GICs: Comparison of intercalated HOPG and carbon fibers

Synthetic Metals. 34 (1989) 549 555

549

ESR STUDIES OF AsF5/GICs: COMPARISON OF INTERCAI2uqS~ HOPG AND CARBON FIBERS.

*

S. LUSKI , C. RETIDRI

**

+

, , D. DAVIDOV

**

*

and H. SELIG

* Institute of Chemistry, Hebrew University ** Racah Institute of Physics, Hebrew University, Jerusalem (Israel)

Abstrac% Order-disorder phase transitions in AsF~/GICs (HOPG, Fibers) can be probed by the hysteresis in the temperature dependence of the CESR linewidth. Evidence for in-plane phase transitions at ~240K and -200K were observed for AsF~/HOPG stage I in rough agreement with X-ray and neutron scattering studies. The high temperature phase transition was also observed in fibers. Quenching to temperatures below the melting point yields time dependent CESR linewidths. This time dependence is associated with domain growth. No time dependence CESR could be observed in stage II AsF~/HOPG or in intercalated fibers.

Introduction There is much theoretical and experimental interest [1,2] in the mechani~n for domain growth and the kinetics of ordering in Graphite Intercalation Ccmpounds (GICs). It was demonstrated recently that time dependent ordering after fast quenching to temperatures below the melting temperature can be probed by using X-rays or Conduction Electron Spin Resonance (CESR) techniques. Hernandez et al. [2] have shown using synchrotron X-rays on Sb~I~-GICs, that the average domain size, L, follows an algebraic growth law, L~t** (n~0.43 + 0.05) immediately after quenching, with evidence for subsequent slower domain growth. Stein, et al. [3] and Rolla, et al. [4] have demonstrated that CESR can probe the in-plane phase transitions in AICI3 and SbCIs-GICs. Furthermore, these authors [5,6] have shown that after quenching, the time broadening of the linewidth follows an algebraic law, namely, AH(t)-AH(0)~tn, with n~3.5.

+

Lady Davis Fellow. Permanent address: Instituto de Fisica, UNICAMP, 13081, Campinas, (SP), Brazil.

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© Elsevier Sequoia/Printed in The Netherlands

550 Th/s paper reports systematic CESR studies of AsFs/GICs with both HOPG and carbon pitch based fibers. These systems have received considerable attention as h/ghly conducting synthetic metals, but no detailed studies of the dc~ain growth or even the phase transitions in fibers existed until now. Previous studies of AsFs/HOPG include the X-ray measurements of Lelaurain, et al. [7] and the neutron scattering studies of Vaknin, et al. [ 8 ] . These authors have shown that below 3CX3K, the various intercalated molecular species, originating from the AsF~ disproportionation upon graphite oxidation, experience in-plane order-disorder phase transformation. Th/s leads to different in-plane structural ordering. Also evidence for progressive appearance of a ~nall Hendricks-Teller (H.T) disorder was observed [7,8].

Exper~ntal Highly Oriented Pyrolytic Graphite (HOPG) and pitch based carbon fibers with different degrees of graphitization (AMOCO P-120, P-I~, P-75 and P-55) [9], were intercalated with AsF~. In order to remove sizing and /mpurities the fibers were heated to 458"C in air for a few minutes and to 958"C in a chlorine atmosphere for three hours. Prior to intercalation all samples were heated to 258°C under dynamic vacut~n for a few hours. Stage I compounds were prepared by exposure of HOPG chips to 8~3mmHg of AsF, for about five days and then sealed in Pyrex tubes under AsF s pressure of about 70QmmHg. Stage II sample was prepared under pressure of 130mmHg of AsF. (the AsF~ reservoir was kept at -88"C to keep constant pressure). The fibers were intercalated as was described elsewhere [9]. The intercalated P-128 fibers were stage I oompound while P-IEX9, P-75 and P-55 samples were mixed stages [stage I+II) [9]. The CESR measurements were conducted on an E-line X-Band Varian spectrometer using a TE,02 rectangular cavity which allows modulation of the magnetic field at 1,18 and i~9 KHz. Temperatures in the range ICXg-3~3K were controlled by a nitrogen flux system. The stability of the cooling system was about 8.5K. The rates of slow cooling/heating cycles were about 1 K/minute and for the temperature quenching higher than 2~3K/minute.

