Tetragonal polymerized phase of C60: experimental artifact or reality?

ELSEVIER

Synthetic Metals 103 (1999) 2415-2416

Tetragonal

polymerized a

V. A. Davydov

, V. Agafonov

phase of Ch,-,: experimental b

, I-I. Allouchi

b

, R. Ceolin

b

artifact

, A.V. Dzyabchenko

or reality ‘and H. Szwarc

? d

a

Institute for High Pressure Physics of the Russian Academy of Sciences, 142092 , Troitsk, hloscow Region, Russian Federation.

b

Laboratoire

de Chimie Physique, Faculte de Pharmacie de 1’Universite de Tours, 31 av, Ivlonge, 37200 Tours, France.

C

Karpov Institute of Physical Chemistry,

d

Laboratoire

de Chimie Physique des Materiaux

ul. Obukha, 10, bfoscow 107120, Russian Federation.

Amorphes,

URA 1 lm, CNRS, Universite Paris XI, 91405, Orsay, France.

Abstract The tetragonal polymerized phase (T) of C60 previousiy considered as metastable is shown experimentally to thermodynamically stable. Three different pressure-temperature paths were followed to prepare it. It has been found that increase of treatment time entails progressive disappearence of impurities to the benefit of the T phase in the p,T-range of stability. The experimental paths leading to almost pure T phase were established. Crystallographic, IR and Raman data related the T phase are presented. Keywords

:fillerenes

and derivatives,

difJlaction,

be the its to

IR and Rantan spectroscopies.

The difficulties of the pure tetragonal fl) polymerized phase synthesis were already noted in the earier studies [l-3]. This phase have been observed only in mixtures with either the orthorhombic (0) or rhombohedral (R) polymerized phases of C, [Z-7]. This fact has become the basis of the assumption that the T phase has no corresponding stability region in the p,T diagram and therefore it can not be obtained in the pure form [S]. It was assumed that, unlike the 0 and R phases, phase T is not a real stable high-pressure phase of the C, system, but rather some growth fault of the 0 or R phase. However, the results of theoretical investigations did not deny the idea of T phase stability at some p,T conditions 19, lo]. The aim of the present work was to obtain firm experimental evidence

supporting the existence of phase T as an individual substance. Three different paths (shown by arrows 1, 2 and 3 in Fig. (1 ) were chosen to reach the point of 2.2 GPa and 873 K accepted as phase T synthesis conditions according to the p,T diagram published earlier [7,11]. Small-crystalline fullerite C6,, with impurity content less than 0.1% was taken as a starting material. “Pistoncylinder” and “toroid” type high pressure devices were used for HPHTT (high pressure - high temperature treatment) of fullerite samples. Other details of HPHTT experiments have been described earlier [$l. Isothermal exposure times of 1000 , lOoo0, 20000 set were used for treating at 22 GPa and 873 K for each synthesis path. The products of HPHTT of fullerite vvere conserved by quenching them down to room temperature

8 P, GPa Fig. 1. Fragment of p,T diagram of Cm with the three paths (marked by 1, 2 and 3) of phase T synthesis. A represents the existence range of atomic carbon M corresponds to the monomeric state. Mp corresponds to the range of T and R polymerized phases of C, 0379-67791991s - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)0069 1-2

Fig. 2 The XRD patterns of phase T

2416

VA Davydov

et al. i Synthetic

under pressure and they were then studied at ambient conditions. The X-ray diffraction (XRD) experiments were carried out by means of an INEL CPSl20 detector using the CuKa, radiation. The IR transmission spectra of sariiples in KBr matrix were recorded with a Specord M80 spectrometer. DILOR XY Raman spectrometer operating with an Ar+ laser (514.5 nm) was used to obtain the Raman spectrum. For paths 1 and 2, practically pure T phase samples wereobtained with treatment times of about 1000 and 20000 set, respectively. For path 3, only a mixture containing about 40% of T and 60% of R phases was obtained after a lC000 set treatmentThe X-ray diffraction pattern of phase T is given in Fig. 2. The refined cell parameters of the final sample: a= 9.097(3), c= 15.04(2) %, , V= 1245(2) A3 are in agreement with those observed previously for phase T obtained at higher pressures [1,4]. The observed infrared spectrum (Fig.3, left) is much more pronounced than that presented in 163 (obtained from a “1: 1 mixture of tetragonal and orthorhombic phases”)

I

Metals

103 (1999)

2415-2416

and differs greatly from it, namely by the presence of a very characteristic band at 932 cm“. The Raman spectrum of the sample (Fig.3, right ) consists of about 35 distinguishable lines. The most intense of them were observed in 161. The differences of our spectra, obtained from purer sample, lie in the band’s relative intensity and in the structure of multiplets in the high-frequency region. The band at 430 cm-’ dominates over the doublet 1447 ! 1464 cm.‘; only a very weak line at 710 cm-’ and no bands at 1192 and 14.59 cm“ were observed, therefore assigned to an impurity. The successful preparation of phase T in pure form showed that this phase has a properregion of existence in the p,T diagram of Cm . However, dramatic differences in the rate -~ of formation of the phase T were observed for the three paths. This rate strongly depends to the mechanisms of T phase formation, which are very different , therefore cannot be correlated with an “onset of molecular rotation” as it was suggested in 181.

I

Wave number (eni’)

Fig. 3, IR (left) and Raman spectrum (right) of phase T.

References

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[7] V.A. Davydov, L.S. Kashevarova, A.V. Rakhmanina, A.V. Dzyabchenko, V. Agafonov, P. Dubois, R. CtZolin and H. Szwarc, JETP Lett, 66 (1997) 120. [S] 3. Sundqvist, Phys. Rev., B, 57 (1998) 3164 [9] C. H. Xu, GE. Scuseria, Phys. Rev. B 74 [1995) 274. [lo] V.A. Davydov, V. Agafonov , A.V. Dzyabchenko, R Ceolin ,H. Szwarc , to by published in J.of Solid State Chem, [ll] I. 0. Bashkin, V.I. Rashchupkin, A.F. Gurov, A.P. Moravsky, O.G. Rybchenko, N.P. Kobelev, Ya. M. Soifer, E.G. Ponyatovsky, J. Phys.: Condens. Matter. 6 (1994) 7491.