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Compressibility of one dimensional phthalocyanine conductors, NiPc(AsF6)0.5 and CoPc(AsF6)0.5
ELSEVIER
Synthetic Metals 86 (1997)
Compressibility
The Graduate University
2167-2168
of One Dimensional Phthalocyanine NiPc(AsF,)o_S and CoPc(AsF,j)0.5 .
Conductors,
Toshihiro HIEJIMA and Kyuya YAKUSHI for Advanced Studies and Institute for Molecular Science,
The Graduate University for Advanced
Okazaki, Aichi, 444, JAPAN
Takafumi ADACHI and Osamu SHIMOMURA Studies and National Laboratory for High Energy Physics,
lchimin SHIROTANI Muroran Institute of Technology, 27-1, Mizumoto, Muroran-shi,
Oho, Tsukuba-shi,
305, JAPAN
050, JAPAN
Abstract Powder X-ray diffraction patterns of phthalocyanine conductors, NiPc(AsF&s and COPC(ASF&~, were measured under high pressure. The crystal symmetries of these compounds were maintained in the pressure range of 0 - 4.1 GPa. The contraction along the stacking axis is larger than that perpendicular to the axis. This diffcrencc becomes clear at the pressure where the metal-ligand charge transfer occurs. The relation between the lattice contraction and the pressure-induced metal-ligand charge transfer is discussed. Keyword: ( Organic conductors
based on radical cation, X-ray emission, diffraction
1. INTRODUCTION One-dimensional conductors based on a metal phthalocyanine (MPc) are the double-chain system which involves two conductive pathways in the same molecular column. In almost all of the MPc salts, the n-HOMO band is partially oxidized and thus this n-band has been ascribed to the conduction band. However, in CoPcI the 3d+band is partially oxidized. It should be noticed that the lattice constants of CoPcI are the smallest among the phthalocaynine conductors with a similar crystal symmetry. The lattice constants seem to be an important factor in determining the oxidation part.[l-31 We have reported that the oxidation part of NiPc(AsF6)u5 and CoPc(AsF,& changes from the macrocycle to the central metal ions ( metal-ligand charge transfer ) when a high pressure is applied.[4] The information on the lattice contraction is important to understand this metalligand charge transfer. We report here the pressure dependent powder X-ray diffraction patterns, and discuss about the reason why the metal-ligand charge transfer is induced by high pressure. 2. EXPERIMENTAL
The crystals of NiPc(AsF,& and CoPc(AsF,&5 were prepared by an electrochemical method.[3] The former belongs to an orthorHombic system and the latter to a tetragonal system. The powder X-ray diffraction of NiPc(AsF& and COP~(ASF&~ were respectively measured using an energy dispersion and angle dispersion methods at the beam lines BL0379-6779/97/$17.00 Q 1997 Elsevier Science S.A All rights r-4 PII SO3796779(%)04789-3
and scattering,
)
13B and BL-6B in the Photon Factory Facility of National Laboratory for High Energy Physics (KEK).[5,6] Each X-ray diffraction line was assigned utilizing the results of the X-ray crystal structure analysis of the single crystal. The lattice constants were calculated by applying the least-square method to the uniquely indexed and well isolated diffraction lines. The crystal symmetry of orthorhombic NiPc(AsF& was approximated to a tetragonal symmetry, since b/2 is very close to u and so (200) and (040), or (141) and (221) was not separated from each other. The maximum error of the d-value is 0.02 % at 3.5 GPa in COPC(ASF&~ and 0.5 % at 4.1 GPa in NiPc(AsF&5. 3. RESULTS AND DISCUSSION Figures 1 and 2 show the pressure-dependence of the powder X-ray diffraction patterns of COPC(ASF,&~.~ and NiPc(AsF&, respectively. Each diffraction line continuously shifts to the high angle side on increasing the pressure. The disappearance, appearance, or splitting of the diffraction line was not observed in any experimental pressure region. These results mean that the metal-ligand charge transfer is not accompanied by a large structural change. Figures 3 and 4 show the pressure dependence of the lattice contraction ratio, [x(O)-x(P)]/x(O) of COPC(ASF&S and NiPc(AsF,&, respectively. As shown in figure 3, the lattice constants of CoPc(AsF&~ decrease linearly up to 2 GPa. The compressibility (x-tdx/dP) in this pressure range is -0.018 and 0.023 GPa-1 for a- and b-axes and c-axis, respectively. On the
T. Hiejima et al. /Synthetic
2168
other hand, both lattice constants of NiPc(AsFb)n,5 decrease nonlinearly from the low pressure range. The contraction ratio can be expressed as [x(O)-x(P)]/x(O) = a P p with a = 0.030, l3 = 0.56 for u-axis and a = 0.045, l3 = 0.58 for c-axis. The compressibility of NiPc(AsF6)nS in low pressure range ( P < 1 GPa ) is larger that that of CoPc(A~Fe)tt,~, but is almost the same in high pressure range. As shown in these figures, the contraction along the c-axis is larger than that perpendicular to the c-axis in both compounds. The results that the compressibility along the molecular stacking axis is larger than other axis is consistent with other organic conductors such as ‘ITF-TCNQ and (TMTSF),
Metals 86 (199:) 2167-2168
theory. When the 3d,2-orbital moves over the Fermi level of the n-band of the macrocycle at a threshold pressure, the electrons of 3d+orbital begins to move to the vacant states of n-band. This selective contraction of the metal-metal distance is the origin of the pressure-induced metal ligand charge transfer. As the metal-metal distance is shortened, the orbital energy moves up to high energy side, and thus the metal-ligand charge transfer occurs continuously. This scenario can explain the continuous change of the optical absorption.
