- Email: [email protected]
Experimental charge density study of (DBr-DCNQI)2Cu for metallic phase by synchrotron X-ray diffraction
ARTICLE IN PRESS Physica B 405 (2010) S321–S323
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Experimental charge density study of (DBr-DCNQI)2Cu for metallic phase by synchrotron X-ray diffraction S. Maki a, E. Nishibori a, H. Okabayashi a, R. Sato a, S. Aoyagi a, H. Sawa a,, R. Kato b a b
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Condensed Molecular Material Laboratory, RIKEN, Wako 351-0198, Japan
a r t i c l e in fo
Keywords: (DBr-DCNQI)2Cu Synchrotron radiation Maximum-entropy method Multipolar refinement Bader topological analysis
abstract An accurate experimental charge density of (DBr-DCNQI)2Cu in the metallic phase has been determined from the high resolution single-crystal diffraction data measured at one of the third generation synchrotron radiation X-ray sources, SPring-8. Two kinds of analytical method were used for determination of the experimental charge density. First, the maximum entropy method (MEM) was used for calculation of a total experimental charge density. Secondly, the multipolar refinement was used for calculation of the static deformation charge density, which represents the aspherical charge density in the molecules without thermal smearing effect. The charge densities indicating bonding electrons of DCNQI molecule were clearly revealed in the static deformation map. We estimated the valence charge for Cu ion by the Bader topological analysis. The determined value was +1.30(13)e, which is consistent with value in the previous studies. & 2010 Elsevier B.V. All rights reserved.
1. Introduction Molecular conductors have attracted interest for their novel electronic structures and intriguing physical properties. The attractive physical phenomena, such as superconductivity, a metal-insulator transition, and a magnetic transition, in the molecular conductors are usually accompanied by the structural changes, such as a charge transfer (CT), a charge ordering (CO), and a charge density wave (CDW). An accurate charge density study from X-ray diffraction is one of the most powerful techniques to determine the structure. However, it is difficult to reveal electronic structures in molecular conductors using X-ray diffraction method, because it is normally required to detect the charge transfer of less than one electron. In addition, the charge density study has to reveal not only the CO and CDW but also aspherical charge-density in the molecule. Both the high-quality diffraction data and the sophisticated analytical techniques are required for charge density study of the molecular conductor. Recently, a large imaging plate (IP) camera for an accurate charge density study has been installed at SPring-8, one of the third generation synchrotron radiation (SR) source. A combination of the SPring-8 data and the precise analytical technique would enable us to carry out charge density study of the molecular conductor with enough accuracy. In this study, we have carried out the charge density study of molecular conductor, (DBr-
Corresponding author. Tel.: + 81 527894453; fax: + 81 527893724.
E-mail address: [email protected] (H. Sawa). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.014
DCNQI)2Cu, in the metallic phase using the SPring-8 data. A molecular conductor (DBr-DCNQI)2Cu system, where DCNQI is the N, N0 -dicyanoquinonediimine, shows the novel metal–insulator (M–I) transition at 160 K [1–3]. The Cu ion is in a mixed-valence state in the metallic phase. During past 20 years, several experimental studies, such as the Li,Zn-doping and X-ray photoemission spectroscopy (XPS), were carried out to reveal the relation between the valency of Cu and the M–I transition [4–7]. These studies revealed that the valency of Cu ion is closely related to the stability in the metallic phase. Required accuracy of the determination of the valence state for Cu ion is o0.1 electron. 2. Experiment The SR single crystal X-ray diffraction experiments using a Weissenberg camera with an IP as a detector were carried out at SPring-8, BL02B1 beamline. Highly monochromatic X-ray beam by a Si(1 1 1) double-crystal monochromator and the collimating and the focusing mirrors were used for the experiment. The wave ˚ The beam size of the X-ray length of incident X-ray was 0.4148 A. was o0.15 0.15 mm2. The size of the single crystal was 0.06 0.02 0.02 mm3. In order to avoid intensity decay in the high angle region by thermal vibration effect, the sample was cooled to 200 K using a helium gas flow low temperature device. ˚ and high counting The data with wide d-spacing range, d 40.40 A, statistics, 107 counts, were obtained from the experiments. Integrated intensities of Bragg reflections were extracted from
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the two-dimensional IP data by the program RAPID AUTO. The intensities were processed by SORTAV software [8]. A crystal structural analysis was performed by SHELX97 software [9]. Table 1 Summary of the crystal data of (DBr-DCNQI)2Cu at 200 K. Formula Formula wt Crystal dimens, (mm3) Space group Cell dimens ˚ a (A)
C16H4N8Br4Cu 691.39 0.06 0.02 0.02 I41/a 21.5768(4) 3.8637(1)
˚ c (A) V (A˚ 3)
1798.78(7)
Z 2y range (deg) Total no. of obsd reflctns No. of unique data (I 44s) Weighted schemea Average redundancy Overall completeness R (I 44s)
4 0–62.7 64 739 4059 a =0.0348, b= 1.3092 12.2 0.715 0.0328
a The weighting scheme is defined as w ¼ 1=½s2 ðFo2 Þ þ ða Fc2 Þ2 þ b Fc2 by SHELX97.
