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Photo-oxidation of tetracyanoplatinate single crystals
582
Journal of Luminescence 31 & 32(1984) 582-585 North-Holland, Anssterdarrr
PHOTO—OXIDATION OF TETRACYANOPLATINATE SINGLE CRYSTALS
Werner A. PFAB aod Volkmar GERHARDT Institut Physik 11 — Fcstkdrperphysik, Universitit Regeoshurg, UniversitEtsstr. 31, 8400 Regensburg, West—Oermaoy
Mex[Pt(CN)
4]~nH2O (Me Sr. Ca, Ba, Mg, Lu ...) crystals can he partially photo—oxidized at 1.7 K. An exciton—exciton---ioteractioo withio the polaroo— like A’iu—excited state removes electrons from antihonding orhitals of the Pt—chain. The electrons may be trapped between the chains, where they are coordinated octahedrically by the crystal—water molecules. From partially oxidized crystals like K2[Pt(CN)d]~Br.lO-3H2O (“Krogmann—Salt”) it is known that they have reduced Pt—Pt—intrachain distance. In comparison, at high excitation—intensities the photo—oxidation should produce domains with shorter Pt—Pt—distance, which are re—arranged by the flow—hack of eiectrons, if the crystal is heated. Coincident to this s thermoluminescence is observed.
The
tetracyanoplatinate crystals have become interesting
linear
chain compounds (e.g.
and electrical anisotropy
.
TcNQ, The Pt
—
like other extended
Polyacetylenel — because of their optical 9 d~—configuration and is coordi-
—ion has
nated planar—quadratically by the CM—ligands. These units are stacked together forming the chain—structure. The
cations and the water molecules
are
posi-
tioned between the chains. The absorption as well as the emission is highly polarized. If the crystals are
observed with light polarized parallel
to
the
Pt—Pt—stacking
axis
(c—axis), they show high reflectivity, which is much lower in the other polarization direction.
The absorption— and emission—hands shift to lower energies 2. in crystals having shorter Pt—Pt—distance Some properties of the crystal already can be derived from the single—electron MO—scheme of the [Pt(CN)
2—comptex: the eight d—electrons of the Pt—ion occupy all possible d—orhitals41except those of the dxc _yc~ They are filled from d—parts of the CN—ligands. The dx~_and the dyz_orbital~s take jart in unoccupied Trz_states of the CN—ligands. This donor—acceptor—binding decouples dzc_orbitals from dx
and dyz~ Consequently,
the
the
spin—orbit—splitting is re-
duced (“Ham—reduction”). The main part of the Pt—Pt—bonding is produced by 5d 2— and Os—orbitals. In good approximation, a nearly one—dimensional valence—band is formed, enlarged2—ion by the demixing—effect of d—orhitals. MO—structure of perpendicular to the c—axis is notTheessentially influthe free [Pt(CN)4J enced within the crystal. But it is important to mention that I. Absorption of light to A’.)
with polarization
0022—231 3/84/$03 00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
~I Ic
leads to Mott—Wannier—type
WA. Pfab, V. Gerhardt! Photo-oxidation of tetraryanoplatinate single crystals
583
oxcitons, because of the strong overlap of dzs_wavefunctions within the c— axis. 2. Absorption of light with polarization E J c leads to Frenkel—type excitons, which have a strong interaction with local vibrations of the CN—ligands (small exciton—polaron—state)
2
frt(CN)
-ION
CRYSTAL
43
~:: ~
Fig.
I
A
_______
£2
~
~
1g A’1j’ without with spin-orbit-coupling
1:
Energy—level diagram of the 2——ion and the tetracyano— platinate EPt(CN)4J crystal.
Fig. I shows the energy—levels of the complex—ion without and with spin—orbit— coupling and the related excitonic bands of the crystal. The observed emissions are marked in this figure. Using the selection—rules
derived from the complex molecule the following
spectroscopic properties should be observable: Aig~A~u_transition: — dipole—allowed, polarization El Ic, absorption and emission; Ajg~E~—transition: s~rsn—forbidden, polarization E J c, absorption and emission; Aig~Aiu_transition: spin— and parity—forbidden; coupling of localized eg— and a 2 —vibrations of the four CN—ligands breaks this selection—rule in emission; ~he emission—band is Ntokes—shifted and long—living. All transitions can be seen in the experiment. The two emissions starting from the E
— and the A —levels form one broad band. The two levels can be separated u lu by time—resolved spectroscopy, because they have different The The ener1
gy difference the E—the and energy of the between local eg~and a the Atu_level is 20 cm 2g~mode is about 300 cm’
~.