PF6.[7,81 0
0.06 -
*
0, om-
*
e
0.02-
. .
3
1
”
”
Prehure
I
0.5
0.0
1.0 1.5 Pn/d(A-‘)
Fig. 1 Pressure dependence patterns of CoPc(AsF,)
2.0
4
E
Fig. 3 Pressure dependence of lattice contractionratios of lines show the guide Iw the eyes CoPc(AsFs ) a5’Thedotted
1
13
3.
( &a )
2.5
of powder X-ray diffraction
0.12Y
,,5
0.10 0
:-O
. ..O c-axis
___....... 7 .:m
‘;; o.OE-
/W.’
;-A
4.1
.C
si.-
a Y 2
wa
0.060.04
-
Y -
0.w:
0.
E
j -.i’...
-k
0.0
..c)-.-~~;;;,is
//O.... ,,:o. P’.P..’
0.5
1.0 1.5 Pn/d(A.‘)
2.5
Fig. 2 Pressure dependence of powderX-ray diffraction pattemofNiPc(AsF,)a,
Let us discuss the relation between the lattice contraction and the metal-ligand charge transfer. The environment of the transition metal ion of CoPc(AsF& and NiPc(AsFg)o.5 is a distorted octahedron elongated along the c-axis. The metal is coordinated by four nitrogen atoms with a distance of 1.91 A and by two metal ions with distances of 3.148 A in COP~(ASF&~ and 3.233 A in NiPc(AsF&S at ambient pressure. When pressure is applied, the metal-metal distance is significantly shortened, for example, the metal-metal distance is 2.95 A at 3.1 GPa in CoPc(AsF&5 and 2.93 A at 4.1 GPa in NiPc(AsFe&. However, the metal-nitrogen distance does not change under this pressure, because the intramolecular covalent band does not change at this pressure range. Therefore the metal-metal distance is selectively shortened. As a result, the elongated octahedron approaches a octahedron. This geometrical change around the transition metal makes a big influence on the splitting of the 3d-orbital, and pushes up the orbital energy of the 3d+orbital according to the ligand field
1
2 3 Pressure ( GPa
)
4
5
Fig. 4 Pressure dependence of lattice contractionratios of NiPc(AsFs) a5’ The dotted lirm show the guide Iw the eyes.
xi
2.0
.B’ .’
Incidentally, this metal-ligand charge transfer is different from the Neutral-to-Ionic (NI) phase transition of TTF-CA tetrathiafulvalene-p-chloranil in several respects.[7] First, the NI phase transition is a first order phase transition and so the charge transfer from TI’F to CA occurs discontinuously at a critical pressure. Second, the NI phase transition is accompanied by a dimerization of the lattice. So the spin Peierls-like mechanism is involved as well. In contrast, the metal-ligand charge transfer occurs continuously from 0.5 GPa in NiPc(AsF&S and 1 .l GPa in CoPc(AsF&~. These differences are related to the different mechanism of charge transfer in both compounds.
REFERENCES [l] [2] [3] [4] [5] [6] [7]
C. J. Schramm et al., J. Am. Chem. Sot., 102,6702 (1980). J. Martinsen et al.,J. Am. Chem. Sot. 107, 6915, (1990). K. Yakushi et al., Bull. Chem. Sot. Jpn., 62,687 (1989). T. Hiejima et al., J. Chem. Phys., 103,395O (1995). I. Shirotani et al., Phys. Lett. A, 205,77 (1995). 0. Shimomura et al., Rev. Sci. Instrum., 63,967 (1992). J. B. Torrance et al., Phys. Rev. Lett., 46, 253 (1981).