Table 2 (I) Bond lengths and angles of DBr-DCNQI molecule, (II) the coordination geometry of Cu of (DBr-DCNQI)2Cu (Using the indices in Ref. [3]). I
II
˚ a (A) ˚ b (A)
1.1618(13)
˚ c (A) ˚ d (A)
1.3279(14)
˚ e (A) ˚ f (A)
1.4320(13)
1.3041(13) 1.4408(13) 1.3582(14)
+N–C–N (deg) +C–N–C (deg) ˚ R (A)
173.05(12) 120.56(9) 1.9673(10)
a (deg)
125.94(5) 168.24(10)
y (deg)
3. Results and discussion The information of the refined structure at 200 K is summarized in Table 1. The values of the completeness and the redundancy for the data at the resolution range d 40.40 A˚ are 71.5% and 12.2, respectively, which are suitable for the accurate charge density study. The reliability factor based on structure factor was 3.28%. Other structural parameters, bond lengths, angles, and the coordination geometry around Cu ion are presented in Table 2, using the indices as shown in Fig. 1(a). The values of the bond length of the DCNQI molecule in Table 2-I are consistent with the previous values within their errors. As presented in Table 2-II, the parameter a which represents the angle +N–Cu–N, is 0.6(3)1 larger than the previously reported one. We calculated the experimental charge density (robs) by using maximum-entropy method (MEM) [10] from observed structure factors. We also calculated the deformation density, rdef, from the following equation, robs rsph, where rsph is the MEM charge density from calculated structure factors using a free spherical atomic model. Fig. 1(b) shows the section of the experimental total charge density, and Fig. 1(c) shows the section of the deformation density. Six carbon atoms of the six-membered ring in DCNQI molecule locate on the figure plane. Other atoms slightly shift on the plane in each map. In Fig. 1(b), there are the charge density indicating hydrogen atoms inside dashed circles. This fact indicates that the present charge density is reliable to discuss o1.0 electron level. The charge densities indicating bonding electrons are clearly seen in Fig. 1(c). We examined the charge density at the bond midpoint labelled as A, B, C, D, and E in Fig. 1(c). The values are as follows: A= 0.379 eA˚ 3, B= 0.167 eA˚ 3, C= 0.243 eA˚ 3, D=0.340 eA˚ 3, E= 0.324 eA˚ 3. The differences of covalency in the C–N triple bond, the C–N single bond, the C–C single bond, and the C–C double bond are clearly revealed by the deformation density. For the quantitative examination of charge-density differences at the bond midpoints, we carried out the multipolar refinement using XD2006 program [11]. In the multipolar refinement, total charge density, r, is divided into the core electron density, rcore, and the valence electron density, rvalence. The valence electron
Fig. 1. (a) A structural formula of DBr-DCNQI molecule with the indices labelling as a, b, c, d, e, f for the bond lengths (top). Schematic tetrahedral structure consist of Cu ion and four N atoms. The indices a for the angle N–Cu–N, and R for the distance between Cu ion and N atom (bottom). (b) The section of an experimental total charge density by the MEM. The contour lines drawn from 0.04 to 2.00 eA˚ 3 with 0.20 eA˚ 3 step width. The charge densities indicating hydrogen atoms are shown inside the dashed circles. (c) The section of the deformation density. The contour lines drawn from 0.50 to 0.40 eA˚ 3 with 0.09 eA˚ 3 step width. The solid lines represent the positive values, and the dashed lines represent the negative values.