Results and Analysis

Figure la shows the temperature dependence of the CESR linewidth for stage I GIC. Very narrow resonances are observed at room temperature, consistent with a motional narrowing and 2D liquid-like phase of the intercalated layers [ 7 ] . For slow cooling and beating rates, the stage I sample clearly shows hysteresis at 240K and between 158K and 26X~K (Fig. la). The CESR linewidth observed at low temperatures upon slow cooling is significantly broader than that at room temperature. Upon fast temperature quenching [from room temperature) a narrow resonance line is observed at T
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The observed "hystereses" were found at approximately the same temperatures where in-plane phase transitions were seen by X-ray diffraction [7] and neutron scattering [8]. These authors [7,8] associated the phase transitions to different intercalated molecular species namely, AsF~ and AsFs, for the high and lOW temperature transition, respectively. Therefore, we associate these hystereses with such in-plane phase transitions. For further understanding of the observed time dependance of the line broadening for the stage I sample, we quenched the sample from room temperature to mlIOK. After that, the temperature was raised quickly to ~I6OK, and the resonance was monitored as a function of time. The time dependence of the linewidth is illustrated in Fig.2.

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The best fit of the data is given by in [~H(t)-~H(@)] = n i n t (I) for two time intervals, with n~0.52+0.05 for 010 min. Here ~H(0) is the initial "residual" ~inewidth at T=I60K and, t, the t/me. It is evident from eq. (1) that the linewidth reveals a scaling behavior, suggesting that the time evolution of the linewidth may be a consequence of domain growth [i]. Similar results have been obtained for AICI3 [5] and SbCI~ [6] -GICs. Also, Monte Carlo simulations [i] of the kinetics of the domain growth in 2D degenerate systems, quenched from T > T to T~0.5T , show a lattice independent power law. The exponents fall gradually from t~e classic value n=0.5 (Ising limit) to a constant value of n=0.41 for large degeneracies (>30). Therefore, we believe that in AsF~-GICs, as well, the time dependent linewidth is caused by the time growing of in-plane microd3mains which were frozen after the process of quenching frcm the disordered phase. The smaller exponent observed after t=10 min is probably associated to pinning and/or interaction between dcmains. At high temperatures the intercalant layers are in the "liquid" phase, the domains ere small enough and the conduction electrons (confined to the graphite) "see" an average homogeneous local field. This leads to a narrow line. At low temperatures the "equilibrium" linewidth (i.e. the CESR linewidth observed upon slow cooling) is significantly broader than that at room temperature. This broadening at low temperatures can be attributed to two factors a) inhomogeneous distribution of g values associated with i n h c ~ e n e ous distribution of dcmain sizes (such g value distributions are expected in the absence of motional narrowing)and b) relaxation effects due to conduction electrons' spin flip and the Elliot mechanism [i0]. Time resolved ESR studies [ii] suggest no strong dependence of T2* on temperature. Also resistivity studies of HOPG/AsF~ indicate that the conduction electrons' momentum relaxation rate is smaller at low temperatures [12]. Thus, we tend to believe that the CESR linewidth at low temperatures is due to inhomogeneous broadening. This is, however, in some conflict with the CESR data of Saint Jean [13], who found that the CESR linewidth of AsF~/HOPG is frequency independent, suggesting hcmogeneous broadening. After fast thermal quenching at low temperatures, the "liquid" state is frozen and a narrow line is still observed. However, domain growth and especially the evolution of inhomogeneous distribution of domain sizes leads to the time evolution of the linewidth (Fig.2).

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Fig. ib exhibits the teni0erature dependence of the CESR linewidth which we found to be reversible (without any hysteresis) and independent of the cooling rate. Thus, even very fast quenching does not lead to a narrow line at low temperatures and no time dependencies could be observed. We did find, however, that the thermal broadening shows significant changes in its slope at T ~260K and T ~I50K (Fig. ib). This is taken as evidence for phase transformations at these temperatures in rough agreement with the X-ray and neutron scattering studies [7,8]. Within the accuracy of the measurements no anomalies were observed either in the resonance lineshape or intensity as a function of temperature. Fast domain growth associated with smaller in-plane density and smaller domains as well as the absence of pinning centers are probably the reason for the absence of time dependencies here. We note that at all t~peratures, the linewidths for stage II sample are narrower than that of stage I, suggesting fewer defects and impurities.

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Figure 3 Temperature dependence of the CESR linewidth for the AsF~ intercalated air heated fibers. Figure 4 Temperature dependence of the CESR linewidth for AsF~ intercalated Cl 2 heat treated fibers.

