The transition from the ordered to the merohedral disordered phase in oxygenated solid C60

9 December 1996

PHYSICS

LETTERS

A

ws Physics Letters A 223 (1996) 273-279

EISEVIER

The transition from the ordered to the merohedral disordered phase in oxygenated solid c6() Min Gu a, Yening Wang a, Tong B. Tang b, Weiyi Zhang a, Chen Hu ‘, Feng Yan a, Duan Feng” a National Lnborotory of Solid State Microstructures and Institute of Solid State Physics, Nanjing University, Nanjing 210093, China h Physics Deportment, Hong Kong Baptist Universitj! Kowloon Tong, Kowloon. Hong Kong

Received 19 June 1996; revised manuscript

received 19 September 1996; accepted for publication 25 September 1996 Communicated by J. Flouquet

Abstract Cflr powder, pellets and single crystals were oxygenated at 0.11 kbar and then studied with MAS ‘sC NMR, DSC and XRD. In agreement with reports by other authors, powder samples exhibited minor NMR resonances, indicative of blocking of fullerene rotation by interstitial oxygen, the distribution of which had yet to attain the equilibrium state. Also, as reported by others, DSC showed two endothermic peaks near 260 K, and both, we found, corresponded to a discontinuous change in the lattice parameter. The peak on the low-temperature side moved up in temperature with elapsed time. Pellets, in which presumably oxygen diffused easily, displayed only one peak but shifted in the same manner. Single crystals, in contrast, had only the higher-temperature “immobile” peak. We suggest that the shifting endotherm originates from the transition of oxygen-intercalated Ca to a merohedral disordered phase, in which the rotation of fullerene molecules is partially impeded by oxygen. A phenomenological model is proposed for this transition. PACS: 61.48.+c;

64.60.Cn; 64.75.+g

1. Introduction Fullerite accommodates a rotational disorder of Cm molecules and adopts a face-centered cubic (fee) lattice having the space group Fm3m, in which the rotations of C6o molecules are hindered due the inlluence of the cubic crystal field [ 11. But below T, = 260 K, the structure changes to simple-cubic (SC) with Pa3 symmetry when the rotations lock into specific orientations. This order-disorder transition is of first order, being associated with an enthalpy of transition and a discontinuity in lattice parameter [ 2,3]. Furthermore, it is significantly affected by the dissolution of gas or solvent molecules, to which the nearly ideal “plastic”

C60 crystal, with a minimum octahedral site spacing as large as 4.12 A at room temperature, displays high affinity [ 41. Thus, in differential scanning calorimetric measurements a two-peak structure has been observed in various cases [ 5-91. The major peak has an onset at 260 K or a somewhat lower temperature, the difference being smaller in samples of higher quality. C60 doped with 25% C7e exhibited two peaks at 255 and at 250 K, the latter attributable to C?n or other impurities [6,7]. If instead HZ was dissolved, they were at 257 K and 253 K; a negative “chemical pressure” effect, i.e. a slight dilation of the lattice, may account for the second, minor peak [ 81. The case of 02 is of special interest. From nuclear

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magnetic resonance (NMR) data, Assink et al. [ lo] have concluded that under high-pressure conditions, oxygen molecules entered into octahedral voids of the lattice until half of those sites become filled, and that this intercalation process is controlled by diffusion kinetics. The distribution of molecular oxygen resembles a superposition of two distributions, one for the region with a high and the other for the region with a low filling fraction [ lo]. The guest molecules do not distort the crystal structure, nevertheless they impede the rotation of C60 at room temperature, as revealed by NMR [ 111. Differential scanning calorimetry (DSC) locates a main transition at 258 K and reveals in addition a broad peak on the low-temperature side, whose exact position and area depend on oxygen concentration [ 91. These findings lead to the important question, whether the part of fullerite intercalated with 02 undergoes the same order-disorder transition as the undoped part. This is the question to be addressed here.

