‘Low-pressure’ orthorhombic phase formed from pressure-treated C60

14 March 1997

CHEMICAL PHYSlCS LETTERS

ELSEVIER

Chemical Physics Letters 267 (I 997) 193- 198

‘Low-pressure’ orthorhombic phase formed from pressure-treated C 6o V. Agafonov a, V.A. Davydov b, L.S. Kashevarova b, A.V. Rakhmanina b, A. Kahn-Harari ‘, P. Dubois a, R. Ctolin a, H. Szwarc d a Lahorutoire de Chimie Physique, J.E. 408, Fucul~h de Phrrrmucie, Universitb de Tours. 31 cluenue Mange, 37200 Tours, France b Instirute jbr High Pressure Physics, Russiun Au&my oj’Science. 142092 Troitsk, Moscow Region, Russian Federation ’ Lahorutoire de Chimie AppliquGe de I’Etat S&de, URA 1466, CNRS, ENSCP, I I rue Pierre et Marie Curie, 75231 Paris C&x 05. France d Lahoratoire de Chimie Physique des Murbriuur Amorphes, URA D 1104, CNRS, B&iment 490. Universid Paris Sud, 91405 Orsuy, France Received 20 June 1996; in final form 1I December 1996

Abstract X-ray and electron diffraction and Raman spectroscopy have shown that the solid formed by a pressure-temperature treatment of C, at 1.5 GPa-723 K has an orthorhombic structure 0’. It may be considered that phases 0’ and 0, the previously known orthorhombic phase formed at 8 GPa-573 K, are the same so that our pressure-temperature treatment provides “the optimal way of obtaining the linear-chain orthorhombic fullerene phase”. However, differences in the respective spectra of these two phases led us to re-examine the structure of the previously described phase 0: it is found that a rhombohedral structure fits the experimental X-ray data at least as well as an orthorhombic one does. In any case, phase 0 may be an intermediary for the formation of the tetragonal high-pressure modification.

1. Introduction When

treated

pressure-temperature

in the

l-8

GPa

domain,

and 3OO- 1000 K

crystalline

C,,

under-

a series of transformations into crystalline materials which do not revert to initial cubic C,, when pressure is released [l-5] and therefore can be studied at room pressure and temperature. Iwasa et al. [ 11 treated C,, at 5 GPa up to 1073 K and observed the formation of two new phases: a fee one between 573 and 673 K and a rhombohedral one between 773 and 1073 K. Spectral shifts in the IR (1428-1422 cm-‘) and Raman spectra (1468-1457 cm-’ ) were reminiscent of the photoinduced polymerization of C,, films [6].

goes

COO9-2614/97/$17.00 Copyright Pff SOOO9-26 14(97)00072-9

Nufiez et al. [2] generated models of crystalline C,, polymers which explain their experimental X-ray results fairly well. They concluded that: (i) at 4 GPa-973 K and 3 GPa-873 K, a mixture of a rhombohedral phase (R) and a tetragonal one (T) is formed; (ii) at 8 GPa-573 K, only an orthorhombic phase (0) forms. Phase 0 and probably phase T formed at 3 GPa-773 K and 3 GPa-973 K, respectively, were also observed by Davydov et al. [4]. There is some disagreement about the structures of the different phases. Sundar et al. [5] found that all the diffractograms of materials formed at 5 GPa(673 and 923 K) and 7.5 GPa-773 K are indexable to an orthorhombic structure. In another case, an R + T mixture could be indexed to a unique mono-

0 1997 Elsevier Science B.V. All rights reserved

194

V. Aguj?wzou et ul./Chemicnl

clinic lattice [3]. Furthermore, kinetics may play a part: through a 2-h treatment at 3 GPa-973 K, Kozlov et al. [7] formed a superhard material different from the expected rhombohedral and/or tetragonal solid. The present work was meant to get more details on the pressure-temperature transformations of C,,. The results obtained in the 2-8 GPa range essentially confirm previous results with the observations of structures 0, T and R. However, when decreasing the treatment pressure to 1.5 GPa at 723 K, an orthorhombic structure 0’ was observed. Its characteristics will be compared to those of phase 0 which we obtained under the same conditions (8 GPa, 573 K) as Nu”nezet al. [2].

