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ESR study under hydrostatic pressure on (BEDT)2SbF6 and AsF6 : Disapperance of the metal-insulator phase transition
Physica 143B (1986) 547-549 North-Holland,Amsterdam
547
ESR STUDYUNDERHYDROSTATIC PRESSUREON (BEDT)2SbF6 AND AsF6 : DISAPPEARANCE OF THE METAL-INSULATOR PHASE TRANSITION G. SEKRETARCZYK, M. KRUPSKI, A. GRAJA, P. DELHAES* and R. LAVERSANNE* Institute of Molecular Physics of the Solid Academy of Sciences, Smoluchowskiego 60.179 POZNAN POLAND *Centre de Recherche Paul Pascal, CNRS, Domaine Universitaire de Bordeaux I , F-33405 TALENCE CEDEX FRANCE (BEDT)2Sb~ and As~ are known to present a sharp metal-insulator phase transition close to room temperatuffe. ESR studies under hydrostatic pressures up to 5kbar presented experimental evidence of the disappearance of this phase transition for pressure larger than 5 kbar. 1. INTRODUCTION
The temperature dependences of the linewidth
The large interest devoted to salts of bis-
AH and of the spin susceptibility Xs of (BEDT)2
ethylenedithiotetrathiafulvalene (BEDT) is due to
AsF6 are presented in Fig.l and 2 respectively.
the rich variety of metallic and superconducting
Similar results have been obtained on the SbF6
properties of this series : B(BEDT)213 is the
salt.
f i r s t S-based organic superconductor at ambient pressure1. We have synthesized and described two salts
The phase transition observed at ambient pressure on the electrical conductivity i s c l e a F ly evidenced by a sharp decrease of both AH and
of BEDT : (BEDT)2SbF6 and AsF6 which undergo a
×s" This effect is weakened and the transition
sharp metal-insulator phase transition at 273
temperature Tc s l i g h t l y lowered with increasing pressure up to 4 kbar. At P= 5 kbar only a small
and 264K respectively2. Moreover the highest electrical conductivity has been found along the b axis where the molecules lay side by side. I t ismoteworthythat KOBAYASHI et al 5 found similar results on B(BEDT)2PF6 (Tc=293K). Besides a (a, 2b,c) superstructure has been evidenced below
anomaly of Xs is observed at much lower Tc, AH decreasing monotonously down to 150 K. The gap in the magnetic excitations deduced from Xs(T) at low temperature is almost pressure independent (Ap ~ 700-800 K).
the phase transition for both the AsF63 and the SbF64 salts. In this communication we investigate the influence of the pressure on this phase transition,
3. COMMENTSAND DISCUSSION 3.1. The high-T conducting phase The room temperature pressure dependences of
using ESR experiments.
both the normalized spin susceptibility and the
2. EXPERIMENTS We have performed ESR experiments on single
slight increase of AH (50 %) with P, but the spin susceptibility remains constant within the
crystals between 0 and 5 kbar in the temperature range 80- 320K, using an experimental set-up al-
accuracy of the experiments. These results are in contrast with those usually found for ld ma-
ready described6, with the magnetic f i e l d orien-
t e r i a l s like (TMTTF)2BF47 or TTF and TMTTF-TCNQ~
ted along the direction of the intermediate
For the l a t t e r compounds Xs is a decreasing func. tion of pressure while the AH increase in the
linewidth have been examined. We observed a
eigen-value of the g-tensor. 0378 - 4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Pubhshing Division) and Yamada Science Foundation
G. Sekretarczyk et al. / ESR study under hydrostatic pressure
548
P : 0 kbar
P : 0 k l~r
P : 1kbar
P = Ikbar
2OO ,qi
20
I. ~
I" f. I 1
'-r
lO •.
"...••• ........
J
l I -I ..'I
•'1 I
•••
.
.
.
P : 4 kl~" C"
I .'l • •" I
.... r ~ ,
, [Tc,
P:5k~r
P- 4kbar
liIq
.
I .•,'.. ;
-."
100
2O0 "2
P : 5 kl~r /
v
100
o•
20
•."