2. Materials

Letters A 223 (1996) 273-279

75 MHz, under the condition of magic-angle spinning (MAS) at the rate of 2.4 kHz and with a delay time of 30 s. The line for pure C6o under MAS was adjusted to an extremely narrow width (N 0.027 ppm = 2 Hz). No samples other than the powder form were studied, due to the difficulty with spinning rigid bulk shapes at high speed. However, some powder and pellets were set aside for in-situ XRD experiments in a variable-temperature chamber, and their lattice parameters then determined from least-squares fit to their five reflections (ill), (220), (311), (222) and (331) off Cu Ka radiation from a D/Max-rA rotating-anode diffractometer fitted with graphite monochromator. In order to calibrate the shift of the diffraction lines not due the sample, a little silicon powder was pressed onto the surface of the sample to function as the standard. Finally, DSC measurements were performed in a Rigaku 8 150 on all three types of materials. Samples of 20-25 mg in mass were examined at the common heating rate of 10 K/min in vacuum, with alumina powder serving as the reference.

and procedures

c60 powder was obtained from the sublimation of 99.9%-pure materials provided by liquid column chromatography of the fullerene mixture extracted with toluene from carbon soot. Powder X-ray diffraction (XRD) , DSC and NMR confirmed that a typical batch of the powder had a fee structure at room temperature and contained negligible chemical impurities; the experimental details can be found in the next paragraph. Its mean grain size was estimated from scanning electron micrographs. Its NMR spectra were recorded for comparison with those subsequent to oxygen treatment. Single crystals of 1.1 x 1.0 x 0.4 mm3 average size were also grown from the vapor phase using the double-temperature-gradient technique [ 121. The powder and single crystals then sat inside a dessicator for 3 months before use. In addition, uniaxial compaction of powder in air under a pressure of 15 kbar for 5 min produced rigid pellets that measured about 1 mm in thickness. Finally, prior to experiments the samples (powder, pellets or single crystals) were subjected to 0.11 kbar of oxygen for 168 h at room temperature. ‘sC NMR measurements proceeded at room temperature in a Bruker MSL-300 spectrometer equipped with a 7-T field and having a resonance frequency of

3. Results Fig. la shows the high-resolution 13C NMR spectrum of pristine powder. At 143.7 ppm a primary resonance appeared, related to freely rotating C60 molecules [ lo], but no other features were discernible over the entire shift range from 81 to 207 ppm. Powder that had sat in air for 3 months then developed a minor resonance at 144.4 ppm (Fig. lb). On the other hand, oxygen charging at 0.11 kbar for 168 h followed by storage in air over 3 days led to the introduction of six minor resonances (Fig. lc) . Fig. 2 depicts the variation with temperature of the lattice parameters derived from XRD data, for both powder and pellet within aday after removal from oxygen loading at 0.11 kbar. They represent mean values, as will be explained in Section 4. In the case of the charged powder (Fig. 2a), two jumps occurred at 236 and 255 K, the latter of which obviously accompanied the well-known order-disorder transition. In the pellet case (Fig. 2b), the first jump was larger. Figs. 3 and 5 present the DSC data taken after various times. The freshly oxygenated powder in Fig. 3a gave two rather broad endothetms with onset temperatures of 236 and N 255 K respectively, the former

M. Gu et al./ Physics Letters A 223 (1996) 273-279

275

clearly corresponding to the first jump in the lattice parameter, affirmed thus to be also a first-order phase transition since it involved enthalpy. As time went on, its position shifted to higher temperature and finally merged with the second endothermic peak, as Figs. 3b-3g indicate. The time-dependent octahedral occupancy by oxygen calculated from the resonance peaks of 13C NMR spectrum [ IO], and critical temperatures T,’ determined from its peaks are plotted in Fig. 4, where a theoretical fit is also included which will be discussed later. The DSC data for an oxygenated pellet, in Fig. 5, exhibited only a single endotherm with onset at 233 K initially, unlike the situation in powder, but it moved up in temperature with the passage of time, as T,’ did in powder. This remarkable case of a single, shifting endothermic peak has apparently not been previously reported. On the other hand, single crystals before and after intercalation each gave a single sharp endotherm, with onsets at 260.9 K and 261.1 K respectively, but which did not change with time. These thermograms are not reproduced here, for brevity.