Synthetic Metals 86 (1997)
Compressibility
The Graduate University
2167-2168
of One Dimensional Phthalocyanine NiPc(AsF,)o_S and CoPc(AsF,j)0.5 .
Conductors,
Toshihiro HIEJIMA and Kyuya YAKUSHI for Advanced Studies and Institute for Molecular Science,
The Graduate University for Advanced
Okazaki, Aichi, 444, JAPAN
Takafumi ADACHI and Osamu SHIMOMURA Studies and National Laboratory for High Energy Physics,
lchimin SHIROTANI Muroran Institute of Technology, 27-1, Mizumoto, Muroran-shi,
Oho, Tsukuba-shi,
305, JAPAN
050, JAPAN
Abstract Powder X-ray diffraction patterns of phthalocyanine conductors, NiPc(AsF&s and COPC(ASF&~, were measured under high pressure. The crystal symmetries of these compounds were maintained in the pressure range of 0 - 4.1 GPa. The contraction along the stacking axis is larger than that perpendicular to the axis. This diffcrencc becomes clear at the pressure where the metal-ligand charge transfer occurs. The relation between the lattice contraction and the pressure-induced metal-ligand charge transfer is discussed. Keyword: ( Organic conductors
based on radical cation, X-ray emission, diffraction
1. INTRODUCTION One-dimensional conductors based on a metal phthalocyanine (MPc) are the double-chain system which involves two conductive pathways in the same molecular column. In almost all of the MPc salts, the n-HOMO band is partially oxidized and thus this n-band has been ascribed to the conduction band. However, in CoPcI the 3d+band is partially oxidized. It should be noticed that the lattice constants of CoPcI are the smallest among the phthalocaynine conductors with a similar crystal symmetry. The lattice constants seem to be an important factor in determining the oxidation part.[l-31 We have reported that the oxidation part of NiPc(AsF6)u5 and CoPc(AsF,& changes from the macrocycle to the central metal ions ( metal-ligand charge transfer ) when a high pressure is applied.[4] The information on the lattice contraction is important to understand this metalligand charge transfer. We report here the pressure dependent powder X-ray diffraction patterns, and discuss about the reason why the metal-ligand charge transfer is induced by high pressure. 2. EXPERIMENTAL
The crystals of NiPc(AsF,& and CoPc(AsF,&5 were prepared by an electrochemical method.[3] The former belongs to an orthorHombic system and the latter to a tetragonal system. The powder X-ray diffraction of NiPc(AsF& and COP~(ASF&~ were respectively measured using an energy dispersion and angle dispersion methods at the beam lines BL0379-6779/97/$17.00 Q 1997 Elsevier Science S.A All rights r-4 PII SO3796779(%)04789-3
and scattering,
)
13B and BL-6B in the Photon Factory Facility of National Laboratory for High Energy Physics (KEK).[5,6] Each X-ray diffraction line was assigned utilizing the results of the X-ray crystal structure analysis of the single crystal. The lattice constants were calculated by applying the least-square method to the uniquely indexed and well isolated diffraction lines. The crystal symmetry of orthorhombic NiPc(AsF& was approximated to a tetragonal symmetry, since b/2 is very close to u and so (200) and (040), or (141) and (221) was not separated from each other. The maximum error of the d-value is 0.02 % at 3.5 GPa in COPC(ASF&~ and 0.5 % at 4.1 GPa in NiPc(AsF&5. 3. RESULTS AND DISCUSSION Figures 1 and 2 show the pressure-dependence of the powder X-ray diffraction patterns of COPC(ASF,&~.~ and NiPc(AsF&, respectively. Each diffraction line continuously shifts to the high angle side on increasing the pressure. The disappearance, appearance, or splitting of the diffraction line was not observed in any experimental pressure region. These results mean that the metal-ligand charge transfer is not accompanied by a large structural change. Figures 3 and 4 show the pressure dependence of the lattice contraction ratio, [x(O)-x(P)]/x(O) of COPC(ASF&S and NiPc(AsF,&, respectively. As shown in figure 3, the lattice constants of CoPc(AsF&~ decrease linearly up to 2 GPa. The compressibility (x-tdx/dP) in this pressure range is -0.018 and 0.023 GPa-1 for a- and b-axes and c-axis, respectively. On the
T. Hiejima et al. /Synthetic
2168
other hand, both lattice constants of NiPc(AsFb)n,5 decrease nonlinearly from the low pressure range. The contraction ratio can be expressed as [x(O)-x(P)]/x(O) = a P p with a = 0.030, l3 = 0.56 for u-axis and a = 0.045, l3 = 0.58 for c-axis. The compressibility of NiPc(AsF6)nS in low pressure range ( P < 1 GPa ) is larger that that of CoPc(A~Fe)tt,~, but is almost the same in high pressure range. As shown in these figures, the contraction along the c-axis is larger than that perpendicular to the c-axis in both compounds. The results that the compressibility along the molecular stacking axis is larger than other axis is consistent with other organic conductors such as ‘ITF-TCNQ and (TMTSF),
Metals 86 (199:) 2167-2168
theory. When the 3d,2-orbital moves over the Fermi level of the n-band of the macrocycle at a threshold pressure, the electrons of 3d+orbital begins to move to the vacant states of n-band. This selective contraction of the metal-metal distance is the origin of the pressure-induced metal ligand charge transfer. As the metal-metal distance is shortened, the orbital energy moves up to high energy side, and thus the metal-ligand charge transfer occurs continuously. This scenario can explain the continuous change of the optical absorption.
