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Pressure dependence of optical properties in halogen-bridged mixed-valence metal complexes
Physica 139 & 140B (1986) 487-490 North-Holland, Amsterdam
PRESSURE DEPENDENCE OF OPTICAL PROPERTIES IN HALOGEN-BRIDGED MIXED-VALENCE METAL COMPLEXES H. TANINO, N. K O S H I Z U K A and K. H O H Electrotechnical Laboratory, Sakura-mura, lbaraki 305, Japan
K. KATO National Institute for Researches in Inorganic Materials, Sakura-mura, lbaraki 305, Japan
M. YAMASHITA Kyushu University, Ropponmatsu, Fukuoka 810, Japan
K. KOBAYASHI Toyama University, Gofuku, Toyama 930, Japan
Some of the halogen-bridged mixed-valence metal complexes are studied under high pressure by the measurements of lattice parameters, Raman frequencies, optical gaps, luminescence peak and X-ray absorption near edge structure. All of them are consistently explained according to the theory on the quasi-one-dimensional electron-phonon system. As the Peierls gap decreases with pressure, the electron-phonon system is continuously changed from a moderate coupling state to a weak coupling state, not by decreasing the electron-phonon coupling energy, but by increasing the transfer energy.
I. Introduction
Halogen-bridged mixed-valence metal complexes (HMMC) [M(AA)2 ] [MX2(AA)2]Y 4 (M = Pt, Pd, Ni, X = C1, Br, I, AA = amines, Y = anions) are the typical qusi-one-dimensional (l-d) insulating compounds. They are made of the linear chains of . . . M ( I I ) . . . X - M ( I V ) X . . . . in which each halogen ion is located not at the midpoint, but at a point closer to M(IV) than to M(II). This structure is understood [1] as the charge density wave with twice the period of the hypothetical metal made of the chain o f - M ( I I I ) X - M ( I I I ) - X - . In HMMC, the charge transfer (CT) absorption band from dz2 of M(II) to d.2 of M(IV) and the luminescence from its self-trapped state are commonly observed. In the case of Wolffram's red salt (EA/CI: defined afterwards), the barrierless relaxation process to the selftrapped state characteristic of a 1-d system is found [1]. All of these interesting phenomena are understood according to the theory on the 1-d electron-phonon system [2]. Even though, details
of the electron-phonon system such as the degree of the valence localization and the lattice distortion are not completely clarified. One way to solve these problems is to study several complexes , changing chemical species systematically. However, this approach is not sufficient because the variety of the complexes is limited and their intrinsic properties may be different from each other. Thus, we report another way here, that is, to study the complexes under pressure. The lattice parameters obtained by the Buerger precession method, absorption spectra, luminescence, Raman scattering and Xray absorption near edge structure (XANES) are studied for several complexes. For simplicity, the symbols EA/X, en/Cl, chxn/X and Pt/X/Pd are used for [Pt(EA)4][PtXz(EA)4]X4.4H20, [Pt(en)2l[PtX2 (en)21(CIO4)4, [Pt(chxn)2)[PtX2(chxn)2](CIO4)4 and
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
488
H. Tanino et al. / Optical properties in halogen-bridged metal complexes 13.5
[Pd(en)2][PtX2 (en)2](C104)4, respectively,
EA/CI
where EA = ethylamine, en = 1,2-diaminoethane and chxn--1,2-diaminocyclohexane. We note that a part of this work has already been reported
rr
LU )-LU
[3].
RT
130
< at<
2. Experimental The samples used here were needle-like fine crystals for absorption, luminescence, Raman scattering and XANES measurements, and a single crystal for X-ray study. Diamond anvil cells (DAC) developed by Yamaoka et al. [4] and Takemura et al. [5] were used with a gasket and a pressure-transmitting medium such as kerosene or n-pentane/i-pentane (1 / 1). Quasi-hydrostaticity is attained in the absorption measurement and complete hydrostaticity in the others. For the XANES measurement, the anvil rotation method was used to subtract the diffraction effect of diamond [6]. The ruby fluorescence method was always used for pressure calibration. All measurements were performed at room temperature. Other details are described elsewhere [3, 7, 8].
