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197Au Mössbauer spectroscopy of the cubic phase in the halogen bridged mixed valence complex Cs2Au2I6
Nuclear Instruments North-Holland
and Methods
in Physics Research
B76 (1993) 321-322
NOMB
Beam Interactions with Materials A Atoms
197AuMijssbauer spectroscopy of the cubic phase in the halogen bridged mixed valence complex Cs,Au,I, N. Kojima a, F. Amita a, H. Kitagawa b, H. Sakai ’ and Yu. Maeda d pDepartment of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan b Institute for Molecular Science, Okazaki 444, Japan ‘Department of Chemistry, Faculty of Science, Hiroshima Unic~ersity,Saijo-cho, Higashi-Hiroshima 724, Japan d Research Reactor Institute, Kyoto Uniuersity, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan
Cs,Au,I, is a halogen bridged three-dimensional mixed valence Au complex. This complex undergoes a semiconductor-to-metal transition and a metal-to-metal transition under high pressures. The second metallic phase appearing at P 2 6.5 GPa and T >- 60°C has a cubic perovskite structure and can be obtained as a metastable state at room temperature and ambient pressure. From an interest in the electronic state of the cubic phase in Cs,Au,I,, we investigated the “‘Au Miissbauer spectrum for the metastable cubic phase in CssAusI,.
Cs,AuzI, is well known as a three-dimensional mixed valence AL’ complex. The crystal has a distorted perovskite structure belonging to the space group 14/mmm [l]. In this crystal, all the bridging halogens are distorted from the midpoint of the Au’ and Au”’ ions, and the linear [Au’I,]and square planar [Au”‘I,]~ complexes are formed alternately. This complex undergoes a semiconductor-to-metal transition and a metal-to-metal transition under high pressures [2]. The second metallic phase appearing at P 2 6.5 GPa and T > 60°C has a cubic perovskite structure, and this phase can be obtained as a metastable state at room temperature (rt) and ambient pressure cap) [2]. In this cubic phase, the Au sites are indistinguishable and the Au valence state is considered to be Au”. Many complexes of gold, whose empirical formulae suggest the presence of the Au” valence, have been shown to be Au’-Au”’ mixed-valence species and little is known of the true Au” polynuclear complex. From an interest in the electronic state of the cubic phase in Cs,Au,I,, we have investigated the 19’Au Mossbauer spectrum for the metastable cubic phase. Mijssbauer spectroscopic measurements of the 77.34 keV transition in 19’Au were carried out with both source and absorber cooled down to 16 K using a NaI(T1) scintillation counter. The data were stored in a microcomputerized 512 multichannel analyser. A 19’Pt source was obtained by neutron irradiation of 98% enriched Pt due to the nuclear reaction ‘96Pt(n, y)‘“‘Pt in the Kyoto University Reactor (KUR). The velocity scale was calibrated by taking spectra of metallic iron against a 57Co source at room temperature. All the 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
isomer shift data were in reference to the Au-metal resonance at 16 K. The metastable cubic phase of Cs,Au,I, was obtained at r.t. and a.p. by decreasing temperature (at a rate of SC/s.) and pressure (at a rate of 0.2 GPa/min.) after increasing pressure up to 7.0 GPa and increasing temperature up to 200°C. This metastable cubic phase was rather unstable and returned to the tetragonal phase at room temperature, therefore it was stored at low temperature. Fig. 1 shows the observed 19’Au Miissbauer spectra in Cs,Au,I,. Fig. la shows the spectrum for the tetragonal phase. In this phase, the states Au’ and Au”’ could clearly be distinguished. The isomer shifts of Au’ and Au”’ states are 1.02 f 0.05 mm/s and 1.54 k 0.05 mm/s, respectively, and the quadrupole splittings of Au’ and Au”’ are 4.58 + 0.05 mm/s and 1.68 k 0.05 mm/s, respectively [3]. Figs. lb and lc show the spectra for the metastable cubic phase of Cs,Au,I,. The samples in figs. lb and lc were stored for a week in a freezer (-2O’C) and in liq. Nz (- 196”C), respectively, after obtaining the metastable phase. As shown in figs. lb and lc, a single resonance line having a broad halfwidth, which corresponds to the cubic phase, appeares at about 0.6 mm/s, while components corresponding to the tetragonal phase formed during the storage are also observed. Strangely, the isomer shift of the Au state for the cubic phase is smaller than those of the Au’ and Au”’ states for the tetragonal phase. In the case of 19’Au Mijssbauer spectra, the change in the square of the nuclear radius between the excited and the ground
B.V. All rights reserved
N. Kojima et al. / 19?4u Miissbauer spectroscopy of Cs, Au, I,
322
$ 1.00
.“a:::,
._
;p
.. _,., “,:,,.
