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Charge density study under high pressure
Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976 www.elsevier.com/locate/jpcs
Charge density study under high pressure Makoto Sakataa,*, Takafumi Itsuboa, Eiji Nishiboria, Yutakata Moritomoa, Norimichi Kojimab, Yasuo Ohishic, Masaki Takatac a
Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Graduate School of Arts and Science, University of Tokyo, Tokyo 153-8902, Japan c Japan Synchrotron Research Institute, Kouto, Mikazuki-cho, Sayo-gun, Hyougo 679-5198, Japan
Abstract The experimental and analytical method of the high-pressure powder experiment at BL10XU, SPring-8, is described. There is no doubt that BL10XU must be one of the most appropriate beam lines for high pressure X-ray diffraction experiment taking advantage of third generation synchrotron source. As an example of the advanced charge density study under high pressure, the structural change of Cs2Au2Br6 by applying pressure is studied by Rietveld/MEM analysis. It reveals that the structural change of Cs2Au2Br6 by applying pressure occurs basically at electron level, such as valence state change and chemical bonding, which may be called the electronic phase transition. q 2004 Elsevier Ltd. All rights reserved. Keywords: C. High pressure; C. X-ray diffraction; D. Crystal structure; D. Electronic structure
1. Introduction It is very well known that physical properties of materials can be sometimes drastically changed by applying pressure, such as emergence of superconductivity of ladder compounds [1]. It is also very common that materials undergo structural phase transitions when they start to show very different physical properties under high pressure. It is, therefore, highly desirable to determine structural changes accurately, preferably at electron density level, under high pressure. In order to produce high-pressure conditions, we have to use high-pressure cell, such as diamond anvil cell (DAC), which limit the amount of specimen to be used in the X-ray diffraction experiment to microgram order. Under such a condition, there is no doubt that third generation Synchrotron Radiation (SR) source with high intensity and high energy X-ray has a great advantage. In this study, an accurate structural analysis of Cs2Au2Br6 by using SR powder data collected at BL10XU, SPring-8 will be described as an example of accurate structure analysis under high pressure obtained by DAC. * Corresponding author. Tel.: C81 52 789 4453; fax: C81 52 789 3724. E-mail address: [email protected] (M. Sakata). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.08.028
2. The experimental condition to have homogeneous intensity of Debye-Scherrer ring For the reliable structure analysis, it is essential to develop the experimental techniques and to establish their conditions to get homogeneous intensity distribution of Debye-Scherrer ring in reality. If the Debye-Scherrer ring shows rather spotty pattern due to inhomogeneous large grain size powder particle distribution as shown in Fig. 1(a), it is not possible to have accurate structural information from powder specimen. In this study, a DAC with large culet is used to have the powder specimen as much as possible. Because of this, the highest pressure to be reached by the DAC has to be limited to a few tenth gigapascal. The powder specimens prepared by the above process are not still good enough for homogeneous Debye-Scherrer ring, which is very good specimens for laboratory X-ray source experiments and probably for second generation SR. Because of extremely parallel beam of third generation SR, it would not possible to have homogeneous intensity distribution for Debye-Scherrer ring without oscillating powder specimens. Therefore, it is essential to have some type of oscillation mechanism for DAC experiment at third generation SR, if the detailed structural information is required under high pressure.
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M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
Fig. 2. The schematic crystal structure model of Cs2Au2Br6 at ambient pressure. The schematic models of AuBr4 and AuBr2 clusters were also shown.
Fig. 1. Two-dimensional powder diffraction pattern of CeO2 recorded on IP (a) without sample oscillation (b) with sample oscillation.
Fortunately, the instrument at BL10XU, SPring-8, has a sample oscillation mechanism along the vertical axis. The Debye rings obtained with 3.58 specimen oscillation was shown in Fig. 1(b). It is clearly understood that the oscillation is very effective to have homogeneous intensity distribution along Debye ring. The powder data are recoded along the horizontal equatorial line because of available direction of oscillation mechanism. The finally optimized experimental conditions in this study are shown in Table 1.