,i
'~1o
I •"
F
.•'I
~...- ,.••"i .•." I
J I
1-
l.• .•-
100
F"
!Tc
200 T (K)300
100
200 T lj'K'300
"I •"•"" •" I
100
~
T (K1300
FIGURE 2 Temperature dependences of the spin s u s c e p t i b i l i t y (X~) under d i f f e r e n t h y d r o s t a t i c pressures f o r a (BEDT~As~ s i n g l e c r y s t a l .
FIGURE 1 Temperature dependences of the E.S.R. l i n e w i d t h AH (Gauss) under d i f f e r e n t h y d r o s t a t i c pressures f o r a (BEDT)2As ~ s i n g l e c r y s t a l • same pressure range is l a r g e r than I00%.
The
d i f f e r e n c e between these behaviors migth be e i t h e r of s t r u c t u r a l or of e l e c t r o n i c o r i g i n . I f we note t h a t the c o m p r e s s i b i l i t y f a c t o r s
p l y and no more anomaly is observed f o r the l i n e w i d t h . In t h i s pressure range the phase t r a n s i t i o n could be second order• By extrapol a t i o n f o r P > 6 kbar, the phase t r a n s i t i o n no
determined on B(BEDT)2139 are about the same
longer occurs, and a low-T conducting regime
order of magnitude as in TTF-TCNQ and r e l a t e d
could be s t a b i l i z e d .
compounds, we can
assume
t h a t the s t r u c t u r a l
behavior under pressure is q u i t e s i m i l a r f o r a l l
3.3 Concerning the phase t r a n s i t i o n The band s t r u c t u r e of ~(BEDT)2PF6 has been
o f these molecular compounds. This assumption
c a l c u l a t e d by MORI et al I0. They show t h a t the
suggests t h a t the e l e c t r o n i c band s t r u c t u r e is
Fermi surface is opened and t h a t t h i s compound
less P dependent in BEDT s a l t s (no apparent
can be considered as Id e l e c t r o n i c system. Accor.
change of s t a t e d e n s i t y at the Fermi l e v e l ) than
ding to these authors, the upper e l e c t r o n i c band
in the TTF or TMTTF series.
is h a l f - f i l l e d
3.2 The (P,T) phase diagram
and the observed phase t r a n s i t i o n
is associated with the condensation of a 2k FCDW.
Figure 3 shows the phase diagram deduced from
More r e c e n t l y , the energy band c a l c u l a t i o n of
t h e a n a l y s i s of the experimental data. For both
(BEDT)2SbF611 has lead to very s i m i l a r conclu-
compounds, we observe two d i f f e r e n t regimes :
sion, in agreement with the observed superstruc-
- For P < 4 kbar, Tc is almost constant and the phase t r a n s i t i o n should be f i r s t
order : a
ture. In a d d i t i o n , the state d e n s i t y at the Fermi l e v e l has been evaluated and the comparison bet-
gap is present f o r both magnetic and e l e c t r o n i c
ween the Pauli paramagnetism and the observed
excitations.
spin paramagnetism at zero Kelvin 12 furnishes an
- For P > 4 kbar, Tc is decreasing very shar-
estimated enhancement f a c t o r of about 2.
G. Sekretarczyk et al. / ESR study under hydrostatic pressure
bed these compounds. Finally, we have shown that
300~Tc(K)
the metal-insulator phase transition observed in
Conductor (a.b.c)
(BEDT)2MF6 can be suppressed on application of a hydrostatic pressure larger than 6 kbar.
4.._____t
REFERENCES 1. E.B. Yagubskii, I.F. Schegolev, V.N. Laukin, P.A. Kononovich, H.V. Kartsovnik, A.V. Ezvarykina and L.I. Buravov, JETP Letters 39, 12 (1984).
200 i I
Insulator
2. R. Laversanne, J. Amiell, P. Delhaes, D. Chasseau and C. Hauw, Solid State Commun 5__22, 177 (1984).
l
(a.2b,c) I I I
100
3. P.C.W. Leung, M.A. Beng, G.S. Blackman, B.R. Coughlin, C.A. Miderski, W. Joss, G.W. Crabtree and J.M. Williams, Acta Cryst. C 4__0_0, 1331 (1984).
(BEDT)2 AsFB L +
(BEDT)2 SbFB
i I I I t
0
i
0
1
549
!