2. Experimental Fullerene C 6. purchased from TermUSA (initial purity 99.9%) was sublimed twice under vacuum at about 800 K to get rid of possible remaining solvents. No special care was taken to protect it from ambient oxygen and it was used within two months after the vacuum-sealed silica container was open. The pressure-temperature treatments were performed using a piston-cylinder pressure device up to 2 GPa, and a toroid tungsten carbide anvil cell [Sl in the 2-8 GPa range. The C,, samples, previously pressed into pellets (typically 2.5 mm diameter and 2.5 mm thick), were wrapped in thin tantalum foils and embedded within a hexagonal boron nitride cylinder meant to protect from the graphite heating system. Catlenite mainly CaCO,) was used as the pressure transmitF60 ting medium. To operate, first, the pressure was increased to the desired value, then the temperature was increased and held at the experimental value ( It:3 K according to chrome]-alumel thermocouples) for lo-15 min. Afterwards, the heating current was switched off and the ensuing temperature quenching proceeded at initial rates of about 60 and 400 K s- ’ for the pistoncylinder apparatus and the toroid device, respectively. Finally, the pressure was released at = 0.050.1 GPa min-’ rates. The resulting materials were all studied at room temperature and pressure.

Physics Letters 267 (1997) 193-198

X-ray diffraction (XRD) profiles were obtained with a powder diffractometer (Siemens D 5000, A(Co K a) = 0.17890 nm, Bragg-Brentano geometry>. Peak positions were determined with the Socabim fitting program PROFIL (PC software package DIFFRACT-AT supplied by Siemens). Electron diffraction experiments were performed with a JEOL lO/ 10 electron microscope at 100 kV. Samples were crushed into powders under water. Drops of powder-containing water were deposited upon a copper grid, covered with polyacetate film and dried. Afterwards, the samples were coated with a Pt film to prevent heating and subsequent transformations. A Bruker FT Raman RFS 100 spectrometer was used to record the spectra. The Nd:YAG laser was operated at 1064 nm with 130 mW power. At least 100 scans at 2 cm-’ maximum resolution were collected. A Ge-diode detector encompassed the 503000 cm-’ spectral range.

3. Results 3.1. XRD analysis Typical XRD profiles of phases 0’ and 0 are shown in Fig. 1 (a and b, respectively). The 0’ profile exhibits more peaks than the 0 one. It could not be indexed using the unit cell parameters of high-pressure cubic, tetragonal or rhombohedral phases. Eleven well-resolved peaks were extracted from the XRD profile and treated using the DICVOL91

Fig. 1. X-ray diffraction profiles of materials obtained by a pressure-temperahre treatment of C,. (a) orthorhombic phase 0 obtained at 8 GPa-573 K; (b) orthorhombic phase 0’ obtained at 1.5 GPa-723 K. Profile (c) was calculated using a modified structure of crystalline RbC, [lo] as a model for phase 0’.

Table I X-ray data for orthorhombic 1.5 GPa and 723 K

V. Agujimov et (II./ Chemicul Physics Letters 267 (1997) 193-198

195

Table 3 Fractional atomic coordinates orthorhombic Dhase 0’

used to simulate the XRD profile of

phase 0’ obtained by treating C,

hkl

cl,, p (nm)

cl,,, (nm)

I

011 101 110 112 200 013 103 121 022 211 202

0.818 0.776 0.669 0.4936 0.4546 0.4392 0.4320 0.4149 0.4086 0.3976 0.3869

0.818 0.774 0.668 0.4945 0.4549 0.4389 0.4318 0.4149 0.4088 0.3975 0.3869

s m w

at

C(I) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9)

s m m m m m w w

x

Y

2

0.0000 0.0795 0.0797 0.1285 0.1562 0.2526 0.2847 0.3427 0.4135

0.0708 0.2628 - 0.3520 0.1441 0.3065 0.0733 0.2332 0.1286

0.2363 0.1566 0.2058 0.0797 0.1764 0.0490 0.0912 0.0563

0.0000

Space group: Immm, 2 = 2.

Intensities (I) are expressed using code: s = strong, m = medium, w = weak.