The increase of the integrated emission—intensity of band 1 and 2 as a function of the pump—power is drawn in Fig. 2. Considering the non—linearity of the Ai—intensity that can be fitted with the equation given in Fig. 3, we conclude that a decay—channel k2 exists, which
reduces
the population of the Aiu_level
by an exciton—exciton—interaction. We interprete the formation of a new absorption—band as a result of this exciton—exciton—(polaron—)interaction.
Fig. 4
shows this new absorption band that can be bleached again using green light and it will also disappear,
if the crystal is heated. As an accompanying phenomenon
584
I
WA. PJdb, V. Gerhardt ! Photo-oxidation of tetraryanoplatinate single crystals
;m9:(C~i,~A,~
~
RFA’~~~IFvFl:
~
Z
+ INTENSITY
OF EXCITATION
2k
2
Fig. 2: Increase in intensity of the A10— and the E,—emission as a function of excitation —intensity.
2k
2k2
Fig. 3: Rate—equation for the Aiu_level.
a thermoluminescence in the region of 2.5
ENERGY 2.2
2.4
,
‘
the A1rA1g_transstson is observed with peaks at 8.5, IS, and about 70 K.
B0CP
:25
z
LeVI
2.0 10.0
P
From “Krogmann—Salt”
20000
19000
18000
known that electrons can be removed
a
from antihonding orbitals of the Pt—Pt—
~
chain.
~
acceptor.
17000
ENERGY
well
<
The Br —ion is the electron— This partial
the chain leads 21000
it is
[1/cm]
Fig. 4: New absorption—band produced by photo— oxidation of BaCP compared to the po— sitson of the A2u_ and Eu_emission. the decay of the absorption as a func— tion of time by bleaching with green -Isght is also shown.
to
oxidation some
of
dramatic
changes of the anisotropic properties of the crystal: The one—dimensional valence—band is no longer completely occupied. It becomes a one—dimensional conduction—band. deed,
In—
the partially oxidized crystals
of this type show metallic reflection and have high conductivity in the direction of the Pt—chains. The anisotropy
is nearly I0~at 205 K. At low temperatures a metal—insulator—transition takes place driven by a Peierls distortion. The conductivity is reduced to 11(Q-cm)1 at 30 K4. 1O With reference to the chemical oxidation we interprete our observations as a photo—oxidation of the Pt—chain: the exciton—exciton—annihilation supplies sufficient energy to remove electrons from the one—dimensional valence—band.
We
presume that a lattice site between the chains, which is octahedricaily coordinated by water molecules,
acts as the acceptor for the electron. We assume that
WA. Pfab, V. Gerhardt
!
Photo-oxidation of tetracyanoplatinate single crystals
585
the new absorption—band belongs to the A’ —. A —transition of the distorted lg 2u chain. Therefore, we can evaluate the shortening AR of the Pt—Pt—distance by 1 the photo—oxidation from the experimental relation VABS/cm 45.5~10~— 8.O105R~/A
_______
Emission—peak
T-I.8 K
A 2ui
B
cm
A
Absorption—peak A
E
E
16.080 16.280 16.250 16.850 19.850
K9[Pt(CN)
3.101 3.109 3.108 3.132 3.262
~.Br .3H,,0 4 .30 K9[Pt(CN)4J.C139.3H2O -
In the table,
A
R5
2u1 cm
A
aR=RE_E~
2u 1
MgCP LuCP YbCP LiKCP BaCP
New Absorption
A
cm
A
18.7OO~
3.102
22.35O1
3.257
15.300 15.750 15.700 16.290 19.170
—
T-295K
A
2.98 2.996 2.994 3.014 3.14
.121 .113 .114 .118 .122
2.89 2.88
E is given for some platinates together with the positions
of
the A’ —tA —absorption and the El Ic—emission. For comparison the Pt—Pt—distance ig 2u of two chemically oxidized salts are given. At high pump—rates an optical astability can be observed: the emission—intensity oscillates with frequencies between I and 10 kHz. Within our can be understood, if we assume domains that are completely
model
this
photo—oxidized.
These domains should undergo a Peierls—transition at low temperatures.
Hence,
the observed oscillations should be produced by the photon—driven fluctuations between the distorted and the undistorted domains.
ACKNOWLEDGEMENT We thank the “Deutsche Forschungsgemeinschaft” for generous support of this project.
REFERENCES 1) J.S. Miller (ed.), Extended Linear Chain Compounds (Plenum Press, New York, 1982). 2) H. Yersin and G. Gliemann, in: J.S. Miller and A.J. Epstein (ed.), Synthesis and Properties of Low Dimensional Materials, Annals of the New York Academy of Sciences, Vol. 313 (1978), p. 539.
3) I. Hidvegi, W. v.Ammon, and C. Gliemann, J.Chem.Phys. 76 (1982), p. 4361. 4) H.R. Zeller and A. Beck, J.Phys.Chem.Solids 35 (1974), p. 77. 5) W. Tuszynski, Dissertation (Regensburg, 1977).