C

4. Discussion

Fig. I. High resolution MA.5 t3C NMR spectra at room temperature: (a) of pristine Cm powder; (b) of a sample in air for 3 months; (c) after exposure to 0.1 I kbar of oxygen for 168 h followed by storage in air for 72 h.

L

“Loo 100

200 Temperature

(K)

Fig. 2. Temperature dependence of lattice parameter as determined from XRD profiles with least-squares method in (a) powder and (b) pellet.

The NMR spectra we have measured of oxygencharged powder matched those reported by Assink et al. [ lo], who attribute the minor resonances to fullerene surrounded, respectively, by one to six 02 molecules occupying octahedral sites. The sample examined by Assink et al. had been exposed to oxygen at 1.0 kbar for 1.75 h, and the fraction of occupied sites reached (or exceeded) 8%, evaluated from the ratios among the peak heights of the one major and the six minor resonances. Later on, Belahmer et al. [ 1 l] studied in greater detail the single minor resonance in a powder kept in air for four months, which obviously had a much lower fractional occupancy. They quantified the hindrance to the fullerene rotation by the 02 molecules through analysis of the 13C resonance shape, and propose that the blocking is not total and some reorientation remains permissible. Following the procedures of Assink et al. [ IO], we calculate the time-dependent octahedral site occupancies of 02 after our sample has been oxygenated at 0.11 kbar for 168 h (Fig. 4), and a octahedral site occupancy 0.7% for powder that has sat in air for 3 months. It may be

276

M. Gu et al. /Physics Letters A 223 (1996) 273-279

a

-BE

-60

-40

-20

Temperature

r”

20

‘C

40

Fig, 3. DSC heating thermogramsof CM) powder after oxygenation at 0.1 I kbar for 168 h, examined 77.5 h; (d) 97 h; (e) 193 h; (f) 33 I h; (g) annealing in dynamic vacuum for 5 h at 22O’C.

2.,040 0

100

200

300

s

Time(h)

Fig. 4. T,‘(t) : ( l) experimental data; (-_) theoretical curve; (0) octahedral occupanciesof 02 (%): the dashed line is an exponantial fit only to guide the eye.

more accurate to calculate with resonance peak areas instead of heights, in which case the corresponding figures are 24% for our sample oxygenated at 0.11 kbar for 168 h, and 2.4%. Assink et al. [lo] have argued that the observed distribution of 02 molecules is kinetically controlled, because it had yet to attain the binomial distribution that would prevail under thermodynamical equilib-

60

1 after (a) 0.5 h; (b) 24.5 h; (c)

rium condition. Qualitatively we can see at once from Fig. Ic that this situation likewise applies to our more oxygenated sample, as the fractions of fullerenes surrounded by a few 02 were not much greater than those for COOwith more, although the fifth and sixth minor resonances did decay faster than those resonance peaks ascribable to species surrounded by fewer OZ. The oxygen distribution resembles therefore a superposition of two, or more (or a continuum of) distribution. One distribution, or one extreme in the continuum of distributions, describes a part of the sample with no oxygen filling; the other, or others, the remainder with various filling ratios. Next we consider these additional features related to oxygen: a jump in the lattice parameter at 236 K (Fig. 2a), and an endotherm with onset at the same temperature (Fig. 3). With elapsed time and thus presumably the out-diffusion of 02, the endotherm shifted towards the position for oxygen-free C60. This correlation implies that both features originate from a new phase transition related with the dissolution of oxygen, from the SC structure below 236 K to a phase different from, but with the same symmetry of, the fee phase in pristine COO. We suggest that this new

