) have been found to be superconductors, a small compact anion salt had not been explored out except the CI- salt [4a].
In this paper the crystal
structure and the physical properties of a Br- complex [5] is presented.
EXPERIMENTAL Black plate-crystals were grown by electrochemical crystallization of BEDTTTF in chlorobenzene and tetrahydrofuran by using tetra-n-butylammonium bromide as a supporting electrolyte during I-2 months.
The lattice parameters of (BEDT-
TTF)3Br2(H20) 2 are; triclinic, PT, a=16.167(2), b=18.125(3), c=7.718(I) ~,
0379-6779/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
A402 ~=91.O5(2), ~=94.09(2), y=148.18(1) °, V=1176.0(4) ~3 and Z=I.
The crystal
structure was solved by the heavy atom method and refined by the block-diagonal least-squares method by using 2574 independent reflections (Cu Ks, 2e<120 °, IFoI >3~(IFoI)).
The final R is 0.082.
The temperature dependence of electrical
resistivity was measured by a standard d.c. four-probe technique with a gold paint as an electrode.
ESR studies were performed by utilizing a JEOL JES-FEIXG
(9.2 GHz) spectrometer over 300-10 K.
The sample crystal was placed at the
center of cylindrical cavity in the TE011 mode.
RESULTS and DISCUSSION Crystal structure The crystal structure of (BEDT-TTF)3Br2(H20) 2 is illustrated in Fig. I. unit cell is composed of three H20 molecules.
The
BEDT-TTF molecules, two bromide anions, and two
The inclusion of H20 was confirmed by the elemental analysis;
Found: C, 26.69; H, 2.09 %. Calcd for Br2S2402C30H28 : C, 26.40; H, 1.97 %.
As
the starting materials were purified with the exclusion of H20 , air humidity had been included into this complex to form the stable single crystals during the course of electroerystallization for I-2 months. (BEDT-TTF)3Br2(H20) 2 has a layered structure: the organic conducting sheets and the inorganic insulator ones stack alternatingly along the [IT0] axis.
In
the latter sheets, Br, Br', 0, and O' of Br2(H20) 2 form a rectangular unit (Fig. o
la).
The distances of Br..O and O''Br' (3.36 and 3.26 A, respectively) are a
little bit longer than the van der Waals contact (3.22 ~) and the angle of Br-OBr' (111°) is close to that of H-O-H (104°), which suggests the hydrogen contact of Br-. H-O-H around an inversion center. TTF)3CI2(H20) 2 [4b].
This part nearly resembles to (BEDT-
A little different point between them is that C14(H20) 4 is
one unit which is composed of a rectangular C12(H20) 2 and the additional hydrogen bonded 2(CI(H20)).
Therefore the unit cell volume of the bromide
complex is a half as large as that of the chloride one.
r~-%~ ~-o~p
o.
~
~ .a ~
b
o k ~
a} Fig. 1. Crystal structure of (BEDT-TTF)3Br2(H20) 2 projected on the a) bc and b) ab plane.
A403
[112]
[221 ] [11Y1 Fig. 2. Molecular arrangement of (BEDT-TTF)3Br2(H20) 2.
In the donor layers two BEDT-TTF molecules are crystallographically independent.
One(A) is on the center of symmetry and the other(B) is on the
general position as shown in Fig. I.
The similar intramolecular bond lengths
and angles of two independent donors indicate no charge separation which leads the donor to be BEDT-TTF 2/3+.
As shown in Fig. 2, there are three interacting
chains of donors: a stacking along the [11~] axis, two side-by-side ones along the [112] and [221] axes.
Among them short S''S contacts (<3.6 ~) are only
found in the [221] direction.
However, this does not mean that the strongest
interaction is observed along [221] direction since an extended H~ckel calculation and reflectance spectra in the (BEDT-TTF)3(C10&)2, which is isostructural to the bromide complex show that the strongest interaction
[6] and
dispersion [7] are detected along another side-by-side direction [112] but not along the direction [221] with short contacts.
The inclusion of H20 made the
complex of a small bromide anion belong to structurally a tetrahedral anion system in BEDT-TTF salts.
Electrical properties The electrical resistivity versus temperature on the (110) plane of a typical crystal is plotted in Fig. 3.
Above 185 K the salt is weakly metallic with a
room-temperature resistivity of 0.O13 ~ cm. to a semiconducting system below 185 K. still below (0.05 ~cm). 0.021 eV.
The material undergoes a transition
The resistivity at this temperature is
The activation energy just below 185 K is obtained as
In order to confirm the metallic state and observe the transition
property, ESR measurements were performed.
A404
OD-=
E u r o
%
%
,L
i
I
100
200
300
TIK
Fig. 3. Electrical resistivity of (BEDT-TTF)3Br2(H20) 2.
5C
<
@
@
@
@ @
@
@
4C o
@
@
@
@
[]
~: 3c
2.014
[] m
2C
[]
,
[]
10
2,002
c' '
'
a~'
25
'
'
2.010 En 2.006
~ ' ' e / degree
;~
'
;eo
Fig. 4. Angular dependence of g-values and linewidths(AH) of (BEDT-TTF) 3 Br2(H20)2 •
Electron spin resonance studies As shown in Fig. Ib, the two-dimensional sheets parallel to the (ITO) plane are sandwiched by the anion sheets (2(H20)"2Br2).
The Lorentzian ESR spectra
were recorded with the applied magnetic field normal to the [110] axis. shows the angular dependence of g-values and peak-to-peak linewidths.