{3_ LU L) pF< ..J
5.5
5.0 I
I I
0
Fig. 1. P r e s s u r e d e p e n d e n c e
energy Eg becomes larger ( E A / C I < e n / C I < chxn/Cl).
Fig. 3 shows the pressure dependence of the
3
GPa
of lattice parameters
(al
of EA/CI.
cb)
£ 30 o , .~,(P{'~Cl )
30
RT 241eV
>: ~320
• • o
05(Pt.CI) w
E A / C I , E A / B r , e n / C l , c h x n / C l and P t / C 1 / P d . The shift JA~oJ becomes larger as the band gap
,
absorption edge of EA/CI and Pt/CI/Pd. The threshold energy E~h is defined as the lower edge of the charge transfer absorption band, which is about 0.4 eV smaller than Eg. The luminescence peak E~ of EA/CI is also shown. The shift of Eth of EA/CI is - 0 . 2 eV/GPa, which is larger than in
EA/CI
Fig. 1 shows the lattice dimensions of EA/CI determined by the precession method. As pressure increases, both the interchain distance and the M(II)-M(IV) distance decreases. Because each chain is surrounded by the ligands and the chains are coupled to each other by van der Waals force, the M(II)-M(IV) shortening will dominantly affect the electronic states. Fig. 2 shows the Raman frequencies w ( M ( I V ) - X ) of the symmetric stretching vibration of halogens around M(IV) in some complexes. The decrease of the phonon energy is observed in the pressure range of a few GPa for
I 2
PRESSURE
TE
3. Results and discussions
J
,a,
•
,,=31o Z <
>: ~320
~
• •e
3000
,~ '~ , ; ,
GPa
PRESSURE.
(c)
GPa
(d)
7 200
~E 3 4 0 f
E
o
~h(N.Pt.N
>:
)
/ ~,~Vc,> [
%°~
(J330~
~190 u.i
~170
chxn/CI RT 241eV
Z <
PRESSURE,
Z <
ee e e
~31o
,oOo• •
=<3o% , ~ , , , ~4 ,
8 ~18o
°
~
" o o o ,=or
~ ~, , c,,(p~Br)
' ~
en/Br RT 241eV L
4L
L
PRESSURE.
;
,E, 3201_ Z ] < F 3;
Pt/Cl/Pd RT 241eV
/
i
GPa
=<310v/
L
~ ' ;,
'
PRESSURE.
~
'
GPa
Fig. 2. Pressure dependence of Raman frequencies ~o (M(IV)-X) of (a) EA/CI, (b) chxn/Cl, (c) en/Br and (d) Pt / CI/Pd. Arrows indicate the values at atmospheric pressure.
489
H. Tanino et al. I Optical properties in halogen-bridged metal complexes
3t
Eth ] EA/CI El J
~2 )-
Br K - - 0 GPa --- 1.9 GPa
o Eth Pt/CI/Pd
I
en/Br
RT
~
=
l
e
f+
~
~
I!
'//~
ee" uJ
z
ktA ml
1
"
i -+ I
0
-8 I
I
2
PRESSURE,
-6
I
I
4 GPa
Fig. 3. Pressure dependence of absorption threshold energy E,h of EA/CI and Pt/CI/Pd, and luminescence peak energy E~ of EA/CI.