*.._... -.
decrease of 5d-6s mixing, the isomer shift of Au in the cubic phase becomes small. This concept is supported by the following fact. Since the Au-I distance (2.62 A) in Au1 is longer than that (2.53 A) in [n-Bu,N][AuI,], [6,7] the isomer shift (- 1.32 mm/s> of Au’ in Au1 is remarkably smaller than that c-to.30 mm/s> of Au’ in [n-Bu,N][AuI,] where the value of isomer shift is relative to gold in platinum. [8,9]. The Au valence state in the cubic phase is considered to be Au”. However, for more detailed discussion of the Au valence state in the cubic phase, the ‘97Au Mossbauer measurements under high pressure is indispensable.
,:.-\
;i
5 n
0.9. t
(C)
.
...,c.,.,A..,,’
-10
;,,,,
0 10 Velocity(mm/s)
<
20
I
Fig. 1. 197AuMiissbauer spectra of Cs,Au,I,. (al tetragonal phase, (b) metastable cubic phase stored at - 20°C for a week, (cl metastable cubic phase stored in liq. N, for a week.
is positive, so that a negative shift in the isomer shift indicates a decrease in the s-electron density p(O) at the nucleus. In the case of Cs,Au,I,, p(O) caused by the 5d-6s mixing is sensitive to the Au-I distance. In the tetragonal phase of Cs,Au,I,, the Au-I distances in the component complexes [Au’IJ and [Au”‘I,]are 2.574 A and 2.634 A, respectively [4]. On the other hand, the Au-I distance in the metastable cubic phase is 2.933 A [5] which is remarkably longer than those in the [Au’I,]and [Au”‘I,]complexes in the tetragonal phase. Because of the elongation in the Au-I distance in the cubic phase, which causes a states
References PI G. Brauer and G. Sleater, J. Less-Common Metals 21 (1970) 283. El N. Kojima, H. Kitagawa, T. Ban, F. Amita and M. Nakahara, Solid State Commun. 73 (19901 743; Synthetic Metals 41-43 (19911 2347. [31 H. Kitagawa, N. Kojima and H. Sakai, J. Chem. Sot. Dalton Trans. (1991) 3211. [41 N. Matsushita, H. Kitagawa and N. Kojima, to be published in J. Chem. Sot. Dalton Trans. El H. Kitagawa, H. Sato, N. Kojima, T. Kikegawa and 0. Shimomura, Solid State Comm. 78 (1991) 989. [61 A.F. Wells, Structural Inorganic Chemistry (Clarendon Press, 1984) p. 411. [71 P. Braunstein, A. Miiller and H. Biigge, Inorg. Chem. 25 (1986) 2104. [81 M.O. Faltens and D.A. Shirley, J. Chem. Phys. 53 (19701 4249. [91 R.V. Parish, Gold Bull. 15 (1982) 51.
and Methods
in Physics Research
B76 (1993) 321-322
NOMB
Beam Interactions with Materials A Atoms
197AuMijssbauer spectroscopy of the cubic phase in the halogen bridged mixed valence complex Cs,Au,I, N. Kojima a, F. Amita a, H. Kitagawa b, H. Sakai ’ and Yu. Maeda d pDepartment of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan b Institute for Molecular Science, Okazaki 444, Japan ‘Department of Chemistry, Faculty of Science, Hiroshima Unic~ersity,Saijo-cho, Higashi-Hiroshima 724, Japan d Research Reactor Institute, Kyoto Uniuersity, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan
Cs,Au,I, is a halogen bridged three-dimensional mixed valence Au complex. This complex undergoes a semiconductor-to-metal transition and a metal-to-metal transition under high pressures. The second metallic phase appearing at P 2 6.5 GPa and T >- 60°C has a cubic perovskite structure and can be obtained as a metastable state at room temperature and ambient pressure. From an interest in the electronic state of the cubic phase in Cs,Au,I,, we investigated the “‘Au Miissbauer spectrum for the metastable cubic phase in CssAusI,.
