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ESR study of dynamics of the phase transition in Wurster's blue perchlorate
Synthetic Metals, 55-57 (1993) 1683-1687
168 3
ESR STUDY OF DYNAMICS OF THE PHASE TRANSITION IN WURSTER'S BLUE PERfiHLORATE
H. AWAN0, H. ARAKI and H. OHIGASHI F a c u l t y of E n g i n e e r i n g , Yamagata U n i v e r s i t y ,
Yonezawa (Japan)
ABSTRACT Wurster's blue perchlorate oxidation,
(WBP) c r y s t a l
was p r e p a r e d by an e l e c t r o c h e m i c a l
and i t s ESR s p e c t r a were measured over t h e r a n g e of 100--350 K.
Each s p e c t r u m shows a s i n g l e l i n e w i t h an e x c h a n g e - u a r r o w e d h o r e n t z shape. ESR s p e c t r a have a maximum i n t h e i n t e n s i t y linewidth near the temperature,
a t 193 K, and a minimum i n t h e
which c o r r e s p o n d s t o t h e well-known
paramagnetic-antiferromagnetic remarkable anisotropy,
The
phase transition.
in particular,
The l i n e w i d t h s show
in the low-temperature region.
The
a n g u l a r d e p e n d e n c e of l i n e w i d t h can be a c c o u n t e d f o r by a o n e - d i m e n s i o n a l s p i n a r r a y a l o n g t h e a - a x i s of WBP c r y s t a l . decrease,
when t h e f i e l d
at the transition
point,
the cation radicals
a-axis than along other axes, radical
The l i n e w i d t h e x h i b i t s
is applied along to the a-axis.
t h e most d r a s t i c
This is because,
would more f l u c t u a t e
along the
s i n c e t h e r e a r r a n g e m e n t of t h e W u r s t e r ' s b l u e
cation occurs at the temperature along the a-axis.
I NTRODUCTI ON Wurster's blue perchlorate
(WBP) i s a c a t i o n r a d i c a l
s a l t which i s p r e p a r e d
by t h e o x i d a t i o n o f N,N,N' , N ' - t e t r a m e t h y l - p - p h e n y l e n e d i a m i n e
(TMPD).
been w e l l known t o e x p e r i e n c e p a r a m a g n e t i c - a n t i f e r r o m a g n e t i c
phase transition
a r o u n d 190 K.
Magnetic s u s c e p t i b i l i t y
workers [ 1 - 5 ] .
Chihara et el.
determined the transition presented the crystal
[6] have measured t h e h e a t c a p a c i t y of WBP, and In 1971, de Boer and Vos [7]
o f t h e r o o m - t e m p e r a t u r e and t h e low-
t e m p e r a t u r e p h a s e s o f WBP. The c r y s t a l room t e m p e r a t u r e ,
of WBP have been measured by many
p o i n t a s 189.9 K.
structures
WBP h a s
of WBP h a s o r t h o r h o m b i c symmetry a t
w h i l e i t h a s m o n o c l i n i c symmetry below t h e t r a n s i t i o n
point.
In t h e r o o m - t e m p e r a t u r e p h a s e , t h e TMPD c a t i o n s a r e s t a c k e d a t e q u a l d i s t a n c e s along the a-axis alternately
(see F i g . l ) .
Below t h e t r a n s i t i o n
r e a r r a n g e d a l o n g t h e a - a x i s by 5 %.
0379-6779/93/$6.00
point,
t h e TMPD c a t i o n s a r e
Consequently,
the
© 1993- Elsevier Sequoia. All rights reserved
1684
%
1
%.
1
%,
Fig. 1. P r o j e c t i o n of the c r y s t a l s t r u c t u r e of the room-temperature phase of WBP onto the ab-plane (reproduced from Ref. 8).
a n t i f e r r o m a g n e t i c behavior of the low-temperature phase can be i n t e r p r e t e d in terms of d i m e r i z a t i o n of the TMPD r a d i c a l c a t i o n s . Thus the nature of the phase t r a n s i o n of WBP, accompanied by a c r y s t a l l o g r a p h i c rearrangement, has been s u c c e s s f u l l y c l a r i f i e d . little
However,
works have been c a r r i e d out on spin dynamics in WBP as yet.