PF6.[7,81 0
0.06 -
*
0, om-
*
e
0.02-
. .
3
1
”
”
Prehure
I
0.5
0.0
1.0 1.5 Pn/d(A-‘)
Fig. 1 Pressure dependence patterns of CoPc(AsF,)
2.0
4
E
Fig. 3 Pressure dependence of lattice contractionratios of lines show the guide Iw the eyes CoPc(AsFs ) a5’Thedotted
1
13
3.
( &a )
2.5
of powder X-ray diffraction
0.12Y
,,5
0.10 0
:-O
. ..O c-axis
___....... 7 .:m
‘;; o.OE-
/W.’
;-A
4.1
.C
si.-
a Y 2
wa
0.060.04
-
Y -
0.w:
0.
E
j -.i’...
-k
0.0
..c)-.-~~;;;,is
//O.... ,,:o. P’.P..’
0.5
1.0 1.5 Pn/d(A.‘)
2.5
Fig. 2 Pressure dependence of powderX-ray diffraction pattemofNiPc(AsF,)a,
Let us discuss the relation between the lattice contraction and the metal-ligand charge transfer. The environment of the transition metal ion of CoPc(AsF& and NiPc(AsFg)o.5 is a distorted octahedron elongated along the c-axis. The metal is coordinated by four nitrogen atoms with a distance of 1.91 A and by two metal ions with distances of 3.148 A in COP~(ASF&~ and 3.233 A in NiPc(AsF&S at ambient pressure. When pressure is applied, the metal-metal distance is significantly shortened, for example, the metal-metal distance is 2.95 A at 3.1 GPa in CoPc(AsF&5 and 2.93 A at 4.1 GPa in NiPc(AsFe&. However, the metal-nitrogen distance does not change under this pressure, because the intramolecular covalent band does not change at this pressure range. Therefore the metal-metal distance is selectively shortened. As a result, the elongated octahedron approaches a octahedron. This geometrical change around the transition metal makes a big influence on the splitting of the 3d-orbital, and pushes up the orbital energy of the 3d+orbital according to the ligand field
1
2 3 Pressure ( GPa
)
4
5
Fig. 4 Pressure dependence of lattice contractionratios of NiPc(AsFs) a5’ The dotted lirm show the guide Iw the eyes.
xi
2.0
.B’ .’
Incidentally, this metal-ligand charge transfer is different from the Neutral-to-Ionic (NI) phase transition of TTF-CA tetrathiafulvalene-p-chloranil in several respects.[7] First, the NI phase transition is a first order phase transition and so the charge transfer from TI’F to CA occurs discontinuously at a critical pressure. Second, the NI phase transition is accompanied by a dimerization of the lattice. So the spin Peierls-like mechanism is involved as well. In contrast, the metal-ligand charge transfer occurs continuously from 0.5 GPa in NiPc(AsF&S and 1 .l GPa in CoPc(AsF&~. These differences are related to the different mechanism of charge transfer in both compounds.
REFERENCES [l] [2] [3] [4] [5] [6] [7]
C. J. Schramm et al., J. Am. Chem. Sot., 102,6702 (1980). J. Martinsen et al.,J. Am. Chem. Sot. 107, 6915, (1990). K. Yakushi et al., Bull. Chem. Sot. Jpn., 62,687 (1989). T. Hiejima et al., J. Chem. Phys., 103,395O (1995). I. Shirotani et al., Phys. Lett. A, 205,77 (1995). 0. Shimomura et al., Rev. Sci. Instrum., 63,967 (1992). J. B. Torrance et al., Phys. Rev. Lett., 46, 253 (1981).