ordinary semiconductors, but the shift of E~ is only -0.05 eV/GPa. Hence, the relaxation energy E g - E I ( b E t h - El) decreases as pressure increases. These results are explained consistently as follows: At 0 GPa, X is strongly bonded to M(IV) in the form of [MX2(AA)2 ] and the X . . . M(II) interaction is rather weak. As the M(II)-M(IV) distance decreases, X would be pulled in the direction of M(II). As a result, the M(IV)-X distance will be elongated slightly, and the force constant of M(IV)-X and the phonon energy will decrease. We note that similar phenomena are observed in O - H . . . O of ice VII [9] and A u ( I I I ) - C I . . . Au(I) in CsAuCI 3 [10]. In this situation, the hybridization between dz2 of M(II) and Pz of X increases under pressure, so the transfer energy T of the charge transfer exciton will increase. Since the electron-phonon coupling energy S is little affected by pressure, the decrease of the relaxation energy originates from the decrease of S / T . We conclude that the system is continuously changed by pressure from a moderate coupling state at 0 GPa ( S - T) to a weak coupling state (S ~ T), not by decreasing the electron-phonon coupling energy S, but by increasing the transfer energy T. At higher pressure, the phonon energy of EA/CI begins to increase. In en/Br and en/I,
-4
-2
ENERGY, eV
"~ I
I
[
-20
O 20 40 ENERGY, eV Fig. 4. XANES spectra at the Br K-edge of en/Br at 0 GPa and 1.9 GPa. The first peak is enlarged in the inset.
where the gap energy Eg is rather small, it increases from the beginning. Since the sum of the ionic radii of M(IV), X and M(II) is comparable to the M(II)-M(IV) distance in this situation, X will be unmovable between the neighbouring metals and the phonon energy will increase. The decrease of the gap energy is also observed here, because the quasi-l-d character and CDW are preserved. Fig. 4 shows XANES spectra at the Br K-edge of en/Br. The first and second peaks are redshifted under pressure, but others are not. It is suggested that these two are related to the core-exciton and others are EXAFS oscillations. The first peak can be assigned as the allowed transition from ls to the 4cr*(p) band. Since the 4~r*(p) band is just the same as the empty dz_, band of Pt(IV), which is the origin of the conduction band in CDW, the red-shift of this peak is consistent with the decrease of the Peierls gap Eg. This tells us that the mixed-valence system is also continuously changed by pressure to the narrow gap state of CDW in the case of &os/a p > O. We note that the origin of the phase transition of Pt/CI/Pd observed in Raman frequency at 2.5 GPa is a problem for the future.
Acknowledgement The authors would like to acknowledge the
490
H. Tanino et al. / Optical properties in halogen-bridged metal complexes
support given by the staff of the Photon Factory of KEK. One of the authors (HT) is thankful to Dr. O. Shimomura for helpful advice on DAC and to Drs. T. Yao, H. Oyanagi and K. Takahashi for their continued interest and encouragement. References [1] H. Tanino and K. Kobayashi, J. Phys. Soc. Jpn. 52 (1983) 1446. [2] K. Nasu, J. Phys. Soc. Jpn. 53 (1984) 427. [3] H. Tanino, N. Koshizuka, K. Kobayashi, M. Yamashita and K. Hoh, J. Phys. Soc. Jpn. 54 (1985) 483.
[4] S. Yamaoka, O. Fukunaga, O. Shimomura and H. Nakazawa, Rev. Sci. Instr. 50 (1979) 1163. [5] K. Takemura, O. Shimomura, K. Tsuji and S. Minomura, High Temp.-High Press. 11 (1979) 311. [6] O. Shimomura, T. Fukamachi, T. Kawamura, S. Hosoya, S. Hunter and A. Bienenstock, Japan. J. Appl. Phys. 17-2 (1978) 221. [7] H. Tanino, K. Kato, M. Tokumoto, H. Anzai and G. Saito, J. Phys. Soc. Jpn. 54 (1985) 2390. [8] H. Tanino, H. Oyanagi, M. Yamashita and K. Kobayashi, Solid St. Commun. 53 (1985) 953. [9] G.E. Walrafen, M. Abebe, F.A. Manet, S. Block, G.J. Piermarini and R. Munro, J. Chem. Phys. 77 (1982) 2166. [10] W. Denner, H. Schulz and H. d'Amour, Acta Cryst. A35 (1979) 360.