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S323
XPS measurements determined the valencies of Cu ions for (DI-DCNQI)2Cu, (DMe-DCNQI)2Cu, (MeCl-DCNQI)2Cu, and (MeBrDCNQI)2Cu. The valencies of Cu ion for the materials without M–I transitions were + 1.28e for (DI-DCNQI)2Cu and + 1.22e for (DMeDCNQI)2Cu. The valencies of Cu ion for (MeCl-DCNQI)2Cu with TMI =210 K and (MeBr-DCNQI)2Cu with TMI = 160 K were + 1.31e and + 1.32e. The valency of Cu ion for (DBr-DCNQI)2Cu with TMI =160 K determined by the present study is + 1.30(13)e. This value is close to that of (MeCl-DCNQI)2Cu. The N–Cu–N bond angle shown as a in Table 2-II is also related to the M–I transition temperature. There are some inconsistencies between the angle a and the valence of Cu ion in the previous studies. The angle a was determined by the structural analysis using the X-ray diffraction data. The valency of Cu ion was determined by the XPS measurement. In the cases of (MeCl-DCNQI)2Cu and (MeBrDCNQI)2Cu, the increase in the average valence from + 1.31 to +1.32e decreases the angle a from 126.21 to 125.31. The present method can determine simultaneously the electronic structure and the atomic coordination of the material, such as the valency of Cu ion and the angle a. The present methods are suitable for the investigation of the relation between the valency of Cu ion and the M–I transition temperature more precisely.
Fig. 2. A contour map of the deformation density by the multipolar refinement. The inset figure shows the molecular structure of DBr-DCNQI molecule. The contour lines were drawn from 4.0 to 3.9 eA˚ 3 with 0.1 eA˚ 3 step width. The solid lines represent the positive values, and the dashed lines represent the negative values. The dotted lines represent the zero values of the charge density.
density is represented by the spherical harmonics functions as the multipolar functions. The multipolar refinement can improve the structure model refined by SHELX97, because it can represent aspherical charge-density contributions between the atoms. The charge density from the multipolar refinement enables us to estimate the static charge density without thermal smearing effect. Fig. 2 shows the section of the static deformation density determined by the multipolar refinement. This map is very similar to Fig. 1(c). The R-factor is improved from 3.56% to 2.75% by using multipolar model. These facts indicate that the multipolar refinement in the present study was successfully performed. The charge density expressed by the multipolar model was determined by this refinement. The deformation density at the bond midpoints were from 0.1 to 0.4 eA˚ 3. The differences between the MEM and multipolar deformation densities at the bond midpoints were within 0.05 eA˚ 3. We have determined the aspherical charge-density distribution in the molecule with an accuracy o0.05 eA˚ 3 by using the high-quality X-ray diffraction data measured at SPring-8. We carried out the Bader topological analysis [12] to estimate valence electrons for copper ion. The Bader analysis is one of the space partitioning-methods for total charge density based on the topology. All the charge densities in the unit cell belong to the independent atoms by the Bader analysis. The partitioned space for each atom is called atomic basin. The number of electrons for each atom is calculated by the charge integration in the atomic basin. The charge in the atomic basin of Cu ion was 27.7e indicating the valence status of Cu ion being + 1.30(13)e. Several studies were carried out to reveal the relation between the valency of Cu ion and the M–I transition temperature [5–7]. The studies of Li,Zn-dopings to the Cu site showed that the M–I transition can be suppressed with Li + substitution while Zn2 + substitution increases the M–I transition temperature, TMI [4]. The
4. Conclusion We have determined the experimental charge density of the molecular conductor (DBr-DCNQI)2Cu at 200 K by using SR single crystal X-ray diffraction data at SPring-8. The valency of Cu ion was determined as + 1.30(13)e with sufficient accuracy to discuss the mechanism of M–I transition. These results confirm that the charge density study using SPring-8 data are suitable to reveal an origin of novel phenomena in the science of molecular conductors.