In a d d i t i o n , the a n i s o t r o p y of ESR of WBP c r y s t a l near the t r a n s i t i o n point has never been i n v e s t i g a t e d .
In the p r e s e n t work, ESR s p e c t r a of WBP c r y s t a l are
i n v e s t i g a t e d , and dynamics of the phase t r a n s i t i o n i s discussed. EXPERIMENTAL WBP crystal was prepared by an electrochemical oxidation [9].
A solution of
0.5 mol dm -3 TMPD and 0.5 tool dm -3 tetrabutylammonium perchlorate in acetonitrile was prepared and poured into a glass cell.
An anode of indium tin
oxide-coated glass and a cathode of silver wire were used.
When a voltage of
4.0 V was applied for 10 minutes at room temperature, dark purple needle-like crystals longer than several milimeters were deposited on the anode. C, 46.05; H, 6.36; N, 9.94%. 10.62.
Found:
Calcd for CIoHI6N204CI: C, 45.55; H, 6.12; N,
The absorption spectrum of the product in acetonitrile coincides
thoroughly with that reported for WBP by Uemura et al. [i0]. X-band ESR spectra of the crystal were obtained over the range of 100--350 K with a JOEL JES-FEIX spectrometer.
Differential scanning calorimetry (DSC) was
measured with an MAC 001 thermal analyzer.
1685 RESISTS AND DISCUSSION Each ESR spectrum shows a single line with an exchange-narrowed Lorentz shaI~.
Figure 2 shows the temperature dependence of integrated intensity,
where the field is applied along three axes.
The intensity has a maximum at
193 K, and it abruptly drops by about 30 %, which corresponds to the well known paramagnetic antiferromagnetic phase transition.
DSC of our specimen was
measured: when the temperature was raised, an endothermic peak was observed at 192 K.
According to Okumura's static measurement [23, the magnetic
succeptibility experiences a maximum at 189 K, and it suddenly decreases by about I0 % of its maximum.
On the other hand, Kawamori et al. [II] have
re~rted a sudden decrease in intensity by more than 30 %, with which our results well agree.
Remarkable anisotropy is observed at the transition point:
the intensity ratio is found t,o be
I//a: I//b: I//c ~
1.0:i.2 :l.0.
[]
c
>, x5
[] ~
¢,
1
~
0
Z~
&
/la
ZX
[
I
1O0
I
[
200
300
Temperature / K
Fig. 2.
T e m p e r a t u r e dependence o f t h e ESR i n t e g r a t e d
intensity
o f WBP.
F i g u r e 3 shows t h e t e m p e r a t u r e dependence of t h e p e a k - t o - p e a k l i n e w i d t h . The l i n e w i d t h h a s a minimum n e a r 193 K, and a b r u p t l y r i s e s w i t h d e c r e a s i n g temperature. ratios
Remarkable a n i s o t r o p y i s a l s o o b s e r v e d i n t h e l i n e w i d t h :
are AH//a:
linewidth,
/%H//b : A H / / c
i t can be. s t a t e d
temperature is significantly Such a r e s u l t radicals
=
1 . 0 : 1.2 : 1.2.
that the spin-spin relaxation larger parallel
corresponds to the fact that,
would more f l u c t u a t e
time a t t h e
to the a-axis than to the b-axis. at the transition
point,
along the a-axis than along other axes,
r e a r r a n g e m e n t of t h e W u r s t e r ' s b l u e r a d i c a l along the a-axis.
its
From t h e a n i s o t r o p y of t h e
the cation since the
cation occurs at the temperature
1686
0.8
z~
0.6 ,,b
-J 0,4
0.2 I
I
i
100
a
I
200
300
Temperature / K
Temperature dependence of the p e a k - t o - p e a k l i n e w i d t h of WBP.