[9] program which yielded an orthorhombic cell with: a = 0.9098(6) nm; b = 0.983(l) nm; c = 1.472(2) nm [figures of merit: M(11) = 22.4; F(11) = 30.5(.0144,25)]. Table 1 records the corresponding X-ray powder diffraction data and indexation. Miller indices suggest space group Immm with 2 = 2, but an accurate assignment would need more Bragg peaks. Table 2 records parameters for phases 0’ and 0. Parameters b are almost equal while parameters a and c for 0’ are respectively shorter and longer than the corresponding parameters for 0. The phase 0’ unit cell volume is larger than the phase 0 one, probably because phase 0’ is a lower-pressure material than phase 0. Moreover, the XRD profile of phase 0’, which contains more and better resolved Bragg peaks than that of phase 0, reveals less disorder in the former material than in the latter one: a lower pressure is likely to be more hydrostatic (or rather less non-hydrostatic) than a higher one and

could induce a more homogeneous polymerization, that is less disorder, but this has to be ascertained. To simulate the phase 0’ XRD profile, the model of C,, polymeric chains proposed for the orthorhombit phase of RbC,, [lo] has been used. The atomic positions modified for the Immm group are reported in Table 3. The resulting profile (LAZY ROUTINE) is represented in Fig. lc. The agreement with the experimental profile is quite satisfactory. It supports the fact that phase 0’ is made of chains polymerized along direction a. It is to be noticed that this calculated X-ray profile looks somewhat different from the phase 0 model profile recorded in Fig. 2, Ref. [2] and also derived from linear C, polymer chains. According to this structural model, the nearestneighbour intermolecular distances are equal to 0.909 nm and the next nearest ones are 0.995 nm, to be compared with 0.926 and = 1.OOnm found by Nu5ez et al. [2] and 0.922 and 0.976 nm as determined by Iwasa et al. [l], respectively.

Table 2 Unit cell parameters

of orthorhombic

Phase

a (nm)

b (nm)

c (nm)

V (nm’)

cy 0 Oa C Tb Rb

0.9098(6) 0.926 0.92% 1) I .36 0.909(l) 0.920(l)

0.983( 1) 0.988 0.981(l)

1.472(2) 1.422 1.408(2)

0.658 0.650 0.641 0.629 0.618 0.602

phase 0’ as compared

to all other known phases obtained by a pressure-temperature

1.497(2) 2.463(2)

V = volume per molecule. a Obtained by us under the same conditions (8 GPa-573 K) as phase 0 was in Ref. 121. b Obtained by us under the conditions (3 GPa-973 K) used in Ref. (71.

treatment of C,

Reference this work

121 this work

111 this work this work

196

V. A&&ou

et ul./ChemicuI

Physics Letters 267 (1997) 193-198

direction b the polymerized C,, chains of phase 0’ until the interatomic distances of carbon atoms lying on two neighbouring C,, molecules attain 0.156 nm. The application of pressure could perform this feat: this could explain why increasing the pressure to 2 GPa at higher temperature onto phase 0’ would induce the formation of phase T, which is the main phase under these conditions. 3.2. Electron diffraction analysis Fig. 2. Structural model of orthorhombic phase 0’ obtained by treating C, at 1.5 GPa-723 K: projection along direction h (mass centres at y = 0.0 and 0.5 (black units)).

Parameters a for phases 0’ and T are nearly equal. To comply with the model proposed by Nuiiez et al. [2] according to which phase T is made of sheets of cross-linked C,, polymerized chains, phase T could be obtained by bringing together along

Fig. 3. Electron diffraction patterns of orthorhombic plane; (b) (3 - I - I)’ reciprocal plane.

According to electron diffraction experiments, phase 0’ can be viewed as a distorted fee phase. For instance, Fig. 3a shows an orthorhombic (OOl)* reciprocal plane for phase 0’ which resembles a (001) * cubic one. Some characteristic distances were measured [d,,, = 0.494(2) nm, dozO= 0.454(2) nm, d,,, = 0.669 nm], which closely correspond to some measured in XRD profiles (Table 1). In the same

phase 0’ formed from the treatment

of C,

at 1.5 GPa-723

K: (a) (001)’