Acknowledgments The authors thank to K. Sugimoto for experimental help at SPring-8. This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘New Frontier of Materials Science Opened by Molecular Degrees of Freedom’’ (No. 20110003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan. The SR single crystal diffraction experiments were carried out at the SPring-8 with the approval of Japan Synchrotron Radiation Research Institute. References ¨ ¨ ¨ [1] A. Aumuller, P. Erk, G. Klebe, S. Hunig, J.U. Schutz, H.-P. Werner, Angew. Chem. Internat. Ed. Engl. 25 (1986) 740. [2] A. Kobayashi, R. Kato, H. Kobayashi, T. Mori, H. Inokuchi, Solid State Commun. 64 (1987) 45. [3] R. Kato, H. Kobayashi, A. Kobayashi, J. Amer. Chem. Soc. 111 (1989) 5224. [4] H. Sawa, M. Tamura, S. Aonuma, M. Kinoshita, R. Kato, J. Phys. Soc. Japan 63 (1994) 4302. [5] H. Kobayashi, A. Miyamoto, R. Kato, F. Sakai, A. Kobayashi, Y. Yamakita, Y. Furukawa, M. Tasumi, T. Watanabe, Phys. Rev. B 47 (1993) 3500. [6] A. Tanaka, A. Chainani, T. Yokoya, T. Takahashi, T. Miyazaki, S. Hasegawa, T. Mori, Phys. Rev. B 52 (1995) 7951. [7] O. Akaki, A. Chainani, T. Takahashi, Y. Kashimura, R. Kato, Phys. Rev. B 57 (1998) 11846. [8] R.H. Blessing, J. Appl. Cryst. 30 (1997) 421. [9] G.M. Sheldrick, Acta Cryst. A 64 (2008) 112. [10] M. Sakata, M. Sato, Acta Cryst. 46 (1990) 66. [11] N.K. Hansen, P. Coppens, Acta Cryst. A 34 (1978) 909. [12] R.F.W. Bader, Atoms in Molecules: a Quantum Theory, The International Series of Monographs on Chemistry, Clarendon Press, Oxford.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Experimental charge density study of (DBr-DCNQI)2Cu for metallic phase by synchrotron X-ray diffraction S. Maki a, E. Nishibori a, H. Okabayashi a, R. Sato a, S. Aoyagi a, H. Sawa a,, R. Kato b a b
Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Condensed Molecular Material Laboratory, RIKEN, Wako 351-0198, Japan
a r t i c l e in fo
Keywords: (DBr-DCNQI)2Cu Synchrotron radiation Maximum-entropy method Multipolar refinement Bader topological analysis
abstract An accurate experimental charge density of (DBr-DCNQI)2Cu in the metallic phase has been determined from the high resolution single-crystal diffraction data measured at one of the third generation synchrotron radiation X-ray sources, SPring-8. Two kinds of analytical method were used for determination of the experimental charge density. First, the maximum entropy method (MEM) was used for calculation of a total experimental charge density. Secondly, the multipolar refinement was used for calculation of the static deformation charge density, which represents the aspherical charge density in the molecules without thermal smearing effect. The charge densities indicating bonding electrons of DCNQI molecule were clearly revealed in the static deformation map. We estimated the valence charge for Cu ion by the Bader topological analysis. The determined value was +1.30(13)e, which is consistent with value in the previous studies. & 2010 Elsevier B.V. All rights reserved.
1. Introduction Molecular conductors have attracted interest for their novel electronic structures and intriguing physical properties. The attractive physical phenomena, such as superconductivity, a metal-insulator transition, and a magnetic transition, in the molecular conductors are usually accompanied by the structural changes, such as a charge transfer (CT), a charge ordering (CO), and a charge density wave (CDW). An accurate charge density study from X-ray diffraction is one of the most powerful techniques to determine the structure. However, it is difficult to reveal electronic structures in molecular conductors using X-ray diffraction method, because it is normally required to detect the charge transfer of less than one electron. In addition, the charge density study has to reveal not only the CO and CDW but also aspherical charge-density in the molecule. Both the high-quality diffraction data and the sophisticated analytical techniques are required for charge density study of the molecular conductor. Recently, a large imaging plate (IP) camera for an accurate charge density study has been installed at SPring-8, one of the third generation synchrotron radiation (SR) source. A combination of the SPring-8 data and the precise analytical technique would enable us to carry out charge density study of the molecular conductor with enough accuracy. In this study, we have carried out the charge density study of molecular conductor, (DBr-
Corresponding author. Tel.: + 81 527894453; fax: + 81 527893724.