F i g . 3.
0.~ ~.
(a) ab-plane A [3cos20-114/3+B A=0.090mT,B=0.38mT _
(b) Ix-plane 0.8
_
°V°
E-
0.6,,
0.1
-5
o
(lz.
0.2
415
[
I
90
135
0"20
180
Angle/deg
o
I
4'5
90
1~
160
Angle/dog
F i g . 4. Angular dependence of the p e a k - t o - p e a k l i n e w i d t h of NBP a t 103 K i n t h e a b - p l a n e (a) and in t h e b c - p l a n e (b). D i f f e r e n t samples were used f o r (a) and (b). F i g u r e 4 shows a n g u l a r dependences of p e a k - t o - p e a k l i n e w i d t h in the lowt e m p e r a t u r e phase.
In (a), the a p p l i e d f i e l d
is parallel
to the a b - p l a n e ,
a b s c i s s a showing the a n g l e between t h e a - a x i s and t h e a p p l i e d f i e l d . the applied field is parallel
to the b c - p l a n e ,
between the c - a x i s and the a p p l i e d f i e l d . anisotropy,
in particular,
the
In (b),
t h e a b s c i s s a showing t h e a n g l e
The l i n e w i d t h s show remarkable
in t h e l o w - t e m p e r a t u r e r e g i o n .
As shown in F i g . 4
(a), t h e a n g u l a r dependence of l i n e w i d t h i n the a b - p l a n e f o l l o w s A H = A I 3 c o s 2 8 --1 14/3+B , which can be accounted f o r as a o n e - d i m e n s i o n a l a n t i f e r r o m a g n e t [12] w i t h a o n e - d i m e n s i o n a l s p i n a r r a y a l o n g t h e a - a x i s o f WHP c r y s t a l . the contrary,
On
as shown in F i g . 4 (b), t h e a n g u l a r dependence of l i n e w i d t h in
t h e b c - p l a n e f o l l o w s A H = A ( cos 2 8 + 1 ) + B , which means t h a t in t h e b e - p l a n e
1687 not one-dimensional but traditional three dimensional spin correlation is applicable.
Deviation of the observed values from the theoretical curve can be
attributed to some defects of the crystal.
Heating toward the transition
point, the anisotropy is rapidly disappearing (see Fig. 3).
Above the
transition, the linewidth does not show such a significant anisotropy.
This
corresponds to the fact that, in the room-temperature phase, the TMPD radical c a t i o n s exist
as radical monomers.
ACKNOWLEDGEMENTS The a u t h o r s a r e i n d e b t e d t o Dr. K. Akiyama of Tohoku U n i v e r s i t y and P r o f . T. Watanabe o f Yamagata U n i v e r s i t y f o r v a l u a b l e d i s c u s s i o n s . grateful
The a u t h o r s a r e
t o I k e t a n i S c i e n c e and Technology F o u n d a t i o n f o r s u p p o r t o f t h i s
research through a grant,
and t ~ e y a l s o wish t o t h a n k Yamagata T e c h n o p o l i s
F o u n d a t i o n f o r accommodating them w i t h t h e ESR s p e c t r o m e t e r .
REFERENCES 1
W. Duffy,
2
K. Okumura,
Jr.,
J. Chem.
J. Phys.
Phys.,
3
D. B. Chesnut,
4
Z. G. Soes and R. C. Hughes,
5
J. Tanaka,
6
H. Chihara,
(1970)
J. Chem.
M. Inoue,
36 (1962)
Soc. Jpn., Phys.,
45 (1966) J. Chem.
M. Mizuno
490.
18 (1963)
69. 4677.
Phys.,
46 (1967)
and K. Horai,
Bull.
253.
Chem.
Soc. Jpn.,
43
1998. M. Nakamura
and S. Seki,
Bull.
Chem.
Soc. Jpn.,
J. L. de Boer and A. Vos, Acta Cryst.,
B28
8
Y. lyechika,
Bull.