I
P (kbar)
FIGURE 3 (P,T) phase diagram for the two BEDT salts.
The p o s s i b i l i t y of a Peierls transition cannot be excluded even i f the transition temperature is unusual. Nevertheless preliminary X-ray investigations at low temperature4 demonstrate the occurence of strongly superstructure spots inconsistent with a classical Peierlsdistortion. 4. CONCLUSION In the ~ and B(BEDT)213 salts, the transverse and longitudinal electronic interactions are equilibrated and a quasi 2d behavior with a closed Fermi surface is observed. In the MF6 salts the transverse overlaps overbalanced the longitudinal ones, giving rise to a quasi ld behavior in the transverse direction. This fact shows the delicate balance of the molecular arrangement and suggests that the size and the symmetry of the counter-ion have to be accounted to descri-
4. J. Gaultier, private communication on p r e l i minary results about the SbF6 salt. 5. H. Kobayashi, T. Mori, R. Kato, A. Kobayashi, Y. Sasaki, G. Saito and H. Inokuchi, Chem. Letters 581, (1983). 6. J. Stankoswki, A. Galezewski, M. Krupski, K. Waplak, H. Gierszac, Rev. Sc. Instr. 4_~7, 128 (1976). 7. L. Forro, G. Sekretarczyk, M. Krupski, K. Kamaras, Mol. Cryst. Liq. Cryst. 12___00,89 (1980). 8. C. Berthier, J.R. Cooper, D. Jerome, G. Soda, C. Weyl, Mol. Cryst. Liq. Cryst. 3__22,267 (1976). 9. H. Tanino, K. Kato, M. Tokumoto, H. Anzai, G. Saito, J. of Phys. Soc. of Japan 5_44, 2390 (1985). 10 T. Mori, A. Kobayashi, Y. Sasaki, R. Kato, H. Kobayashi, Solid State Commun 53(8) p. 267 (1985). 11A. Ducasse, A. Abderraba (private communication). 12 An ESR absolute determination on a (BEDT)2 . SbF6 single crystal has given : Xs = 7.5104 emuCGS/mole at room temperature ; using the T and P dependences (Figure 2), we have estimated Xs(O.K) : 4-510 -4 emuCGS/mole.
547
ESR STUDYUNDERHYDROSTATIC PRESSUREON (BEDT)2SbF6 AND AsF6 : DISAPPEARANCE OF THE METAL-INSULATOR PHASE TRANSITION G. SEKRETARCZYK, M. KRUPSKI, A. GRAJA, P. DELHAES* and R. LAVERSANNE* Institute of Molecular Physics of the Solid Academy of Sciences, Smoluchowskiego 60.179 POZNAN POLAND *Centre de Recherche Paul Pascal, CNRS, Domaine Universitaire de Bordeaux I , F-33405 TALENCE CEDEX FRANCE (BEDT)2Sb~ and As~ are known to present a sharp metal-insulator phase transition close to room temperatuffe. ESR studies under hydrostatic pressures up to 5kbar presented experimental evidence of the disappearance of this phase transition for pressure larger than 5 kbar. 1. INTRODUCTION
The temperature dependences of the linewidth
The large interest devoted to salts of bis-
AH and of the spin susceptibility Xs of (BEDT)2
ethylenedithiotetrathiafulvalene (BEDT) is due to
AsF6 are presented in Fig.l and 2 respectively.
the rich variety of metallic and superconducting
Similar results have been obtained on the SbF6
properties of this series : B(BEDT)213 is the
salt.
f i r s t S-based organic superconductor at ambient pressure1. We have synthesized and described two salts
The phase transition observed at ambient pressure on the electrical conductivity i s c l e a F ly evidenced by a sharp decrease of both AH and
of BEDT : (BEDT)2SbF6 and AsF6 which undergo a
×s" This effect is weakened and the transition
sharp metal-insulator phase transition at 273
temperature Tc s l i g h t l y lowered with increasing pressure up to 4 kbar. At P= 5 kbar only a small
and 264K respectively2. Moreover the highest electrical conductivity has been found along the b axis where the molecules lay side by side. I t ismoteworthythat KOBAYASHI et al 5 found similar results on B(BEDT)2PF6 (Tc=293K). Besides a (a, 2b,c) superstructure has been evidenced below
anomaly of Xs is observed at much lower Tc, AH decreasing monotonously down to 150 K. The gap in the magnetic excitations deduced from Xs(T) at low temperature is almost pressure independent (Ap ~ 700-800 K).