M. Gu et al./Physics

-3

I -60

-60

Letters A 223 (1996) 273-279

-40

-20

Temperature

F

20

‘C

40

1

Fig. 5. DSC thermograms of a pellet that has been charged at 0.11 kbar for 168 h, after (a) 1 h; (b) 945 h; ( f) 19 1 h; (g) 330 h; (h) annealing in dynamic vacuum for 5 h at 22O’C.

high-temperature phase is characterized by merohedral disorder in which fullerene molecules blocked by oxygen are allowed to take up only two standard orientations. This model has been proposed by Stephens et al. [ 131 for K&ha based on a Rietveld refinement of the room-temperature X-ray diffraction pattern, and is supported by the deduction of Belahmer et al. [ II] that oxygen hindered molecular rotation of C60 at room temperature. Individual molecules being randomly assigned to two orientations, the lattice retains the symmetry Fmgm, identical to that when each assumes completely random orientations [ 141, so that mixed-indices reflections like (120) remained forbidden. Later on, some NMR and DTA measurements [ 151 have observed the occurrence of a lowtemperature structural modification below 200 K in KsC60. The absence of Pa3 phase in KsC60 is basically due to strong short-range repulsive interactions between tetrahedral K ions and C60 molecules [ 141. If our interpretation is correct, what happened in our powder was that, near 236 K, oxygenated C60 transformed to an fee phase with merohedral disorder, and then oxygen-free C6a transformed, above 255 K, to the fee phase with orientational disorder. In between these

18 h; (c) 23.5 h; (d) 64.5 h; (e)

two critical temperatures, double peaks should appear in XRD profiles indicating two distinct (but close) lattice parameters, as had indeed been reported by Heiney et al. [ 161 who studied fullerite powder, using X-ray of 1.17027 A selected from synchrotron radiation by a Si channel-cut monochromator. In our case, however, such a double-peak structure was not resolved, probably due to the natural width (- 0.01 A) of Cu Ka1,2 and the aperture of the scattered X-ray detector. Hence our calculated lattice parameters should really be average values for the two co-existing phases that gave rise to broadened XRD peaks. This was perhaps the same situation in the powder ground from single crystals by Li et al. [ 171, who stipulated intermediate values for XRD-derived lattice parameters around 250 K. The additional DSC endotherm corroborates with the picture of two sample regimes each with its own phase transitions. The “immobile” endotherm at N 25.5 K comes from that part of the sample to which oxygen had not diffused, or from which oxygen had already been depleted. The additional one was always displaced forwards the low-temperature direction, but the magnitude of its shift varied in proportion to the total oxygen content remaining in the sample (Fig. 4).

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Letters A 223 (1996) 273-279

The same held for pellets, Fig. 5, where its lone presence indicates that no part of the pellet was oxygenfree at any time. No doubt the mechanical compaction under 15 kbar induced volumic defects that greatly increased the diffusion length. In contrast, it was absent in single crystals, because oxygen had diffused into negligible portions in them. It may be noted in passing that the immobile endotherm in Fig. 3 has a fine structure of double peaks separated by N 3 K. Using “modulated DSC” Fisher et al. [ 181 have detected a splitting of - 0.3 K in the endotherms from powder and single crystals with low concentration of impurities, level of crystallographic defects, and exposure to air. If these two features share the same, yet uncertain, origin, our observation of a greater separation for the dual peaks, the relative intensities of which varied with time, may help in elucidating the underlying mechanism. The orientational melting transition in our single crystals possibly also consisted of two steps, but their separation in temperature, if N 0.3 K, would not be resolvable. Finally we turn to the question as to why the transition in oxygenated COO is suppressed in temperature. Oxygen molecules with a van der Waals radius of 1.4 A being much smaller than the minimum octahedral site spacing of 4.12 A, their dissolution will not distort the crystal lattice, a deduction confirmed by molecular dynamics calculations [ IO]. Indeed we did not detect any distortion in our XRD measurements within their resolution. This suppression in T, is therefore unlikely to be attributable to negative pressure effect. We consider instead the effect of oxygenation on the free energy for fullerene, which consists of its static energy and the free energy of three-dimensional rotors [ 191 at temperatures above the transition. But in the case of merohedral disorder, the latter is lowered because its degree of freedom decreases from three to one. This free energy equals the free energy in SCphase when T = T,, which is thereby reduced to Tf. Indeed some NMR measurements have shown the uniaxial rotational motion of COO molecules in K3C60 [ 15 1. Below we attempt a phenomenological explanation for the change of T,’ with time. For a thermodynamic system composed of C60 rotors, the free energy Go can be expressed in terms of the order parameter A as

+ K( dA/dx)*

.