E-mail address: [email protected] (H. Sawa). 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.02.014
DCNQI)2Cu, in the metallic phase using the SPring-8 data. A molecular conductor (DBr-DCNQI)2Cu system, where DCNQI is the N, N0 -dicyanoquinonediimine, shows the novel metal–insulator (M–I) transition at 160 K [1–3]. The Cu ion is in a mixed-valence state in the metallic phase. During past 20 years, several experimental studies, such as the Li,Zn-doping and X-ray photoemission spectroscopy (XPS), were carried out to reveal the relation between the valency of Cu and the M–I transition [4–7]. These studies revealed that the valency of Cu ion is closely related to the stability in the metallic phase. Required accuracy of the determination of the valence state for Cu ion is o0.1 electron. 2. Experiment The SR single crystal X-ray diffraction experiments using a Weissenberg camera with an IP as a detector were carried out at SPring-8, BL02B1 beamline. Highly monochromatic X-ray beam by a Si(1 1 1) double-crystal monochromator and the collimating and the focusing mirrors were used for the experiment. The wave ˚ The beam size of the X-ray length of incident X-ray was 0.4148 A. was o0.15 0.15 mm2. The size of the single crystal was 0.06 0.02 0.02 mm3. In order to avoid intensity decay in the high angle region by thermal vibration effect, the sample was cooled to 200 K using a helium gas flow low temperature device. ˚ and high counting The data with wide d-spacing range, d 40.40 A, statistics, 107 counts, were obtained from the experiments. Integrated intensities of Bragg reflections were extracted from
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the two-dimensional IP data by the program RAPID AUTO. The intensities were processed by SORTAV software [8]. A crystal structural analysis was performed by SHELX97 software [9]. Table 1 Summary of the crystal data of (DBr-DCNQI)2Cu at 200 K. Formula Formula wt Crystal dimens, (mm3) Space group Cell dimens ˚ a (A)
C16H4N8Br4Cu 691.39 0.06 0.02 0.02 I41/a 21.5768(4) 3.8637(1)
˚ c (A) V (A˚ 3)
1798.78(7)
Z 2y range (deg) Total no. of obsd reflctns No. of unique data (I 44s) Weighted schemea Average redundancy Overall completeness R (I 44s)
4 0–62.7 64 739 4059 a =0.0348, b= 1.3092 12.2 0.715 0.0328
a The weighting scheme is defined as w ¼ 1=½s2 ðFo2 Þ þ ða Fc2 Þ2 þ b Fc2 by SHELX97.