9
H. Awano, H. Murakami, T. Yamashita, H. 0higashi and T. Ogata, Synth. Met.,
and H. Kuroda,
Chem.
835: Soc.
ibid.,
1776.
7
K. Yakushi
(1972)
38 (1965)
Jpn.,
839. 53 (1980)
603.
39 (1991) 327. i0
K. Uemura, S. Nakayama, Y. Seo, K. Suzuki and Y. 0oshika, Bull. Chem. Soc. Jpn., 39 (1966) 1348.
ii A. Kawamori, A. Honda, N. Joo, K. Suzuki and Y. 0oshika, J. Chem. Phys., 44 (1966) 4363. 12 R. E. Dietz, F. R. Merritt, R. Dingle, D. Hone, B. G. Silbernagel and P. M. Richards, Phys. Rev. Lett., 26 (1971) 1186.
168 3
ESR STUDY OF DYNAMICS OF THE PHASE TRANSITION IN WURSTER'S BLUE PERfiHLORATE
H. AWAN0, H. ARAKI and H. OHIGASHI F a c u l t y of E n g i n e e r i n g , Yamagata U n i v e r s i t y ,
Yonezawa (Japan)
ABSTRACT Wurster's blue perchlorate oxidation,
(WBP) c r y s t a l
was p r e p a r e d by an e l e c t r o c h e m i c a l
and i t s ESR s p e c t r a were measured over t h e r a n g e of 100--350 K.
Each s p e c t r u m shows a s i n g l e l i n e w i t h an e x c h a n g e - u a r r o w e d h o r e n t z shape. ESR s p e c t r a have a maximum i n t h e i n t e n s i t y linewidth near the temperature,
a t 193 K, and a minimum i n t h e
which c o r r e s p o n d s t o t h e well-known
paramagnetic-antiferromagnetic remarkable anisotropy,
The
phase transition.
in particular,
The l i n e w i d t h s show
in the low-temperature region.
The
a n g u l a r d e p e n d e n c e of l i n e w i d t h can be a c c o u n t e d f o r by a o n e - d i m e n s i o n a l s p i n a r r a y a l o n g t h e a - a x i s of WBP c r y s t a l . decrease,
when t h e f i e l d
at the transition
point,
the cation radicals
a-axis than along other axes, radical
The l i n e w i d t h e x h i b i t s
is applied along to the a-axis.
t h e most d r a s t i c
This is because,
would more f l u c t u a t e
along the
s i n c e t h e r e a r r a n g e m e n t of t h e W u r s t e r ' s b l u e
cation occurs at the temperature along the a-axis.
I NTRODUCTI ON Wurster's blue perchlorate
(WBP) i s a c a t i o n r a d i c a l
s a l t which i s p r e p a r e d
by t h e o x i d a t i o n o f N,N,N' , N ' - t e t r a m e t h y l - p - p h e n y l e n e d i a m i n e
(TMPD).
been w e l l known t o e x p e r i e n c e p a r a m a g n e t i c - a n t i f e r r o m a g n e t i c
phase transition
a r o u n d 190 K.
Magnetic s u s c e p t i b i l i t y
workers [ 1 - 5 ] .
Chihara et el.
determined the transition presented the crystal
[6] have measured t h e h e a t c a p a c i t y of WBP, and In 1971, de Boer and Vos [7]
o f t h e r o o m - t e m p e r a t u r e and t h e low-
t e m p e r a t u r e p h a s e s o f WBP. The c r y s t a l room t e m p e r a t u r e ,
of WBP have been measured by many
p o i n t a s 189.9 K.
structures
WBP h a s
of WBP h a s o r t h o r h o m b i c symmetry a t
w h i l e i t h a s m o n o c l i n i c symmetry below t h e t r a n s i t i o n
point.
In t h e r o o m - t e m p e r a t u r e p h a s e , t h e TMPD c a t i o n s a r e s t a c k e d a t e q u a l d i s t a n c e s along the a-axis alternately
(see F i g . l ) .