PRESSURE DEPENDENCE OF OPTICAL PROPERTIES IN HALOGEN-BRIDGED MIXED-VALENCE METAL COMPLEXES H. TANINO, N. K O S H I Z U K A and K. H O H Electrotechnical Laboratory, Sakura-mura, lbaraki 305, Japan
K. KATO National Institute for Researches in Inorganic Materials, Sakura-mura, lbaraki 305, Japan
M. YAMASHITA Kyushu University, Ropponmatsu, Fukuoka 810, Japan
K. KOBAYASHI Toyama University, Gofuku, Toyama 930, Japan
Some of the halogen-bridged mixed-valence metal complexes are studied under high pressure by the measurements of lattice parameters, Raman frequencies, optical gaps, luminescence peak and X-ray absorption near edge structure. All of them are consistently explained according to the theory on the quasi-one-dimensional electron-phonon system. As the Peierls gap decreases with pressure, the electron-phonon system is continuously changed from a moderate coupling state to a weak coupling state, not by decreasing the electron-phonon coupling energy, but by increasing the transfer energy.
I. Introduction
Halogen-bridged mixed-valence metal complexes (HMMC) [M(AA)2 ] [MX2(AA)2]Y 4 (M = Pt, Pd, Ni, X = C1, Br, I, AA = amines, Y = anions) are the typical qusi-one-dimensional (l-d) insulating compounds. They are made of the linear chains of . . . M ( I I ) . . . X - M ( I V ) X . . . . in which each halogen ion is located not at the midpoint, but at a point closer to M(IV) than to M(II). This structure is understood [1] as the charge density wave with twice the period of the hypothetical metal made of the chain o f - M ( I I I ) X - M ( I I I ) - X - . In HMMC, the charge transfer (CT) absorption band from dz2 of M(II) to d.2 of M(IV) and the luminescence from its self-trapped state are commonly observed. In the case of Wolffram's red salt (EA/CI: defined afterwards), the barrierless relaxation process to the selftrapped state characteristic of a 1-d system is found [1]. All of these interesting phenomena are understood according to the theory on the 1-d electron-phonon system [2]. Even though, details
of the electron-phonon system such as the degree of the valence localization and the lattice distortion are not completely clarified. One way to solve these problems is to study several complexes , changing chemical species systematically. However, this approach is not sufficient because the variety of the complexes is limited and their intrinsic properties may be different from each other. Thus, we report another way here, that is, to study the complexes under pressure. The lattice parameters obtained by the Buerger precession method, absorption spectra, luminescence, Raman scattering and Xray absorption near edge structure (XANES) are studied for several complexes. For simplicity, the symbols EA/X, en/Cl, chxn/X and Pt/X/Pd are used for [Pt(EA)4][PtXz(EA)4]X4.4H20, [Pt(en)2l[PtX2 (en)21(CIO4)4, [Pt(chxn)2)[PtX2(chxn)2](CIO4)4 and
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
488
H. Tanino et al. / Optical properties in halogen-bridged metal complexes 13.5
[Pd(en)2][PtX2 (en)2](C104)4, respectively,
EA/CI
where EA = ethylamine, en = 1,2-diaminoethane and chxn--1,2-diaminocyclohexane. We note that a part of this work has already been reported
rr
LU )-LU
[3].
RT
130
< at<
2. Experimental The samples used here were needle-like fine crystals for absorption, luminescence, Raman scattering and XANES measurements, and a single crystal for X-ray study. Diamond anvil cells (DAC) developed by Yamaoka et al. [4] and Takemura et al. [5] were used with a gasket and a pressure-transmitting medium such as kerosene or n-pentane/i-pentane (1 / 1). Quasi-hydrostaticity is attained in the absorption measurement and complete hydrostaticity in the others. For the XANES measurement, the anvil rotation method was used to subtract the diffraction effect of diamond [6]. The ruby fluorescence method was always used for pressure calibration. All measurements were performed at room temperature. Other details are described elsewhere [3, 7, 8].