the phase transition for both the AsF63 and the SbF64 salts. In this communication we investigate the influence of the pressure on this phase transition,
3. COMMENTSAND DISCUSSION 3.1. The high-T conducting phase The room temperature pressure dependences of
using ESR experiments.
both the normalized spin susceptibility and the
2. EXPERIMENTS We have performed ESR experiments on single
slight increase of AH (50 %) with P, but the spin susceptibility remains constant within the
crystals between 0 and 5 kbar in the temperature range 80- 320K, using an experimental set-up al-
accuracy of the experiments. These results are in contrast with those usually found for ld ma-
ready described6, with the magnetic f i e l d orien-
t e r i a l s like (TMTTF)2BF47 or TTF and TMTTF-TCNQ~
ted along the direction of the intermediate
For the l a t t e r compounds Xs is a decreasing func. tion of pressure while the AH increase in the
linewidth have been examined. We observed a
eigen-value of the g-tensor. 0378 - 4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Pubhshing Division) and Yamada Science Foundation
G. Sekretarczyk et al. / ESR study under hydrostatic pressure
548
P : 0 kbar
P : 0 k l~r
P : 1kbar
P = Ikbar
2OO ,qi
20
I. ~
I" f. I 1
'-r
lO •.
"...••• ........
J
l I -I ..'I
•'1 I
•••
.
.
.
P : 4 kl~" C"
I .'l • •" I
.... r ~ ,
, [Tc,
P:5k~r
P- 4kbar
liIq
.
I .•,'.. ;
-."
100
2O0 "2
P : 5 kl~r /
v
100
o•
20
•."
,i
'~1o
I •"
F
.•'I
~...- ,.••"i .•." I
J I
1-
l.• .•-
100
F"
!Tc
200 T (K)300
100
200 T lj'K'300
"I •"•"" •" I
100
~
T (K1300
FIGURE 2 Temperature dependences of the spin s u s c e p t i b i l i t y (X~) under d i f f e r e n t h y d r o s t a t i c pressures f o r a (BEDT~As~ s i n g l e c r y s t a l .
FIGURE 1 Temperature dependences of the E.S.R. l i n e w i d t h AH (Gauss) under d i f f e r e n t h y d r o s t a t i c pressures f o r a (BEDT)2As ~ s i n g l e c r y s t a l • same pressure range is l a r g e r than I00%.
The
d i f f e r e n c e between these behaviors migth be e i t h e r of s t r u c t u r a l or of e l e c t r o n i c o r i g i n . I f we note t h a t the c o m p r e s s i b i l i t y f a c t o r s
p l y and no more anomaly is observed f o r the l i n e w i d t h . In t h i s pressure range the phase t r a n s i t i o n could be second order• By extrapol a t i o n f o r P > 6 kbar, the phase t r a n s i t i o n no
determined on B(BEDT)2139 are about the same
longer occurs, and a low-T conducting regime
order of magnitude as in TTF-TCNQ and r e l a t e d
could be s t a b i l i z e d .
compounds, we can
assume
t h a t the s t r u c t u r a l
behavior under pressure is q u i t e s i m i l a r f o r a l l
3.3 Concerning the phase t r a n s i t i o n The band s t r u c t u r e of ~(BEDT)2PF6 has been
o f these molecular compounds. This assumption
c a l c u l a t e d by MORI et al I0. They show t h a t the
suggests t h a t the e l e c t r o n i c band s t r u c t u r e is
Fermi surface is opened and t h a t t h i s compound
less P dependent in BEDT s a l t s (no apparent
can be considered as Id e l e c t r o n i c system. Accor.
change of s t a t e d e n s i t y at the Fermi l e v e l ) than
ding to these authors, the upper e l e c t r o n i c band
in the TTF or TMTTF series.
is h a l f - f i l l e d
3.2 The (P,T) phase diagram
and the observed phase t r a n s i t i o n
is associated with the condensation of a 2k FCDW.