3. Accurate structure analysis of Cs2Au2Br6 under high pressure Gold atoms in Cs2Au2Br6 are a mixed valence state of AuC and Au3C at ambient pressure. It is said that AuC ions form AuBrK 2 linear cluster in the crystal. On the other hand, Au3C ions form AuBrK planar clusters. The crystal 4 structure of Cs2Au2Br6 at ambient pressure is shown in Fig. 2 including both clusters. Recently, it is found from the Raman scattering experiment under high pressure that Raman spectrum of Cs2Au2Br6 change greatly around
7.5 GPa [2]. From this experimental result, it is presumed that a mixed valence state of AuC and Au3C at ambient pressure become single valence state of Au2C due to charge transfer between Au3C and AuC at 7.5 GPa. It is also suggested that the translational displacement of Br atoms causes the charge transfer. These are, however, all speculations. In order to demonstrate a usefulness of an accurate structural study under high pressure, Cs2Au2Br6 would be a very good example. The powder specimen is loaded in 0.1!0.4f gasket hole, in which about 0.007 mm3 specimen could be contained. Water free pentane/iso-pentane is used as pressure media. The mixing ratio was 1:1. The pressure applied to the specimen was determined by measuring the wavelength shift of ruby in the DAC with the specimen. The wavelength of incident X-ray was 25 keV. The powder X-ray diffraction experiments were performed at 1.2, 2.2, 2.9, 4.5 and 8.1 GPa. The unit cell parameters are determined at each pressure point. In order to show the discrepancy of cubic symmetry, O2a/c at each pressure point is shown in Fig. 3. In the figure, the shaded area corresponds to phase transition pressure observed by Raman
Table 1 The finally optimized experimental conditions to have homogeneous intensity of Debye-Scherrer ring Sample pellet thickness Beam size Sample oscillation angle Sample particle size
50–100 mm 100!100 mm w3.58 0.5–3 mm
Fig. 3. Pressure dependence of the axial ratio O2a/c of Cs2Au2Br6.
M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
1975
and above the transition pressure region, which are 2.2 and 8.1 GPa in the present study. The specimen was oscillated about 3.58 at both pressure points. The collected data at 2.2 and 8.1 GPa are analysed by MEM/Rietveld analysis [3] to obtain the experimental charge density distributions after absorption corrections for DAC, sample, air and pressure medium. In the Rietveld refinements, which are done as a preliminary analysis before MEM, the R-factors based on integrated Bragg intensities became as small as 1.28 and 1.05% at 2.2 and 8.1 GPa, respectively. The fitting results of Rietveld refinements are shown in Fig. 4 for both 2.2 and 8.1 GPa. The lattice parameters and obtained structure parameters are shown in Table 2. Anisotropic thermal parameters have been determined in spite of the high pressure conditions using DAC. It has to be admitted that the refinement is done satisfactorily to proceed on further MEM analysis.
4. MEM charge density distribution of Cs2Au2Br6
Fig. 4. The fitting results of Rietveld refinement of Cs2Au2Br6 at (a) 2.2 GPa and (b) 8.1 GPa.
spectra, which has a hysteresis. It is very clear that a kind of structure change occurs through the transition pressure region and the crystal symmetry at 8.1 GPa become close to cubic symmetry. Since the transition temperature has been determined, the accurate X-ray powder diffraction data were collected under
Following Rietveld analysis, MEM charge densities are calculated by using 202 and 195 structure factors data for ˚ 2.2 and 8.1 GPa, respectively. This corresponds to 0.8 A resolution. In the MEM calculation, the parallel computing program ENIGMA [4] is used and one unit cell is divided to 90!90!132 pixels. The reliability factors based on the MEM charge densities became 0.67 and 0.58% at 2.2 and 8.1 GPa, respectively. The obtained MEM charge density distribution maps of (110) plane are shown in Fig. 5 for both 2.2 and 8.1 GPa. Contour lines are drawn between 0.0 and 4.0 e/A3 with 0.2 e/A3 intervals. It is clearly seen that AuBrK 2 linear cluster is formed by Au(1)–Br(1) covalent bond and AuBrK 4 planar cluster is also formed by Au(2)–Br(2) covalent bonds. Judging from the electron densities at bond midpoint, which are 0.60 e/A3 for Au(1)–Br(1) covalent bond and 0.82 e/A3
Table 2 The structure parameters determined by the Rietveld analysis for Cs2Au2Br6 at (a) 2.2 GPa and (b) 8.1 GPa Site
Occ.