Below t h e t r a n s i t i o n
r e a r r a n g e d a l o n g t h e a - a x i s by 5 %.
0379-6779/93/$6.00
point,
t h e TMPD c a t i o n s a r e
Consequently,
the
© 1993- Elsevier Sequoia. All rights reserved
1684
%
1
%.
1
%,
Fig. 1. P r o j e c t i o n of the c r y s t a l s t r u c t u r e of the room-temperature phase of WBP onto the ab-plane (reproduced from Ref. 8).
a n t i f e r r o m a g n e t i c behavior of the low-temperature phase can be i n t e r p r e t e d in terms of d i m e r i z a t i o n of the TMPD r a d i c a l c a t i o n s . Thus the nature of the phase t r a n s i o n of WBP, accompanied by a c r y s t a l l o g r a p h i c rearrangement, has been s u c c e s s f u l l y c l a r i f i e d . little
However,
works have been c a r r i e d out on spin dynamics in WBP as yet.
In a d d i t i o n , the a n i s o t r o p y of ESR of WBP c r y s t a l near the t r a n s i t i o n point has never been i n v e s t i g a t e d .
In the p r e s e n t work, ESR s p e c t r a of WBP c r y s t a l are
i n v e s t i g a t e d , and dynamics of the phase t r a n s i t i o n i s discussed. EXPERIMENTAL WBP crystal was prepared by an electrochemical oxidation [9].
A solution of
0.5 mol dm -3 TMPD and 0.5 tool dm -3 tetrabutylammonium perchlorate in acetonitrile was prepared and poured into a glass cell.
An anode of indium tin
oxide-coated glass and a cathode of silver wire were used.
When a voltage of
4.0 V was applied for 10 minutes at room temperature, dark purple needle-like crystals longer than several milimeters were deposited on the anode. C, 46.05; H, 6.36; N, 9.94%. 10.62.
Found:
Calcd for CIoHI6N204CI: C, 45.55; H, 6.12; N,
The absorption spectrum of the product in acetonitrile coincides
thoroughly with that reported for WBP by Uemura et al. [i0]. X-band ESR spectra of the crystal were obtained over the range of 100--350 K with a JOEL JES-FEIX spectrometer.
Differential scanning calorimetry (DSC) was
measured with an MAC 001 thermal analyzer.
1685 RESISTS AND DISCUSSION Each ESR spectrum shows a single line with an exchange-narrowed Lorentz shaI~.
Figure 2 shows the temperature dependence of integrated intensity,
where the field is applied along three axes.
The intensity has a maximum at
193 K, and it abruptly drops by about 30 %, which corresponds to the well known paramagnetic antiferromagnetic phase transition.
DSC of our specimen was
measured: when the temperature was raised, an endothermic peak was observed at 192 K.
According to Okumura's static measurement [23, the magnetic
succeptibility experiences a maximum at 189 K, and it suddenly decreases by about I0 % of its maximum.
On the other hand, Kawamori et al. [II] have
re~rted a sudden decrease in intensity by more than 30 %, with which our results well agree.
Remarkable anisotropy is observed at the transition point:
the intensity ratio is found t,o be
I//a: I//b: I//c ~
1.0:i.2 :l.0.
[]
c
>, x5
[] ~
¢,
1
~
0
Z~
&
/la
ZX
[
I
1O0
I
[
200
300
Temperature / K
Fig. 2.
T e m p e r a t u r e dependence o f t h e ESR i n t e g r a t e d
intensity
o f WBP.