Figure 3 shows the phase diagram deduced from
More r e c e n t l y , the energy band c a l c u l a t i o n of
t h e a n a l y s i s of the experimental data. For both
(BEDT)2SbF611 has lead to very s i m i l a r conclu-
compounds, we observe two d i f f e r e n t regimes :
sion, in agreement with the observed superstruc-
- For P < 4 kbar, Tc is almost constant and the phase t r a n s i t i o n should be f i r s t
order : a
ture. In a d d i t i o n , the state d e n s i t y at the Fermi l e v e l has been evaluated and the comparison bet-
gap is present f o r both magnetic and e l e c t r o n i c
ween the Pauli paramagnetism and the observed
excitations.
spin paramagnetism at zero Kelvin 12 furnishes an
- For P > 4 kbar, Tc is decreasing very shar-
estimated enhancement f a c t o r of about 2.
G. Sekretarczyk et al. / ESR study under hydrostatic pressure
bed these compounds. Finally, we have shown that
300~Tc(K)
the metal-insulator phase transition observed in
Conductor (a.b.c)
(BEDT)2MF6 can be suppressed on application of a hydrostatic pressure larger than 6 kbar.
4.._____t
REFERENCES 1. E.B. Yagubskii, I.F. Schegolev, V.N. Laukin, P.A. Kononovich, H.V. Kartsovnik, A.V. Ezvarykina and L.I. Buravov, JETP Letters 39, 12 (1984).
200 i I
Insulator
2. R. Laversanne, J. Amiell, P. Delhaes, D. Chasseau and C. Hauw, Solid State Commun 5__22, 177 (1984).
l
(a.2b,c) I I I
100
3. P.C.W. Leung, M.A. Beng, G.S. Blackman, B.R. Coughlin, C.A. Miderski, W. Joss, G.W. Crabtree and J.M. Williams, Acta Cryst. C 4__0_0, 1331 (1984).
(BEDT)2 AsFB L +
(BEDT)2 SbFB
i I I I t
0
i
0
1
549
!
I
P (kbar)
FIGURE 3 (P,T) phase diagram for the two BEDT salts.
The p o s s i b i l i t y of a Peierls transition cannot be excluded even i f the transition temperature is unusual. Nevertheless preliminary X-ray investigations at low temperature4 demonstrate the occurence of strongly superstructure spots inconsistent with a classical Peierlsdistortion. 4. CONCLUSION In the ~ and B(BEDT)213 salts, the transverse and longitudinal electronic interactions are equilibrated and a quasi 2d behavior with a closed Fermi surface is observed. In the MF6 salts the transverse overlaps overbalanced the longitudinal ones, giving rise to a quasi ld behavior in the transverse direction. This fact shows the delicate balance of the molecular arrangement and suggests that the size and the symmetry of the counter-ion have to be accounted to descri-
4. J. Gaultier, private communication on p r e l i minary results about the SbF6 salt. 5. H. Kobayashi, T. Mori, R. Kato, A. Kobayashi, Y. Sasaki, G. Saito and H. Inokuchi, Chem. Letters 581, (1983). 6. J. Stankoswki, A. Galezewski, M. Krupski, K. Waplak, H. Gierszac, Rev. Sc. Instr. 4_~7, 128 (1976). 7. L. Forro, G. Sekretarczyk, M. Krupski, K. Kamaras, Mol. Cryst. Liq. Cryst. 12___00,89 (1980). 8. C. Berthier, J.R. Cooper, D. Jerome, G. Soda, C. Weyl, Mol. Cryst. Liq. Cryst. 3__22,267 (1976). 9. H. Tanino, K. Kato, M. Tokumoto, H. Anzai, G. Saito, J. of Phys. Soc. of Japan 5_44, 2390 (1985). 10 T. Mori, A. Kobayashi, Y. Sasaki, R. Kato, H. Kobayashi, Solid State Commun 53(8) p. 267 (1985). 11A. Ducasse, A. Abderraba (private communication). 12 An ESR absolute determination on a (BEDT)2 . SbF6 single crystal has given : Xs = 7.5104 emuCGS/mole at room temperature ; using the T and P dependences (Figure 2), we have estimated Xs(O.K) : 4-510 -4 emuCGS/mole.