nanosegregation in na 2 c 60 g. klupp, p. matus, d. quintavalle*, l. f. kiss, É. kováts, n. m....
TRANSCRIPT
Nanosegregation in Na2C60
G. Klupp, P. Matus, D. Quintavalle*, L. F. Kiss, É. Kováts, N. M. Nemes+, K. Kamarás, S. Pekker, A. Jánossy*Research Institute for Solid State Physics and Optics, P. O. Box 49, H-1525 Budapest, Hungary, email: [email protected]
*Department of Experimental Physics, Budapest University of Technology and Economics, Budapest, Hungary+NIST Center for Neutron Research, Gaithersburg, MD, USA; Department of Materials Science & Engineering, University of Maryland, College Park, MD, USA
FundingOTKA T 034198, T 049338, T 046700
Preparation
350 oC 23 d, 450 oC 7 d4 regrinds
2 Na + C60 Na2C60
Sample preparation and measurements were done in inert atmosphere
Motivation
Study of Mott – Jahn – Teller insulating state
C604- C60
2-
K2C60, Rb2C60, Cs2C60 do not exist
X-ray diffraction
10 20 30 40 50 600
10000
20000
30000
40000
50000
60000
Inte
nsi
ty (
cou
nts
)
2 (deg)
Single phase
simple cubic
Pa3
a = 14.19 Å
The same as in [1]
Solubility
C60 could be extracted with toluene from Na2C60 in 11 days
Concentration of the obtained C60 solution was measured with HPLC
26-33% of the sample is neutral C60
0
20
40
60
80
100
50 100 150 200 250 300
0
20
40
60
80
100
Inte
nsi
ty (
arb
. un
its)
293 K repetition time:
1 s
Frequency (ppm)
293 Krepetition time:
250 s
25 % C60
13C-NMR
Peak with fast T1: the same as in [4], due to the metallic phase
Peak with slow T1: 25 ± 5 % C60 [5]
Infrared spectroscopy
Room T: C60+ C60 [2], the latter in a metallic phase
High T: C60 distorted by molecular Jahn – Teller effect [3]
Reversible change
C60 present at room T is not from off-stoichiometry
Retransformation is complete after ~2 weeks
slow Na+ diffusion even at room T
3-
2-
470 K
A
C60, D3d/D5d2-
C60C603-
References[1] T. Yildirim et. al., Phys Rev Lett. 71, 1383 (1993)[2] T. Pichler et. al., Phys. Rev. B 49, 15879 (1994)[3] K. Kamarás et. al., Phys. Rev. B 65, 052103 (2002)[4] V. Brouet et. al., Phys Rev B 66, 155122 (2002)[5] R. Tycko et. al., Phys. Rev. Lett. 67, 1886 (1991)[6] A. Meyer et. al., Rev. Sci. Instrum. 74, 2759 (2003)[7] T. Becker et. al., Phys. Rev E 67, 021904 (2003) [8] G. Faigel et. al., Phys. Rev. B 52, 3199 (1995)[9] R. W. Schurko et. al., J. Solid State Chem. 177, 2255 (2004)
Conclusions
~3-10 nm
: C60
: Na
Nanosegregation
Room T:
XRD: Na2C60
IR, NMR, ESR, HPLC: ~ 70 % Na3C60 + ~ 30 % C60
Structure is similar to that of the intermediate phase of KC60 [8]
Main phases in Na2C60: insulating C60 + metallic Na3C60
Size range of structural coherence: upper limit: XRD, lower: metallicity:
Segregation was also seen in nominally Na3C60 samples [9]
On heating: jump-diffusion of Na+, segregation disappears
At high T: homogenous Na2C60
C60 is D3d/D5d distorted because of the molecular JTE
2-
ESR
Static is the same as in [4]
200-300K: 4 phases: 3 metallic + 1 insulator
Curie paramagnet: ~ 1 % C60 embedded in C60, separated from metallic phases
amount of C60 is nonnegligible
-
0 100 200 300
0.0
2.0x10-7
4.0x10-7
6.0x10-7
8.0x10-7
1.0x10-6
1.2x10-6
Sta
tic s
usc
ep
tibili
ty (
em
u/g
Oe
)
Temperature (K)
Phase1C Phase2C Phase3 Phase4
insulator metals
ESR intensity was normalized to static measured by SQUID
8.11 8.12 8.13
345 350 355 360
Magnetic Field (T)
High Field ESR225 K
X-Band ESRRoom T
Magnetic Field (mT)
Neutron scattering
400 K: Na+ jump diffusion becomes faster than 1 ns
The different Ttrans in IR is due to the different timescales
Q = 1 Å-1 jump distance is 4.5 Å
jump between T and off-centered O site
Measurements were done on the High Flux Backscattering Spectrometer of NIST [6,7]
Na+ has the same nearest neighbour distances
0.033
0.034
0.035
0.036
0.037
0.038
0.039
Na2C
60 @ Q=1A-1
300 400 500 600
0.010
0.011
Empty sample holder
background @ Q=1 A-1
Inte
nsity
(a.
u.)
T (K)
0 1 20.0
0.2
0.4
Inte
nsity
(a.
u.)
Q (A-1)