F i g u r e 3 shows t h e t e m p e r a t u r e dependence of t h e p e a k - t o - p e a k l i n e w i d t h . The l i n e w i d t h h a s a minimum n e a r 193 K, and a b r u p t l y r i s e s w i t h d e c r e a s i n g temperature. ratios
Remarkable a n i s o t r o p y i s a l s o o b s e r v e d i n t h e l i n e w i d t h :
are AH//a:
linewidth,
/%H//b : A H / / c
i t can be. s t a t e d
temperature is significantly Such a r e s u l t radicals
=
1 . 0 : 1.2 : 1.2.
that the spin-spin relaxation larger parallel
corresponds to the fact that,
would more f l u c t u a t e
time a t t h e
to the a-axis than to the b-axis. at the transition
point,
along the a-axis than along other axes,
r e a r r a n g e m e n t of t h e W u r s t e r ' s b l u e r a d i c a l along the a-axis.
its
From t h e a n i s o t r o p y of t h e
the cation since the
cation occurs at the temperature
1686
0.8
z~
0.6 ,,b
-J 0,4
0.2 I
I
i
100
a
I
200
300
Temperature / K
Temperature dependence of the p e a k - t o - p e a k l i n e w i d t h of WBP.
F i g . 3.
0.~ ~.
(a) ab-plane A [3cos20-114/3+B A=0.090mT,B=0.38mT _
(b) Ix-plane 0.8
_
°V°
E-
0.6,,
0.1
-5
o
(lz.
0.2
415
[
I
90
135
0"20
180
Angle/deg
o
I
4'5
90
1~
160
Angle/dog
F i g . 4. Angular dependence of the p e a k - t o - p e a k l i n e w i d t h of NBP a t 103 K i n t h e a b - p l a n e (a) and in t h e b c - p l a n e (b). D i f f e r e n t samples were used f o r (a) and (b). F i g u r e 4 shows a n g u l a r dependences of p e a k - t o - p e a k l i n e w i d t h in the lowt e m p e r a t u r e phase.
In (a), the a p p l i e d f i e l d
is parallel
to the a b - p l a n e ,
a b s c i s s a showing the a n g l e between t h e a - a x i s and t h e a p p l i e d f i e l d . the applied field is parallel
to the b c - p l a n e ,
between the c - a x i s and the a p p l i e d f i e l d . anisotropy,
in particular,
the
In (b),
t h e a b s c i s s a showing t h e a n g l e
The l i n e w i d t h s show remarkable
in t h e l o w - t e m p e r a t u r e r e g i o n .
As shown in F i g . 4
(a), t h e a n g u l a r dependence of l i n e w i d t h i n the a b - p l a n e f o l l o w s A H = A I 3 c o s 2 8 --1 14/3+B , which can be accounted f o r as a o n e - d i m e n s i o n a l a n t i f e r r o m a g n e t [12] w i t h a o n e - d i m e n s i o n a l s p i n a r r a y a l o n g t h e a - a x i s o f WHP c r y s t a l . the contrary,
On
as shown in F i g . 4 (b), t h e a n g u l a r dependence of l i n e w i d t h in
t h e b c - p l a n e f o l l o w s A H = A ( cos 2 8 + 1 ) + B , which means t h a t in t h e b e - p l a n e
1687 not one-dimensional but traditional three dimensional spin correlation is applicable.
Deviation of the observed values from the theoretical curve can be
attributed to some defects of the crystal.
Heating toward the transition
point, the anisotropy is rapidly disappearing (see Fig. 3).
Above the
transition, the linewidth does not show such a significant anisotropy.
This
corresponds to the fact that, in the room-temperature phase, the TMPD radical c a t i o n s exist
as radical monomers.
ACKNOWLEDGEMENTS The a u t h o r s a r e i n d e b t e d t o Dr. K. Akiyama of Tohoku U n i v e r s i t y and P r o f . T. Watanabe o f Yamagata U n i v e r s i t y f o r v a l u a b l e d i s c u s s i o n s . grateful
The a u t h o r s a r e
t o I k e t a n i S c i e n c e and Technology F o u n d a t i o n f o r s u p p o r t o f t h i s
research through a grant,
and t ~ e y a l s o wish t o t h a n k Yamagata T e c h n o p o l i s
F o u n d a t i o n f o r accommodating them w i t h t h e ESR s p e c t r o m e t e r .
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