x
y
z
˚ 2) B (A
(a) 2.2 GPa. Lattice parameters: aZbZ7.38726(8) (A˚), cZ10.7239(2) (A˚) CsC 4d 1.0 0.0 0.5 0.25 Au3C 2a 1.0 0.0 0.0 0.0 AuC 2b 1.0 0.0 0.0 0.5 1.13(4) 8h 1.0 0.2349(3) 0.2349(3) 0.0 BrK (1) BrK (2) 4e 1.0 0.0 0.0 0.2736(3) (b) 8.1 GPa. Lattice parameters: aZbZ7.2278(1) (A˚), cZ10.2812(4) (A˚) CsC 4d 1.0 0.0 0.5 0.25 1.30(3) 2a 0.5 0.0 0.0 0.0 0.69(2) Au3C AuC 2a 0.5 0.0 0.0 0.0 0.69(2) Au3C 2b 0.5 0.0 0.0 0.5 0.69(2) 2b 0.5 0.0 0.0 0.5 0.69(2) AuC BrK (1) 8h 1.0 0.247(1) 0.247(1) 0.0 BrK (2) 4e 1.0 0.0 0.0 0.2664(5) 1.30(3)
˚ 2) U11 (A
˚ 2) U22 (A
0.0206(7) 0.009(1)
0.0206(7) 0.016(2)
0.018(1) 0.027(1)
0.029(1) 0.016(3)
0.0095(8)
0.0095(8)
˚ 2) U33 (A
˚ 2) U12 (A
˚ 2) U23 (A
˚ 2) U13 (A
0.026(1) 0.0 0.016(2) 0.0
0.0 0.0
0.0 0.0
K0.010(1) 0.0 0.0 0.0
0.0 0.0
0.0 0.0
0.0
0.0
0.028(2) K0.005(2)
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M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
Fig. 5. The MEM charge densities of Cs2Au2Br6 for (110) section at ˚3 (a) 2.2 GPa and (b) 8.1 GPa. The contour lines drawn from 0.0 to 4.0 e/A 3 ˚ with 0.2 e/A intervals.
for Au(2)–Br(2) covalent bond, respectively, the covalency to form AuBrK 2 linear cluster is fairly strong. On the other hand, such a linear cluster could not be recognized in the MEM density map at 8.1 GPa. This can be explained by the drastic change of nature of chemical bond between Au(1)– Br(2) atoms. The MEM charge density at bond midpoint density between Au(1)–Br(2) are greatly increased from 0.21 to 0.49 e/A3. This is an experimental evidence that the nature of chemical bond between Au(1)–Br(2) atoms becomes much more covalent than that at 2.2 GPa. On the other hand, the covalency between Au(2)–Br(2) is practically unchanged, i.e. 0.59 e/A3, which is almost the same to 0.60 e/A3 at 2.2 GPa. The same type of difference of bond nature is also recognized for AuBrK 4 planar cluster, which is shown in Fig. 6, which is the MEM charge density map of Cs2Au2Br6 (002) plane. Contour lines are the same as Fig. 5. In Fig. 6(a), AuBrK 4 planar cluster can be easily recognized at 2.2 GPa, where Br(1) atoms are covalently connected to Au(2) atoms only. At 8.1 GPa, however, Br(1) atoms are all connected to not only Au(1) but also Au(2) atoms. Due to the change of chemical bonding by applying pressure, both linear and planar clusters in Cs2Au2Br6 in the crystalline state forms network structure and hence Cs2Au2Br6 becomes more or less one of two-dimensional network substances at 8.1 GPa with monovalent state.
Fig. 6. The MEM charge densities of Cs2Au2Br6 for (002) section at (a) ˚ 3 with 2.2 GPa and (b) 8.1 GPa. The contour lines drawn from 0.0 to 4.0 e/A 3 ˚ 0.2 e/A intervals.
It is demonstrated in this study that the most precise structural work, which we believe is the accurate charge density study, can be performed by taking advantage of third generation SR.
Acknowledgements This work has been supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology of Japan. The Nippon Sheet Glass Foundation for Materials Science and Engineering also supported this work. The SR experiments were performed at SPring-8 BL010XU with the approval of the Japan SR Research Institute (JASRI).
References [1] M. Uehara, T. Nagata, J. Akimitsu, H. Takahashi, N. Moˆri, K. Kinoshita, J. Phys. Soc. Jpn 65 (1996) 2764–2767. [2] X.J. Liu, K. Matsuda, Y. Moritomo, A. Nakamura, N. Kojima, Phys. Rev. B 59 (1999) 7925–7930. [3] M. Takata, E. Nishibori, M. Sakata, Z. Kristallogr. 216 (2001) 71–86. [4] H. Tanaka, M. Takata, E. Nishibori, K. Kato, T. Iishi, M. Sakata, J. Appl. Cryst. 35 (2002) 282–286.