AsF,/Carbon pitch based fibers Figures 3 and 4 show the temperature dependence of the CESR linewidth for the intercalated air and C12 treated fibers, respectively. At room temperature for the two sets of fibers, the linewidth increases as the fiber disorder increases, consistent with shorter conduction electron mean free path [9]. For the ordered fibers (P-120 and P-100) the Cl~ treated fibers show narrower resonances than the air treated fibers. This may suggest that impurities present in the pristine fibers cause CESR relaxation end broadening which is absent in the C12 treated fibers where impurities are partially removed. No temperature dependence of the g-value and intensity of the resonance was detected, oonsistent with Pauli-like susceptibility. In both sets of fibers a broadening of the linewidth is observed at about the same temperature at which a phase transition appears in the HOPG based samples. This indicates that in-plane phase transitions also occur in the intercalated fibers. In both sets of fibers (namely, chlorinated and air treated before

554 intercalation) no narrowing of the resonance was observed upon quenching, and consequently, no time dependence of the linewidth could be detected. The major differences between both sets of fibers is the absence of hysteresis in chlorinated fibers and its presence in the air treated fibers. Exceptional case are air treated P 55 fibers which exhibit very small domains [9] and consequently no hysteresis is expected. The presence of impurities in the air-treated fibers suggests that the hysteresis in the linewidth may be associated with domain pinning by impurities. Discussion and Conclusions This work presents a systematic study of the phase transitions in AsFs/GICs using different types of host graphite materials and for d i f f e r AsF 5 densities. Time dependence of the CESR linewidth after fast quenching to low temperatures was observed for HOPG/AsFs (stage I). Ass~ning that the low temperature linewidth is due to i n ~ e n e i t i e s associated with different domain sizes, the time evolution in Fig.2 gives the time dependence of the domain size distribution. This is different from X-rays [2] which measure the time growth of the average domain size. The observation of hysteresis and time dependencies in the CESR linewidth of stage I HOPG/AsF s is attributed to domain pinning and high intercalants densities. It should be noted that previous X-ray and neutron scattering measurements indicate a progressive appearance of a ~nall amount of stage II at low temperatures, (i.e., H.T disorder) for stage I samples end for slow cooling rates. Upon heating, hysteresis was observed and the pure stage at room temperature appears only twelve hours after the complete t e ~ r a t u r e cycle [7]. Such a behavior was not observed in our experiment, i.e., the CESR linewidth at room temperature is independent of time or temperature cycling. This suggests that H.T effects are not responsible for the time dependencies in Fig. i. The absence of hysteresis in stage II AsFs/HOPG may suggest faster domain evolution due to much fewer pinning centers and probably smaller domains. Evidence that impurities and defects play the role of pinning centers is provided by our results on the fibers. No hysteresis in the CESR linewidth was observed for the C12 treated fibers (which exhibit fewer impurities) but hysteresis was observed in the air treated fibers. In conclusion, we have shown that CESR is a sensitive tool for detecting the presence of in-plane phase transitions in AsF~ intercalated HOPG and carbon fibers with different degrees of order. We have shown that the kinetics of ordering can be studied using the time dependence of the resonance linewidths. The time dependency is interpreted by the time growing of an inhomogeneous distribution of domain sizes. References i) 2) 3) 4) 5) 6) 7) 8)

P.S. Sahni, D.J. Srolovitz, G.S. Grest, M.P. Anderson and S.A. Safran. Phys. Rev. B28, 2705 (1983) P. Hernandez, F. Lamelas, R. Clarke, P. Dimon, E.B. Sirota and S.K. Sinha. Phys. Rev. Letters, 59, 1220 (1987). R.M. Stein, L. Walmsley, G.M. Gualberto and C. Rettori, Phys. Rev. B22, 4774 (1985). S. Rolla, L. Walmsley, H. Suematsu, I. Torriani, Y. Yosida and C. Rettori. Solid State Comm. 58, 333 (1986). R. Stein, L. Walmsley, S. Rolla and C. Rettori. Phys. Rev. B33, 6524 (1986 ). S. Rolla, L. Walmsley, H. Suematsu, C. Rettori and Y. Yosida. Phys. Rev. B36, 2893 (1987). M. Lelaurain, J.F. Mar~ch4, E. McRae, G. Furdin and A. H4rold, J. Mater. Res. 3 (i), 87 (1988). D. Vaknin, J.E. Fischer, D. Djurado, J. Ma and J.W. Milliken, Extended Abstract MRS- Graphite In%mrcalaticn C c ~ , p.301, Boston 1988. D. Vaknin and J.E. Fischer, Synthetic Mel~is 23, 181, (1988).

555

9) I0) ii) 12)

13)

S. Luski, H. Selig, I. Ohana, C. Rettori and D. Davidov, 251 (1989). R.J. Elliot. Phys. Rev. 96, 266 (1954). D. Davidov, A. Grupp, H. Kass and P. Hoffer. Synth. (1988). E. McRae, J.F. Mareche, M. L e l a u r a i n , G. Furdin and H. C~em. Solid, 48, 957 (1987). M. Saint Jean. Ph.D. Thesis, unpublished. Universite' Paris, France.

Synth. Metals 31, Metals 23,

291

Herold. J. Phys.

de Paris, 1989,