Journal of Luminescence 31 & 32(1984) 582-585 North-Holland, Anssterdarrr
PHOTO—OXIDATION OF TETRACYANOPLATINATE SINGLE CRYSTALS
Werner A. PFAB aod Volkmar GERHARDT Institut Physik 11 — Fcstkdrperphysik, Universitit Regeoshurg, UniversitEtsstr. 31, 8400 Regensburg, West—Oermaoy
Mex[Pt(CN)
4]~nH2O (Me Sr. Ca, Ba, Mg, Lu ...) crystals can he partially photo—oxidized at 1.7 K. An exciton—exciton---ioteractioo withio the polaroo— like A’iu—excited state removes electrons from antihonding orhitals of the Pt—chain. The electrons may be trapped between the chains, where they are coordinated octahedrically by the crystal—water molecules. From partially oxidized crystals like K2[Pt(CN)d]~Br.lO-3H2O (“Krogmann—Salt”) it is known that they have reduced Pt—Pt—intrachain distance. In comparison, at high excitation—intensities the photo—oxidation should produce domains with shorter Pt—Pt—distance, which are re—arranged by the flow—hack of eiectrons, if the crystal is heated. Coincident to this s thermoluminescence is observed.
The
tetracyanoplatinate crystals have become interesting
linear
chain compounds (e.g.
and electrical anisotropy
.
TcNQ, The Pt
—
like other extended
Polyacetylenel — because of their optical 9 d~—configuration and is coordi-
—ion has
nated planar—quadratically by the CM—ligands. These units are stacked together forming the chain—structure. The
cations and the water molecules
are
posi-
tioned between the chains. The absorption as well as the emission is highly polarized. If the crystals are
observed with light polarized parallel
to
the
Pt—Pt—stacking
axis
(c—axis), they show high reflectivity, which is much lower in the other polarization direction.
The absorption— and emission—hands shift to lower energies 2. in crystals having shorter Pt—Pt—distance Some properties of the crystal already can be derived from the single—electron MO—scheme of the [Pt(CN)
2—comptex: the eight d—electrons of the Pt—ion occupy all possible d—orhitals41except those of the dxc _yc~ They are filled from d—parts of the CN—ligands. The dx~_and the dyz_orbital~s take jart in unoccupied Trz_states of the CN—ligands. This donor—acceptor—binding decouples dzc_orbitals from dx
and dyz~ Consequently,
the
the
spin—orbit—splitting is re-
duced (“Ham—reduction”). The main part of the Pt—Pt—bonding is produced by 5d 2— and Os—orbitals. In good approximation, a nearly one—dimensional valence—band is formed, enlarged2—ion by the demixing—effect of d—orhitals. MO—structure of perpendicular to the c—axis is notTheessentially influthe free [Pt(CN)4J enced within the crystal. But it is important to mention that I. Absorption of light to A’.)
with polarization
0022—231 3/84/$03 00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
~I Ic
leads to Mott—Wannier—type
WA. Pfab, V. Gerhardt! Photo-oxidation of tetraryanoplatinate single crystals
583
oxcitons, because of the strong overlap of dzs_wavefunctions within the c— axis. 2. Absorption of light with polarization E J c leads to Frenkel—type excitons, which have a strong interaction with local vibrations of the CN—ligands (small exciton—polaron—state)
2
frt(CN)
-ION
CRYSTAL
43
~:: ~
Fig.
I
A
_______
£2
~
~
1g A’1j’ without with spin-orbit-coupling
1:
Energy—level diagram of the 2——ion and the tetracyano— platinate EPt(CN)4J crystal.
Fig. I shows the energy—levels of the complex—ion without and with spin—orbit— coupling and the related excitonic bands of the crystal. The observed emissions are marked in this figure. Using the selection—rules
derived from the complex molecule the following
spectroscopic properties should be observable: Aig~A~u_transition: — dipole—allowed, polarization El Ic, absorption and emission; Ajg~E~—transition: s~rsn—forbidden, polarization E J c, absorption and emission; Aig~Aiu_transition: spin— and parity—forbidden; coupling of localized eg— and a 2 —vibrations of the four CN—ligands breaks this selection—rule in emission; ~he emission—band is Ntokes—shifted and long—living. All transitions can be seen in the experiment. The two emissions starting from the E
— and the A —levels form one broad band. The two levels can be separated u lu by time—resolved spectroscopy, because they have different The The ener1
gy difference the E—the and energy of the between local eg~and a the Atu_level is 20 cm 2g~mode is about 300 cm’
~.
The increase of the integrated emission—intensity of band 1 and 2 as a function of the pump—power is drawn in Fig. 2. Considering the non—linearity of the Ai—intensity that can be fitted with the equation given in Fig. 3, we conclude that a decay—channel k2 exists, which
reduces
the population of the Aiu_level
by an exciton—exciton—interaction. We interprete the formation of a new absorption—band as a result of this exciton—exciton—(polaron—)interaction.