Table 2 (I) Bond lengths and angles of DBr-DCNQI molecule, (II) the coordination geometry of Cu of (DBr-DCNQI)2Cu (Using the indices in Ref. [3]). I
II
˚ a (A) ˚ b (A)
1.1618(13)
˚ c (A) ˚ d (A)
1.3279(14)
˚ e (A) ˚ f (A)
1.4320(13)
1.3041(13) 1.4408(13) 1.3582(14)
+N–C–N (deg) +C–N–C (deg) ˚ R (A)
173.05(12) 120.56(9) 1.9673(10)
a (deg)
125.94(5) 168.24(10)
y (deg)
3. Results and discussion The information of the refined structure at 200 K is summarized in Table 1. The values of the completeness and the redundancy for the data at the resolution range d 40.40 A˚ are 71.5% and 12.2, respectively, which are suitable for the accurate charge density study. The reliability factor based on structure factor was 3.28%. Other structural parameters, bond lengths, angles, and the coordination geometry around Cu ion are presented in Table 2, using the indices as shown in Fig. 1(a). The values of the bond length of the DCNQI molecule in Table 2-I are consistent with the previous values within their errors. As presented in Table 2-II, the parameter a which represents the angle +N–Cu–N, is 0.6(3)1 larger than the previously reported one. We calculated the experimental charge density (robs) by using maximum-entropy method (MEM) [10] from observed structure factors. We also calculated the deformation density, rdef, from the following equation, robs rsph, where rsph is the MEM charge density from calculated structure factors using a free spherical atomic model. Fig. 1(b) shows the section of the experimental total charge density, and Fig. 1(c) shows the section of the deformation density. Six carbon atoms of the six-membered ring in DCNQI molecule locate on the figure plane. Other atoms slightly shift on the plane in each map. In Fig. 1(b), there are the charge density indicating hydrogen atoms inside dashed circles. This fact indicates that the present charge density is reliable to discuss o1.0 electron level. The charge densities indicating bonding electrons are clearly seen in Fig. 1(c). We examined the charge density at the bond midpoint labelled as A, B, C, D, and E in Fig. 1(c). The values are as follows: A= 0.379 eA˚ 3, B= 0.167 eA˚ 3, C= 0.243 eA˚ 3, D=0.340 eA˚ 3, E= 0.324 eA˚ 3. The differences of covalency in the C–N triple bond, the C–N single bond, the C–C single bond, and the C–C double bond are clearly revealed by the deformation density. For the quantitative examination of charge-density differences at the bond midpoints, we carried out the multipolar refinement using XD2006 program [11]. In the multipolar refinement, total charge density, r, is divided into the core electron density, rcore, and the valence electron density, rvalence. The valence electron
Fig. 1. (a) A structural formula of DBr-DCNQI molecule with the indices labelling as a, b, c, d, e, f for the bond lengths (top). Schematic tetrahedral structure consist of Cu ion and four N atoms. The indices a for the angle N–Cu–N, and R for the distance between Cu ion and N atom (bottom). (b) The section of an experimental total charge density by the MEM. The contour lines drawn from 0.04 to 2.00 eA˚ 3 with 0.20 eA˚ 3 step width. The charge densities indicating hydrogen atoms are shown inside the dashed circles. (c) The section of the deformation density. The contour lines drawn from 0.50 to 0.40 eA˚ 3 with 0.09 eA˚ 3 step width. The solid lines represent the positive values, and the dashed lines represent the negative values.
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XPS measurements determined the valencies of Cu ions for (DI-DCNQI)2Cu, (DMe-DCNQI)2Cu, (MeCl-DCNQI)2Cu, and (MeBrDCNQI)2Cu. The valencies of Cu ion for the materials without M–I transitions were + 1.28e for (DI-DCNQI)2Cu and + 1.22e for (DMeDCNQI)2Cu. The valencies of Cu ion for (MeCl-DCNQI)2Cu with TMI =210 K and (MeBr-DCNQI)2Cu with TMI = 160 K were + 1.31e and + 1.32e. The valency of Cu ion for (DBr-DCNQI)2Cu with TMI =160 K determined by the present study is + 1.30(13)e. This value is close to that of (MeCl-DCNQI)2Cu. The N–Cu–N bond angle shown as a in Table 2-II is also related to the M–I transition temperature. There are some inconsistencies between the angle a and the valence of Cu ion in the previous studies. The angle a was determined by the structural analysis using the X-ray diffraction data. The valency of Cu ion was determined by the XPS measurement. In the cases of (MeCl-DCNQI)2Cu and (MeBrDCNQI)2Cu, the increase in the average valence from + 1.31 to +1.32e decreases the angle a from 126.21 to 125.31. The present method can determine simultaneously the electronic structure and the atomic coordination of the material, such as the valency of Cu ion and the angle a. The present methods are suitable for the investigation of the relation between the valency of Cu ion and the M–I transition temperature more precisely.