Charge density study under high pressure Makoto Sakataa,*, Takafumi Itsuboa, Eiji Nishiboria, Yutakata Moritomoa, Norimichi Kojimab, Yasuo Ohishic, Masaki Takatac a
Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Graduate School of Arts and Science, University of Tokyo, Tokyo 153-8902, Japan c Japan Synchrotron Research Institute, Kouto, Mikazuki-cho, Sayo-gun, Hyougo 679-5198, Japan
Abstract The experimental and analytical method of the high-pressure powder experiment at BL10XU, SPring-8, is described. There is no doubt that BL10XU must be one of the most appropriate beam lines for high pressure X-ray diffraction experiment taking advantage of third generation synchrotron source. As an example of the advanced charge density study under high pressure, the structural change of Cs2Au2Br6 by applying pressure is studied by Rietveld/MEM analysis. It reveals that the structural change of Cs2Au2Br6 by applying pressure occurs basically at electron level, such as valence state change and chemical bonding, which may be called the electronic phase transition. q 2004 Elsevier Ltd. All rights reserved. Keywords: C. High pressure; C. X-ray diffraction; D. Crystal structure; D. Electronic structure
1. Introduction It is very well known that physical properties of materials can be sometimes drastically changed by applying pressure, such as emergence of superconductivity of ladder compounds [1]. It is also very common that materials undergo structural phase transitions when they start to show very different physical properties under high pressure. It is, therefore, highly desirable to determine structural changes accurately, preferably at electron density level, under high pressure. In order to produce high-pressure conditions, we have to use high-pressure cell, such as diamond anvil cell (DAC), which limit the amount of specimen to be used in the X-ray diffraction experiment to microgram order. Under such a condition, there is no doubt that third generation Synchrotron Radiation (SR) source with high intensity and high energy X-ray has a great advantage. In this study, an accurate structural analysis of Cs2Au2Br6 by using SR powder data collected at BL10XU, SPring-8 will be described as an example of accurate structure analysis under high pressure obtained by DAC. * Corresponding author. Tel.: C81 52 789 4453; fax: C81 52 789 3724. E-mail address: [email protected] (M. Sakata). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.08.028
2. The experimental condition to have homogeneous intensity of Debye-Scherrer ring For the reliable structure analysis, it is essential to develop the experimental techniques and to establish their conditions to get homogeneous intensity distribution of Debye-Scherrer ring in reality. If the Debye-Scherrer ring shows rather spotty pattern due to inhomogeneous large grain size powder particle distribution as shown in Fig. 1(a), it is not possible to have accurate structural information from powder specimen. In this study, a DAC with large culet is used to have the powder specimen as much as possible. Because of this, the highest pressure to be reached by the DAC has to be limited to a few tenth gigapascal. The powder specimens prepared by the above process are not still good enough for homogeneous Debye-Scherrer ring, which is very good specimens for laboratory X-ray source experiments and probably for second generation SR. Because of extremely parallel beam of third generation SR, it would not possible to have homogeneous intensity distribution for Debye-Scherrer ring without oscillating powder specimens. Therefore, it is essential to have some type of oscillation mechanism for DAC experiment at third generation SR, if the detailed structural information is required under high pressure.
1974
M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
Fig. 2. The schematic crystal structure model of Cs2Au2Br6 at ambient pressure. The schematic models of AuBr4 and AuBr2 clusters were also shown.
Fig. 1. Two-dimensional powder diffraction pattern of CeO2 recorded on IP (a) without sample oscillation (b) with sample oscillation.
Fortunately, the instrument at BL10XU, SPring-8, has a sample oscillation mechanism along the vertical axis. The Debye rings obtained with 3.58 specimen oscillation was shown in Fig. 1(b). It is clearly understood that the oscillation is very effective to have homogeneous intensity distribution along Debye ring. The powder data are recoded along the horizontal equatorial line because of available direction of oscillation mechanism. The finally optimized experimental conditions in this study are shown in Table 1.