Fig. 4
shows this new absorption band that can be bleached again using green light and it will also disappear,
if the crystal is heated. As an accompanying phenomenon
584
I
WA. PJdb, V. Gerhardt ! Photo-oxidation of tetraryanoplatinate single crystals
;m9:(C~i,~A,~
~
RFA’~~~IFvFl:
~
Z
+ INTENSITY
OF EXCITATION
2k
2
Fig. 2: Increase in intensity of the A10— and the E,—emission as a function of excitation —intensity.
2k
2k2
Fig. 3: Rate—equation for the Aiu_level.
a thermoluminescence in the region of 2.5
ENERGY 2.2
2.4
,
‘
the A1rA1g_transstson is observed with peaks at 8.5, IS, and about 70 K.
B0CP
:25
z
LeVI
2.0 10.0
P
From “Krogmann—Salt”
20000
19000
18000
known that electrons can be removed
a
from antihonding orbitals of the Pt—Pt—
~
chain.
~
acceptor.
17000
ENERGY
well
<
The Br —ion is the electron— This partial
the chain leads 21000
it is
[1/cm]
Fig. 4: New absorption—band produced by photo— oxidation of BaCP compared to the po— sitson of the A2u_ and Eu_emission. the decay of the absorption as a func— tion of time by bleaching with green -Isght is also shown.
to
oxidation some
of
dramatic
changes of the anisotropic properties of the crystal: The one—dimensional valence—band is no longer completely occupied. It becomes a one—dimensional conduction—band. deed,
In—
the partially oxidized crystals
of this type show metallic reflection and have high conductivity in the direction of the Pt—chains. The anisotropy
is nearly I0~at 205 K. At low temperatures a metal—insulator—transition takes place driven by a Peierls distortion. The conductivity is reduced to 11(Q-cm)1 at 30 K4. 1O With reference to the chemical oxidation we interprete our observations as a photo—oxidation of the Pt—chain: the exciton—exciton—annihilation supplies sufficient energy to remove electrons from the one—dimensional valence—band.
We
presume that a lattice site between the chains, which is octahedricaily coordinated by water molecules,
acts as the acceptor for the electron. We assume that
WA. Pfab, V. Gerhardt
!
Photo-oxidation of tetracyanoplatinate single crystals
585
the new absorption—band belongs to the A’ —. A —transition of the distorted lg 2u chain. Therefore, we can evaluate the shortening AR of the Pt—Pt—distance by 1 the photo—oxidation from the experimental relation VABS/cm 45.5~10~— 8.O105R~/A
_______
Emission—peak
T-I.8 K
A 2ui
B
cm
A
Absorption—peak A
E
E
16.080 16.280 16.250 16.850 19.850
K9[Pt(CN)
3.101 3.109 3.108 3.132 3.262
~.Br .3H,,0 4 .30 K9[Pt(CN)4J.C139.3H2O -
In the table,
A
R5
2u1 cm
A
aR=RE_E~
2u 1
MgCP LuCP YbCP LiKCP BaCP
New Absorption
A
cm
A
18.7OO~
3.102
22.35O1
3.257
15.300 15.750 15.700 16.290 19.170
—
T-295K
A
2.98 2.996 2.994 3.014 3.14
.121 .113 .114 .118 .122
2.89 2.88
E is given for some platinates together with the positions
of
the A’ —tA —absorption and the El Ic—emission. For comparison the Pt—Pt—distance ig 2u of two chemically oxidized salts are given. At high pump—rates an optical astability can be observed: the emission—intensity oscillates with frequencies between I and 10 kHz. Within our can be understood, if we assume domains that are completely
model
this
photo—oxidized.
These domains should undergo a Peierls—transition at low temperatures.
Hence,
the observed oscillations should be produced by the photon—driven fluctuations between the distorted and the undistorted domains.
ACKNOWLEDGEMENT We thank the “Deutsche Forschungsgemeinschaft” for generous support of this project.
REFERENCES 1) J.S. Miller (ed.), Extended Linear Chain Compounds (Plenum Press, New York, 1982). 2) H. Yersin and G. Gliemann, in: J.S. Miller and A.J. Epstein (ed.), Synthesis and Properties of Low Dimensional Materials, Annals of the New York Academy of Sciences, Vol. 313 (1978), p. 539.
3) I. Hidvegi, W. v.Ammon, and C. Gliemann, J.Chem.Phys. 76 (1982), p. 4361. 4) H.R. Zeller and A. Beck, J.Phys.Chem.Solids 35 (1974), p. 77. 5) W. Tuszynski, Dissertation (Regensburg, 1977).