reciprocal

V. Agafonou ef ul./Chemicul

way, Fig. 3b, which corresponds to (3 - 1 - 1) * of 0’, could be a distorted (112) * cubic plane with three distances [da_ ,, = 0.820(2) nm, d,,, = 0.495(2) nm, d,ox = 0.434(2) nm] recorded in Table 1 for 0’. Thus, the local structure viewed at the electron diffraction scale is in excellent agreement with the mean structure as viewed through X-ray measurements. 3.3. Raman spectroscopy Fig. 4 exhibits Raman spectra corresponding to phases 0 (Fig. 4a) and 0’ (Fig. 4b). The Ag(2) ‘pentagonal-pinch’ mode, which is lying at 1469 cm-’ in pristine C,,, is shifted to 1457 cm-’ in both 0’ and 0, which is characteristic of polymerized C,, [3,6]. However, a number of differences are observed in the spectra of 0’ and 0: - a strong 118 cm-’ line, observed for 0, is seen only in an enlarged spectrum for 0’; - a weak line at 172 cm-’ in the spectrum of 0’ is not observed for phase 0; - the Ag(1) mode is split into two components at 255 and 275 cm-’ for phase 0’ and into three lines at 255, 269 and 292 cm-’ for phase 0; _ a line at 1620 cm- ’ in phase 0 is not observed in the phase 0’ spectrum.

C................l 500 loo

loo0 R-shift

1500 (crCi

)

Fig. 4. m-Raman spectra of materials obtained by a prcssuretemperature treatment of C,: (a) of orthorhombic phase 0 obtained at 8 GPa-573 K; (b) of orthorhombic phase 0’ obtained at 1.5 GPa-723 K.

Physics Letters 267 (1997) 193-198

197

4. Discussion Because of the differences we just described in the Raman spectra and X-ray profiles of phases 0’ and 0, respectively, we assumed that these two phases are different. However, the referee insists that “the 0’ phase is the same as phase 0: they have the same structure with no new distortion. The 0 and 0 are the same structures obtained at different pressures. The 0, obtained at high pressure, is more disordered due to incipient linking between the chains, and thus has a slightly smaller volume than the much better ordered 0’ samples obtained at low pressure. The authors have just found a better way of obtaining the 0 phase.” As a matter of fact, the Raman spectrum of Fig. 4a which corresponds to phase 0 includes that of phase 0’, with the exception of the 172 cm-’ line which is weak and could have eluded observation. However, it could also be understood in terms of a mixture of polymeric organizations including that of phase 0’ or of some other structure. Because of the latter possibility, we examined whether the X-ray pattern of phase 0 could be described in terms of another structure than an orthorhombic one. We saw above that the 11 Bragg peaks which were recorded for phase 0’ were narrow enough to use DICVOL91 for indexing purposes. In the case of phase 0, the peaks are too broad for this method to be applied and we get the same accurate criteria for evaluating the resulting indexation. We found that the X-ray profile of Fig. la can indeed be indexed in terms of an orthorhombit structure but also in terms of a rhombohedral one and that the latter indexation fits the experimental results somewhat better (Table 4). Moreover, the density of samples of C,, treated at 8 GPa and 573 K was determined by means of a float method previously calibrated with respect to C,, ( Pexp = 1.69 g crne3) and to graphite ( pexp = 2.24 g cm -3 instead of 2.26). It became pexp = 1.96 g cme3, which is closer to the density of the rhombohedral form (1.92) than to that of the orthorhombic one (1.86). To conclude, XRD, electron diffraction and Raman measurements have shown that a pressure-temperature treatment at 1.5 GPa and 723 K leads to an orthorhombic phase 0’ which is less dense than the

V. A~c?fi,nou et ul./Chemicol

198

Table 4 X-ray data for phase 0 obtained by treating C,, at 8 GPa and 573 K hkl (i) considered 011 101 110 020 200 121 022 222 u (nm) 0.939(5)

derp (nm) as orthorhombic 0.80 0.775 0.678 0.486 0.472 0.416 0.407 0.307 h (nm) 0.981(6)

d,,, (nm)

0.802 0.779 0.678 0.49 1 0.469 0.415 0.401 0.305 c (nm) I .394(20)

Physics Letters 267 (19971 193-198

ment which would be the result of the cross-linking process that the higher pressure favours as the referee stressed. Anyway, phase 0’ can be viewed as an intermediate stage for the tetragonal phase which is preferentially formed at higher pressure and temperature.

Acknowledgement The present work has been supported INTAS-93-2133.

by contract

V (rims) I ,284 14)

References (ii) considered 003 101 102 104 110 105 006 008 u (nm) 0.945(3)

as rhombohedral 0.80 0.775 0.678 0.486 0.472 0.416 0.407 0.307 c (nm) 2.417(8)

0.806 0.775 0.678 0.486 0.473 0.416 0.403 0.302 V (nm3) 1.871(13)

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