3. Accurate structure analysis of Cs2Au2Br6 under high pressure Gold atoms in Cs2Au2Br6 are a mixed valence state of AuC and Au3C at ambient pressure. It is said that AuC ions form AuBrK 2 linear cluster in the crystal. On the other hand, Au3C ions form AuBrK planar clusters. The crystal 4 structure of Cs2Au2Br6 at ambient pressure is shown in Fig. 2 including both clusters. Recently, it is found from the Raman scattering experiment under high pressure that Raman spectrum of Cs2Au2Br6 change greatly around
7.5 GPa [2]. From this experimental result, it is presumed that a mixed valence state of AuC and Au3C at ambient pressure become single valence state of Au2C due to charge transfer between Au3C and AuC at 7.5 GPa. It is also suggested that the translational displacement of Br atoms causes the charge transfer. These are, however, all speculations. In order to demonstrate a usefulness of an accurate structural study under high pressure, Cs2Au2Br6 would be a very good example. The powder specimen is loaded in 0.1!0.4f gasket hole, in which about 0.007 mm3 specimen could be contained. Water free pentane/iso-pentane is used as pressure media. The mixing ratio was 1:1. The pressure applied to the specimen was determined by measuring the wavelength shift of ruby in the DAC with the specimen. The wavelength of incident X-ray was 25 keV. The powder X-ray diffraction experiments were performed at 1.2, 2.2, 2.9, 4.5 and 8.1 GPa. The unit cell parameters are determined at each pressure point. In order to show the discrepancy of cubic symmetry, O2a/c at each pressure point is shown in Fig. 3. In the figure, the shaded area corresponds to phase transition pressure observed by Raman
Table 1 The finally optimized experimental conditions to have homogeneous intensity of Debye-Scherrer ring Sample pellet thickness Beam size Sample oscillation angle Sample particle size
50–100 mm 100!100 mm w3.58 0.5–3 mm
Fig. 3. Pressure dependence of the axial ratio O2a/c of Cs2Au2Br6.
M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
1975
and above the transition pressure region, which are 2.2 and 8.1 GPa in the present study. The specimen was oscillated about 3.58 at both pressure points. The collected data at 2.2 and 8.1 GPa are analysed by MEM/Rietveld analysis [3] to obtain the experimental charge density distributions after absorption corrections for DAC, sample, air and pressure medium. In the Rietveld refinements, which are done as a preliminary analysis before MEM, the R-factors based on integrated Bragg intensities became as small as 1.28 and 1.05% at 2.2 and 8.1 GPa, respectively. The fitting results of Rietveld refinements are shown in Fig. 4 for both 2.2 and 8.1 GPa. The lattice parameters and obtained structure parameters are shown in Table 2. Anisotropic thermal parameters have been determined in spite of the high pressure conditions using DAC. It has to be admitted that the refinement is done satisfactorily to proceed on further MEM analysis.
4. MEM charge density distribution of Cs2Au2Br6
Fig. 4. The fitting results of Rietveld refinement of Cs2Au2Br6 at (a) 2.2 GPa and (b) 8.1 GPa.
spectra, which has a hysteresis. It is very clear that a kind of structure change occurs through the transition pressure region and the crystal symmetry at 8.1 GPa become close to cubic symmetry. Since the transition temperature has been determined, the accurate X-ray powder diffraction data were collected under
Following Rietveld analysis, MEM charge densities are calculated by using 202 and 195 structure factors data for ˚ 2.2 and 8.1 GPa, respectively. This corresponds to 0.8 A resolution. In the MEM calculation, the parallel computing program ENIGMA [4] is used and one unit cell is divided to 90!90!132 pixels. The reliability factors based on the MEM charge densities became 0.67 and 0.58% at 2.2 and 8.1 GPa, respectively. The obtained MEM charge density distribution maps of (110) plane are shown in Fig. 5 for both 2.2 and 8.1 GPa. Contour lines are drawn between 0.0 and 4.0 e/A3 with 0.2 e/A3 intervals. It is clearly seen that AuBrK 2 linear cluster is formed by Au(1)–Br(1) covalent bond and AuBrK 4 planar cluster is also formed by Au(2)–Br(2) covalent bonds. Judging from the electron densities at bond midpoint, which are 0.60 e/A3 for Au(1)–Br(1) covalent bond and 0.82 e/A3
Table 2 The structure parameters determined by the Rietveld analysis for Cs2Au2Br6 at (a) 2.2 GPa and (b) 8.1 GPa Site
Occ.