{3_ LU L) pF< ..J
5.5
5.0 I
I I
0
Fig. 1. P r e s s u r e d e p e n d e n c e
energy Eg becomes larger ( E A / C I < e n / C I < chxn/Cl).
Fig. 3 shows the pressure dependence of the
3
GPa
of lattice parameters
(al
of EA/CI.
cb)
£ 30 o , .~,(P{'~Cl )
30
RT 241eV
>: ~320
• • o
05(Pt.CI) w
E A / C I , E A / B r , e n / C l , c h x n / C l and P t / C 1 / P d . The shift JA~oJ becomes larger as the band gap
,
absorption edge of EA/CI and Pt/CI/Pd. The threshold energy E~h is defined as the lower edge of the charge transfer absorption band, which is about 0.4 eV smaller than Eg. The luminescence peak E~ of EA/CI is also shown. The shift of Eth of EA/CI is - 0 . 2 eV/GPa, which is larger than in
EA/CI
Fig. 1 shows the lattice dimensions of EA/CI determined by the precession method. As pressure increases, both the interchain distance and the M(II)-M(IV) distance decreases. Because each chain is surrounded by the ligands and the chains are coupled to each other by van der Waals force, the M(II)-M(IV) shortening will dominantly affect the electronic states. Fig. 2 shows the Raman frequencies w ( M ( I V ) - X ) of the symmetric stretching vibration of halogens around M(IV) in some complexes. The decrease of the phonon energy is observed in the pressure range of a few GPa for
I 2
PRESSURE
TE
3. Results and discussions
J
,a,
•
,,=31o Z <
>: ~320
~
• •e
3000
,~ '~ , ; ,
GPa
PRESSURE.
(c)
GPa
(d)
7 200
~E 3 4 0 f
E
o
~h(N.Pt.N
>:
)
/ ~,~Vc,> [
%°~
(J330~
~190 u.i
~170
chxn/CI RT 241eV
Z <
PRESSURE,
Z <
ee e e
~31o
,oOo• •
=<3o% , ~ , , , ~4 ,
8 ~18o
°
~
" o o o ,=or
~ ~, , c,,(p~Br)
' ~
en/Br RT 241eV L
4L
L
PRESSURE.
;
,E, 3201_ Z ] < F 3;
Pt/Cl/Pd RT 241eV
/
i
GPa
=<310v/
L
~ ' ;,
'
PRESSURE.
~
'
GPa
Fig. 2. Pressure dependence of Raman frequencies ~o (M(IV)-X) of (a) EA/CI, (b) chxn/Cl, (c) en/Br and (d) Pt / CI/Pd. Arrows indicate the values at atmospheric pressure.
489
H. Tanino et al. I Optical properties in halogen-bridged metal complexes
3t
Eth ] EA/CI El J
~2 )-
Br K - - 0 GPa --- 1.9 GPa
o Eth Pt/CI/Pd
I
en/Br
RT
~
=
l
e
f+
~
~
I!
'//~
ee" uJ
z
ktA ml
1
"
i -+ I
0
-8 I
I
2
PRESSURE,
-6
I
I
4 GPa
Fig. 3. Pressure dependence of absorption threshold energy E,h of EA/CI and Pt/CI/Pd, and luminescence peak energy E~ of EA/CI.