Figure When
the magnetic field is applied along the a' axis (~[110]) which is closely parallel to the long axis of BEDT-TTF molecules, ga,(=2.014) shows the largest g value.
On the other hand, with the applied field along the c' axis (±[110])
which is nearly parallel to the donor's stacking axis the smallest value (gc,=2.003)
was obtained.
These g-values indicate that these signals are
ascribed to the BEDT-TTF cation radicals [8]. The temperature dependence of the g-values, the peak-to-peak linewidths, and the spin susceptibility
from 300 K to 10 K are depicted in Figs. 5a) and b) when
the magnetic field is applied along the a' axis.
Over the entire temperature
A405 2.014 ~2.012 •
0000
O0000
O0
O0
0 O0
O oo
• 000~0
O0
1£
OO
~ OO
O
00000
O
5O
°O °
°
~0.8
000@000
0 O 0
O0 0
4C
0 0
O°
o
O
0
~3c
°
~0.4 x
0
2C
0 0
o
02 o ~ °
0
1£
~b
o
0
00
IlK
e
0
~0
T/K
~0 a)
300
b)
Fig. 5. Temperature dependence of a) g-values, linewidths(AH), and b) ESR magnetic susceptibility (X) of (BEDT-TTF)3Br2(H20) 2.
range the g-value (2.O013) does not change significantly.
In the high
temperature region above around 200 K no change of the linewidth (50 G) is observed.
Below it the linewidth gradually narrows with decreasing temperature
to 3.5 G at 12 K. x 10 -4 emu/mol)
The spin susceptibility above 200 K is relatively large (9.7
and independent of temperature, which is consistent with the
Pauli-paramagnetism expected for a metal.
This behavior is agreeable to the
metallic behavior of electrical resistivity above 185 K.
Assuming the one-
dimensional tight-binding model, magnetic susceptibility can be described as 2 X=PB NA/ztsin(wN /2) !
where
N A is Avogadoro's
transfer
per donor
is estimated spin
molecule.
as 0.038
susceptibility
number,
eV.
U B is Bohr From
magneton,
the above
equation,
In the low temperature
decreases
with
exponential
N
region
equation
is the amount the transfer below
of charge integral
around
t
200 K, the
of the form,
x=O/T'exp(-J/kBT) in which X is the spin susceptibility, C is the Curie constant, J is the magnetic activation energy, and k B is the Boltzman constant. fitting gives J of 0.017 eV.
The calculated
The decrease in both the spin susceptibility and
the linewidths below around 200 K is consistent with the metal-semiconductor transition of electrical resistivity.
The behavior of the linewidth and the
magnetic susceptibility is similar to that of (BEDT-TTF)3(CI04) 2.
However, the
g-value of the 6104 salt changes clearly at the transition temperature from metal to semiconductor, whereas that of the bromide salt is constant.
Therefore
the further study is necessary to clarify whether the transition is originated from Peierls instability or not.
A406
REFERENCES I
H. Urayama, H. Yamochi, G. Saito, K. Nozawa, T. Sugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto, and J. Tanaka, Chem. Lett., 1988 55.
2
E. B. Yagubskii, I. F° Shchegolev, V. N. Laukhin, P. A. Kononovich, M. V. Karatsovnic, A. V. Zvarykiha, and L. I. Buravov, Pis'ma Zh. Eksp. Theor. Fiz. 39 (1984) 12; H. H. Wang, H. A. Beno, V. Geiser, M. A. Firestone, K. S. Webb, L. Nunez, G. W. Grabtree, K. D. Carlson, J. M. Williams, L. J. Azevedo, J. F. Kwak, J. E. Schirber, Inorg. Chem., 24 (1985) 2466; J. M. Williams, H. H. Wang, M. A. Beno, T. J. Emge, L. M. Sowa, P. T. Copps, F. Behroozi, L. N. Hall, K. D. Carlson, G. W. Crabtree, Inorg. Chem., 23 (1984) 3839. R. N. Lynbovskaya, R. B. Lyubovskii, R. P. Shibaeva, M. Z. Aldoshina, L. M. Gol'denberg, L. P. Rozenber, M. L. Khidekel, and Yu. F. Shul'pyakov, Pis'ma Zh. Eksp. Teor. Fiz., 42 (1985) 380; R. N. Lyubovskaya, E. A. Zhilyaeva, A. V. Zvarykina, V. N. laukhin, R. B. Lyubovskii, and S. I. Pesotskii, ibid. r 45 (1987) 416.
4
a) T. Mori and H. Inokuchi, Solid State Commun., 64 (1987) 335; b) T. Mori and H. Inokuchi, Chem. Lett., 1987 1657; D. Chasseau, D. Watkin, M. Rosseinsky, M. Kurmoo, and P. Day, Synth. Met., 24 (1988)
5
H. Urayama, O. Saito, A. Kawamoto, and J. Tanaka, Chem. Lett., 1987, 1753.
6
H. Kobayashi, R. Kato, T. Mori, A. Kobayashi, Y. Sasaki, G. Saito, T. Enoki,
7
H. Kuroda, K. Yakushi, H. Tajima, and G. Saito, Mol. Cryst. Liq. Cryst. 125
and H. Inokuchi, Chem. Lett., 1984, 179.
(1985) 135. T. 8ugano, G. Saito, and M. Kinoshita, Phy. Rev. B 34 (1986) 117. T. Enoki, K. Imaeda, M. Kobayashi, H. Inokuchi, and G. Saito, Phys. Rev. B 33 (1986) 1553.