Fig. 2. A contour map of the deformation density by the multipolar refinement. The inset figure shows the molecular structure of DBr-DCNQI molecule. The contour lines were drawn from 4.0 to 3.9 eA˚ 3 with 0.1 eA˚ 3 step width. The solid lines represent the positive values, and the dashed lines represent the negative values. The dotted lines represent the zero values of the charge density.
density is represented by the spherical harmonics functions as the multipolar functions. The multipolar refinement can improve the structure model refined by SHELX97, because it can represent aspherical charge-density contributions between the atoms. The charge density from the multipolar refinement enables us to estimate the static charge density without thermal smearing effect. Fig. 2 shows the section of the static deformation density determined by the multipolar refinement. This map is very similar to Fig. 1(c). The R-factor is improved from 3.56% to 2.75% by using multipolar model. These facts indicate that the multipolar refinement in the present study was successfully performed. The charge density expressed by the multipolar model was determined by this refinement. The deformation density at the bond midpoints were from 0.1 to 0.4 eA˚ 3. The differences between the MEM and multipolar deformation densities at the bond midpoints were within 0.05 eA˚ 3. We have determined the aspherical charge-density distribution in the molecule with an accuracy o0.05 eA˚ 3 by using the high-quality X-ray diffraction data measured at SPring-8. We carried out the Bader topological analysis [12] to estimate valence electrons for copper ion. The Bader analysis is one of the space partitioning-methods for total charge density based on the topology. All the charge densities in the unit cell belong to the independent atoms by the Bader analysis. The partitioned space for each atom is called atomic basin. The number of electrons for each atom is calculated by the charge integration in the atomic basin. The charge in the atomic basin of Cu ion was 27.7e indicating the valence status of Cu ion being + 1.30(13)e. Several studies were carried out to reveal the relation between the valency of Cu ion and the M–I transition temperature [5–7]. The studies of Li,Zn-dopings to the Cu site showed that the M–I transition can be suppressed with Li + substitution while Zn2 + substitution increases the M–I transition temperature, TMI [4]. The
4. Conclusion We have determined the experimental charge density of the molecular conductor (DBr-DCNQI)2Cu at 200 K by using SR single crystal X-ray diffraction data at SPring-8. The valency of Cu ion was determined as + 1.30(13)e with sufficient accuracy to discuss the mechanism of M–I transition. These results confirm that the charge density study using SPring-8 data are suitable to reveal an origin of novel phenomena in the science of molecular conductors.
Acknowledgments The authors thank to K. Sugimoto for experimental help at SPring-8. This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘New Frontier of Materials Science Opened by Molecular Degrees of Freedom’’ (No. 20110003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan. The SR single crystal diffraction experiments were carried out at the SPring-8 with the approval of Japan Synchrotron Radiation Research Institute. References ¨ ¨ ¨ [1] A. Aumuller, P. Erk, G. Klebe, S. Hunig, J.U. Schutz, H.-P. Werner, Angew. Chem. Internat. Ed. Engl. 25 (1986) 740. [2] A. Kobayashi, R. Kato, H. Kobayashi, T. Mori, H. Inokuchi, Solid State Commun. 64 (1987) 45. [3] R. Kato, H. Kobayashi, A. Kobayashi, J. Amer. Chem. Soc. 111 (1989) 5224. [4] H. Sawa, M. Tamura, S. Aonuma, M. Kinoshita, R. Kato, J. Phys. Soc. Japan 63 (1994) 4302. [5] H. Kobayashi, A. Miyamoto, R. Kato, F. Sakai, A. Kobayashi, Y. Yamakita, Y. Furukawa, M. Tasumi, T. Watanabe, Phys. Rev. B 47 (1993) 3500. [6] A. Tanaka, A. Chainani, T. Yokoya, T. Takahashi, T. Miyazaki, S. Hasegawa, T. Mori, Phys. Rev. B 52 (1995) 7951. [7] O. Akaki, A. Chainani, T. Takahashi, Y. Kashimura, R. Kato, Phys. Rev. B 57 (1998) 11846. [8] R.H. Blessing, J. Appl. Cryst. 30 (1997) 421. [9] G.M. Sheldrick, Acta Cryst. A 64 (2008) 112. [10] M. Sakata, M. Sato, Acta Cryst. 46 (1990) 66. [11] N.K. Hansen, P. Coppens, Acta Cryst. A 34 (1978) 909. [12] R.F.W. Bader, Atoms in Molecules: a Quantum Theory, The International Series of Monographs on Chemistry, Clarendon Press, Oxford.