ordinary semiconductors, but the shift of E~ is only -0.05 eV/GPa. Hence, the relaxation energy E g - E I ( b E t h - El) decreases as pressure increases. These results are explained consistently as follows: At 0 GPa, X is strongly bonded to M(IV) in the form of [MX2(AA)2 ] and the X . . . M(II) interaction is rather weak. As the M(II)-M(IV) distance decreases, X would be pulled in the direction of M(II). As a result, the M(IV)-X distance will be elongated slightly, and the force constant of M(IV)-X and the phonon energy will decrease. We note that similar phenomena are observed in O - H . . . O of ice VII [9] and A u ( I I I ) - C I . . . Au(I) in CsAuCI 3 [10]. In this situation, the hybridization between dz2 of M(II) and Pz of X increases under pressure, so the transfer energy T of the charge transfer exciton will increase. Since the electron-phonon coupling energy S is little affected by pressure, the decrease of the relaxation energy originates from the decrease of S / T . We conclude that the system is continuously changed by pressure from a moderate coupling state at 0 GPa ( S - T) to a weak coupling state (S ~ T), not by decreasing the electron-phonon coupling energy S, but by increasing the transfer energy T. At higher pressure, the phonon energy of EA/CI begins to increase. In en/Br and en/I,
-4
-2
ENERGY, eV
"~ I
I
[
-20
O 20 40 ENERGY, eV Fig. 4. XANES spectra at the Br K-edge of en/Br at 0 GPa and 1.9 GPa. The first peak is enlarged in the inset.
where the gap energy Eg is rather small, it increases from the beginning. Since the sum of the ionic radii of M(IV), X and M(II) is comparable to the M(II)-M(IV) distance in this situation, X will be unmovable between the neighbouring metals and the phonon energy will increase. The decrease of the gap energy is also observed here, because the quasi-l-d character and CDW are preserved. Fig. 4 shows XANES spectra at the Br K-edge of en/Br. The first and second peaks are redshifted under pressure, but others are not. It is suggested that these two are related to the core-exciton and others are EXAFS oscillations. The first peak can be assigned as the allowed transition from ls to the 4cr*(p) band. Since the 4~r*(p) band is just the same as the empty dz_, band of Pt(IV), which is the origin of the conduction band in CDW, the red-shift of this peak is consistent with the decrease of the Peierls gap Eg. This tells us that the mixed-valence system is also continuously changed by pressure to the narrow gap state of CDW in the case of &os/a p > O. We note that the origin of the phase transition of Pt/CI/Pd observed in Raman frequency at 2.5 GPa is a problem for the future.
Acknowledgement The authors would like to acknowledge the
490
H. Tanino et al. / Optical properties in halogen-bridged metal complexes
support given by the staff of the Photon Factory of KEK. One of the authors (HT) is thankful to Dr. O. Shimomura for helpful advice on DAC and to Drs. T. Yao, H. Oyanagi and K. Takahashi for their continued interest and encouragement. References [1] H. Tanino and K. Kobayashi, J. Phys. Soc. Jpn. 52 (1983) 1446. [2] K. Nasu, J. Phys. Soc. Jpn. 53 (1984) 427. [3] H. Tanino, N. Koshizuka, K. Kobayashi, M. Yamashita and K. Hoh, J. Phys. Soc. Jpn. 54 (1985) 483.
[4] S. Yamaoka, O. Fukunaga, O. Shimomura and H. Nakazawa, Rev. Sci. Instr. 50 (1979) 1163. [5] K. Takemura, O. Shimomura, K. Tsuji and S. Minomura, High Temp.-High Press. 11 (1979) 311. [6] O. Shimomura, T. Fukamachi, T. Kawamura, S. Hosoya, S. Hunter and A. Bienenstock, Japan. J. Appl. Phys. 17-2 (1978) 221. [7] H. Tanino, K. Kato, M. Tokumoto, H. Anzai and G. Saito, J. Phys. Soc. Jpn. 54 (1985) 2390. [8] H. Tanino, H. Oyanagi, M. Yamashita and K. Kobayashi, Solid St. Commun. 53 (1985) 953. [9] G.E. Walrafen, M. Abebe, F.A. Manet, S. Block, G.J. Piermarini and R. Munro, J. Chem. Phys. 77 (1982) 2166. [10] W. Denner, H. Schulz and H. d'Amour, Acta Cryst. A35 (1979) 360.