(1)

Here p is a function of temperature and strain, K, the elastic constant and L, a crystal-field that hinders rotation, and which is modified by interstitial oxygen. Near T,, the third term SA4 is so small that it can be ignored. If the system is homogeneous, at equilibrium aGo(T,A)/dA=2pA-L=O.LettingP=b(T-T,) in the neighbourhood of T,, we get L L/2b Ao=~=~_~,.

(2)

Now we attempt to obtain the transition temperature of a non-homogeneous system with a gradient of the order parameter A. Considering a plate structure because our powder consisted of rectangular platelets, oxygen molecules under high pressure can diffuse into the plate from both planes. So L varies with x after imcomplete oxygenation, whence L = Lof(x) and A = Aof so that

GotT,A) = Go(T,O) + P(f*)A;

- Lodo

+ K(
(3)

and

A’ = T - Tc + (K/b)((df/dx)*)/(f*)

’

(4)

where angular brackets denote averages. The oxygen diffuses exponentially, therefore -d

< x 6 d.

(5)

Here AD stands for diffusion length of oxygen; P, its hindrance to rotation; and d, the sample size. For powder and pressed pellets AD > d, thus T,’ = T,

1 - Ad*

P* (1 -P)2

, >

A=&. 3bh;T,

(6)

Because oxygen occupancy is incomplete at the beginning and drops further as time goes on, as the simplest assumption P = PO - cxut,where (Y is a material constant and PO a sample parameter. Finally

T’(t) C TC

= 1

-Ad*

(I- A)-*.

(7)

M. Gu et al. /Physics

We performed a curve fit to our data in Fig. 4 with the help of the PC software SigmaPlot 1.O. Scanning electron microscopy showed that our powder consisted of rectangular platelets with a fairly uniform thickness of 20 ,um, so they approximated a one-dimensional system with d = 10 pm. By setting Tc to 255 K, we obtained A = 9.1 x 10m7 prnp2, a = 6.2~10~~ hh’ and PO = 0.962. As a rough check on the plausibility of these values, we repeated the operation with data published in the literature [ 201 for powder of 75185 ,ummesh and oxygenated at 1 kbar for 48 h. Peak instead of onset temperatures were reported, but a comparison would still be meaningful because a fit to peak temperatures for our sample yielded values within 30% of those just stated. (However, we analyzed the data in Ref. [20] up to t = 680 h only, and excluded the two remaining points for 1000 and 1248 h to avoid the probable inconsistency with the assumption of linear decrease in P.) The results were: Ad2 = 6.0~ 10e6, LY= 5.5x 10m5 h-r and PO = 0.981. We could not determine A because the other dimensions of the grains were not specified [20], but at least (Y had a value close to ours, lending support for the proposed model.

Letters A 223 (I 9961273-279 [2]

P.A. Heiney, A.M.

279

J.E. Fischer,

Denenstein,

J.P

Makhija ]4]

G. Dabbagh,

and SM.

PA. Heiney,

[ 51 G.A. [6]

W.J. Romanow.

Jr. and A.B.

Smith

111.

I I.

29

R.M.

Zahurak,

Fleming,

R.C. Haddon,

Hansen, R.A.

Assink.

A.V.

( 1991) 1886.

Phys. Rev. Len. 67

1. Phys. Chem. Solids S3 (1992)

Samara, L.V.

Schirber

McGhie,

1333.