x
y
z
˚ 2) B (A
(a) 2.2 GPa. Lattice parameters: aZbZ7.38726(8) (A˚), cZ10.7239(2) (A˚) CsC 4d 1.0 0.0 0.5 0.25 Au3C 2a 1.0 0.0 0.0 0.0 AuC 2b 1.0 0.0 0.0 0.5 1.13(4) 8h 1.0 0.2349(3) 0.2349(3) 0.0 BrK (1) BrK (2) 4e 1.0 0.0 0.0 0.2736(3) (b) 8.1 GPa. Lattice parameters: aZbZ7.2278(1) (A˚), cZ10.2812(4) (A˚) CsC 4d 1.0 0.0 0.5 0.25 1.30(3) 2a 0.5 0.0 0.0 0.0 0.69(2) Au3C AuC 2a 0.5 0.0 0.0 0.0 0.69(2) Au3C 2b 0.5 0.0 0.0 0.5 0.69(2) 2b 0.5 0.0 0.0 0.5 0.69(2) AuC BrK (1) 8h 1.0 0.247(1) 0.247(1) 0.0 BrK (2) 4e 1.0 0.0 0.0 0.2664(5) 1.30(3)
˚ 2) U11 (A
˚ 2) U22 (A
0.0206(7) 0.009(1)
0.0206(7) 0.016(2)
0.018(1) 0.027(1)
0.029(1) 0.016(3)
0.0095(8)
0.0095(8)
˚ 2) U33 (A
˚ 2) U12 (A
˚ 2) U23 (A
˚ 2) U13 (A
0.026(1) 0.0 0.016(2) 0.0
0.0 0.0
0.0 0.0
K0.010(1) 0.0 0.0 0.0
0.0 0.0
0.0 0.0
0.0
0.0
0.028(2) K0.005(2)
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M. Sakata et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1973–1976
Fig. 5. The MEM charge densities of Cs2Au2Br6 for (110) section at ˚3 (a) 2.2 GPa and (b) 8.1 GPa. The contour lines drawn from 0.0 to 4.0 e/A 3 ˚ with 0.2 e/A intervals.
for Au(2)–Br(2) covalent bond, respectively, the covalency to form AuBrK 2 linear cluster is fairly strong. On the other hand, such a linear cluster could not be recognized in the MEM density map at 8.1 GPa. This can be explained by the drastic change of nature of chemical bond between Au(1)– Br(2) atoms. The MEM charge density at bond midpoint density between Au(1)–Br(2) are greatly increased from 0.21 to 0.49 e/A3. This is an experimental evidence that the nature of chemical bond between Au(1)–Br(2) atoms becomes much more covalent than that at 2.2 GPa. On the other hand, the covalency between Au(2)–Br(2) is practically unchanged, i.e. 0.59 e/A3, which is almost the same to 0.60 e/A3 at 2.2 GPa. The same type of difference of bond nature is also recognized for AuBrK 4 planar cluster, which is shown in Fig. 6, which is the MEM charge density map of Cs2Au2Br6 (002) plane. Contour lines are the same as Fig. 5. In Fig. 6(a), AuBrK 4 planar cluster can be easily recognized at 2.2 GPa, where Br(1) atoms are covalently connected to Au(2) atoms only. At 8.1 GPa, however, Br(1) atoms are all connected to not only Au(1) but also Au(2) atoms. Due to the change of chemical bonding by applying pressure, both linear and planar clusters in Cs2Au2Br6 in the crystalline state forms network structure and hence Cs2Au2Br6 becomes more or less one of two-dimensional network substances at 8.1 GPa with monovalent state.
Fig. 6. The MEM charge densities of Cs2Au2Br6 for (002) section at (a) ˚ 3 with 2.2 GPa and (b) 8.1 GPa. The contour lines drawn from 0.0 to 4.0 e/A 3 ˚ 0.2 e/A intervals.
It is demonstrated in this study that the most precise structural work, which we believe is the accurate charge density study, can be performed by taking advantage of third generation SR.
Acknowledgements This work has been supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology of Japan. The Nippon Sheet Glass Foundation for Materials Science and Engineering also supported this work. The SR experiments were performed at SPring-8 BL010XU with the approval of the Japan SR Research Institute (JASRI).
References [1] M. Uehara, T. Nagata, J. Akimitsu, H. Takahashi, N. Moˆri, K. Kinoshita, J. Phys. Soc. Jpn 65 (1996) 2764–2767. [2] X.J. Liu, K. Matsuda, Y. Moritomo, A. Nakamura, N. Kojima, Phys. Rev. B 59 (1999) 7925–7930. [3] M. Takata, E. Nishibori, M. Sakata, Z. Kristallogr. 216 (2001) 71–86. [4] H. Tanaka, M. Takata, E. Nishibori, K. Kato, T. Iishi, M. Sakata, J. Appl. Cryst. 35 (2002) 282–286.