Cs,AuzI, is well known as a three-dimensional mixed valence AL’ complex. The crystal has a distorted perovskite structure belonging to the space group 14/mmm [l]. In this crystal, all the bridging halogens are distorted from the midpoint of the Au’ and Au”’ ions, and the linear [Au’I,]and square planar [Au”‘I,]~ complexes are formed alternately. This complex undergoes a semiconductor-to-metal transition and a metal-to-metal transition under high pressures [2]. The second metallic phase appearing at P 2 6.5 GPa and T > 60°C has a cubic perovskite structure, and this phase can be obtained as a metastable state at room temperature (rt) and ambient pressure cap) [2]. In this cubic phase, the Au sites are indistinguishable and the Au valence state is considered to be Au”. Many complexes of gold, whose empirical formulae suggest the presence of the Au” valence, have been shown to be Au’-Au”’ mixed-valence species and little is known of the true Au” polynuclear complex. From an interest in the electronic state of the cubic phase in Cs,Au,I,, we have investigated the 19’Au Mossbauer spectrum for the metastable cubic phase. Mijssbauer spectroscopic measurements of the 77.34 keV transition in 19’Au were carried out with both source and absorber cooled down to 16 K using a NaI(T1) scintillation counter. The data were stored in a microcomputerized 512 multichannel analyser. A 19’Pt source was obtained by neutron irradiation of 98% enriched Pt due to the nuclear reaction ‘96Pt(n, y)‘“‘Pt in the Kyoto University Reactor (KUR). The velocity scale was calibrated by taking spectra of metallic iron against a 57Co source at room temperature. All the 0168-583X/93/$06.00
0 1993 - Elsevier
Science
Publishers
isomer shift data were in reference to the Au-metal resonance at 16 K. The metastable cubic phase of Cs,Au,I, was obtained at r.t. and a.p. by decreasing temperature (at a rate of SC/s.) and pressure (at a rate of 0.2 GPa/min.) after increasing pressure up to 7.0 GPa and increasing temperature up to 200°C. This metastable cubic phase was rather unstable and returned to the tetragonal phase at room temperature, therefore it was stored at low temperature. Fig. 1 shows the observed 19’Au Miissbauer spectra in Cs,Au,I,. Fig. la shows the spectrum for the tetragonal phase. In this phase, the states Au’ and Au”’ could clearly be distinguished. The isomer shifts of Au’ and Au”’ states are 1.02 f 0.05 mm/s and 1.54 k 0.05 mm/s, respectively, and the quadrupole splittings of Au’ and Au”’ are 4.58 + 0.05 mm/s and 1.68 k 0.05 mm/s, respectively [3]. Figs. lb and lc show the spectra for the metastable cubic phase of Cs,Au,I,. The samples in figs. lb and lc were stored for a week in a freezer (-2O’C) and in liq. Nz (- 196”C), respectively, after obtaining the metastable phase. As shown in figs. lb and lc, a single resonance line having a broad halfwidth, which corresponds to the cubic phase, appeares at about 0.6 mm/s, while components corresponding to the tetragonal phase formed during the storage are also observed. Strangely, the isomer shift of the Au state for the cubic phase is smaller than those of the Au’ and Au”’ states for the tetragonal phase. In the case of 19’Au Mijssbauer spectra, the change in the square of the nuclear radius between the excited and the ground
B.V. All rights reserved
N. Kojima et al. / 19?4u Miissbauer spectroscopy of Cs, Au, I,
322
$ 1.00
.“a:::,
._
;p
.. _,., “,:,,.
*.._... -.
decrease of 5d-6s mixing, the isomer shift of Au in the cubic phase becomes small. This concept is supported by the following fact. Since the Au-I distance (2.62 A) in Au1 is longer than that (2.53 A) in [n-Bu,N][AuI,], [6,7] the isomer shift (- 1.32 mm/s> of Au’ in Au1 is remarkably smaller than that c-to.30 mm/s> of Au’ in [n-Bu,N][AuI,] where the value of isomer shift is relative to gold in platinum. [8,9]. The Au valence state in the cubic phase is considered to be Au”. However, for more detailed discussion of the Au valence state in the cubic phase, the ‘97Au Mossbauer measurements under high pressure is indispensable.
,:.-\
;i
5 n
0.9. t
(C)
.
...,c.,.,A..,,’
-10
;,,,,
0 10 Velocity(mm/s)
<
20
I
Fig. 1. 197AuMiissbauer spectra of Cs,Au,I,. (al tetragonal phase, (b) metastable cubic phase stored at - 20°C for a week, (cl metastable cubic phase stored in liq. N, for a week.
is positive, so that a negative shift in the isomer shift indicates a decrease in the s-electron density p(O) at the nucleus. In the case of Cs,Au,I,, p(O) caused by the 5d-6s mixing is sensitive to the Au-I distance. In the tetragonal phase of Cs,Au,I,, the Au-I distances in the component complexes [Au’IJ and [Au”‘I,]are 2.574 A and 2.634 A, respectively [4]. On the other hand, the Au-I distance in the metastable cubic phase is 2.933 A [5] which is remarkably longer than those in the [Au’I,]and [Au”‘I,]complexes in the tetragonal phase. Because of the elongation in the Au-I distance in the cubic phase, which causes a states
References PI G. Brauer and G. Sleater, J. Less-Common Metals 21 (1970) 283. El N. Kojima, H. Kitagawa, T. Ban, F. Amita and M. Nakahara, Solid State Commun. 73 (19901 743; Synthetic Metals 41-43 (19911 2347. [31 H. Kitagawa, N. Kojima and H. Sakai, J. Chem. Sot. Dalton Trans. (1991) 3211. [41 N. Matsushita, H. Kitagawa and N. Kojima, to be published in J. Chem. Sot. Dalton Trans. El H. Kitagawa, H. Sato, N. Kojima, T. Kikegawa and 0. Shimomura, Solid State Comm. 78 (1991) 989. [61 A.F. Wells, Structural Inorganic Chemistry (Clarendon Press, 1984) p. 411. [71 P. Braunstein, A. Miiller and H. Biigge, Inorg. Chem. 25 (1986) 2104. [81 M.O. Faltens and D.A. Shirley, J. Chem. Phys. 53 (19701 4249. [91 R.V. Parish, Gold Bull. 15 (1982) 51.