B. Morosin.

J.E.

and D. Loy, Phys. Rev. B 47 ( 1993) 47.56.

T. Atake,

T. Tanaka,

H. Kawaji.

K. Kikuchi.

Suzuki. I. lkemoto and Y. Achira.

K. Saito.

S.

( 199 I )

Physica C I 8% I89

427. ]7

1 M.

Gu and T.B. Tang, J. Phys.: Condens.

Matter

7 (1995)

7475.

[ 81

J.E.

Fisher,

D.E.

Ricketts-Foot, Heiney,

D. Li, A.L.

Brand and A.B.

[ 91

A. Dworkin. (1993)

[IO]

Luzzi,

W.R.

K.

Kinaz,

Romanow. Smith,

Smith

R.M.

III,

H. Szwarc

R.A. Assink.

D.A. PA.

M.A.

Cichy.

Phys. Rev. B 47 (1993)

and R. Cedin.

J.E. Schirber,

L.

14614.

Europhys.

Len

D.A. Loy, B. Morosin

J. Mater, Res. 7 (1992)

] I1 ] Z. Belahmer,

22

P. Bernier,

Haluska,

Fejdi,

Appl.

L. Firlej,

H. Kuzmany. L.

Mihaly,

M.

Lambelt

Vybornov,

and M.

and P.

P Rogl

161.

P.L. Lee,

Huang, R. Kaner, E Deiderich (1991)

J.M.

15980.

Phys. A 56 (1993)

P.W. Stephens,

and G.A.

2136.

Ribet, Phys. Rev. B 47 ( 1993)

[ 131

Strongin,

McGhie, Vaughan,

35.

Carlson,

[ 121M.

A.R.

G.B.M.

R.L.

Whetten,

and K. Holczer.

SM.

Nature 351

632.

[ 141

J.E. Fisher and PA. Heiney, J. Phys. Chem. Solids 54 ( 1993)

I 151

Y. Yoshinari,

1725.

Acknowledgements

H. Alloul,

Len. 71 (1993)

We thank Professors Xin Jin, Jinsong Zhu and Yiping Zhu, Mr. Jiaben Ji, and Miss Lili Pan for helpful discussions and technical assistance. This work was jointly supported by a grant from the Jiangsu Provincial Foundation of Natural Science and another from the Hong Kong Baptist University.

Cox,

J.R.D.

K.M. Ill,

[ I7 J J.

Vaughan,

Copley.

D.A.

W.A.

Creegan. D.M. Cox, J.P. McCauley

Li,

Kishio

S. Komiya, A.R. and

transition

Jr. and A.B.

C. Nagasaki,

ter

Smith

J. Kihara,

Physica C 195 (1992)

McGhie,

H.-U.

D.E.

Kamitakahara,

4544.

T. Tamura,

and K. Kitazawa,

1181 J.E. Fisher.

Phys. Rev.

J.E. Fisher, N. Coustel.

Neumann,

Phys. Rev. B 45 (1992)

orientational

References

G. Kriza and K. Holczer,

2413.

1161 P.A. Heiney. G.B.M.

Kuzmany

J.K.

Estmda,

Meer,

Heat

M.

Haluska,

capacity

in solid CM,. submitted

K.

205. and

H. the

to Phys. Rev.

B.

j I9 J J.P. Lu,

[ 1 ] PC. Chow, X. Jiang, G. Reiter. P. Wochner. Axe, J.C. Hanson, R.K. McMulan, Phys. Rev. Len. 69 ( 1992)

I)

Phys. Rev. Len. 66 ( 199 131 R. Tycko,

A.R.

McCauley

SC. Moss, J.D.

R.L. Meng and C.W. Chu,

2943, and references

therein.

X.-P. Li and R.M. Martin.

Phys. Rev. Lctt. 68

( 1992)

1551. 1201 J.E. Schirber,

R.A.

Assink,

G.A.

D. Joy. Phys. Rev. B 51 (1995)

Samara, B. Morrosin 15552.

and