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ressure Hydrofullerites V. E. Antonova, A. V. Bazhenova, *, I. O. Bashkina, L. V. Zorinaa, A. I. Kolesnikovb, S. S. Khasanova, V. K. Fedotova, and T. N. Fursovaa, ** a

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia bNeutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831 United States *e-mail: [email protected] **e-mail: [email protected] Received January 12, 2020; revised February 26, 2020; accepted February 28, 2020

Abstract—This paper is a brief summary of the results of long-term experimental studies of C60Hx hydrofullerites with a hydrogen content of up to x ~ 60 or more, obtained by loading C60 fullerites with hydrogen at pressures up to 9 GPa and temperatures up to 500°C. Basically, this is an overview of already published data. Some results for hydrofullerites with compositions of x ~ 60 and x ~ 90 are presented for the first time. Keywords: fullerene, hydrofullerite, hydrogen, high pressures, X-ray diffraction, infrared spectroscopy, inelastic neutron scattering DOI: 10.1134/S1027451020050237

INTRODUCTION The techniques for synthesis of hydrides in a hydrogen-gas atmosphere under pressures of up to 9 GPa and temperatures of up to 800°С with subsequent quenching the samples under pressure to a temperature of 80 K, which were developed at the Institute of Solid State Physics, Russian Academy of Sciences, provide the opportunity to obtain relatively large single-phase and homogeneous samples with the minimum number of defects and to save them for subsequent study in a metastable state under atmospheric pressure [1, 2]. The amount of the substance synthesized in one experiment varies from 30 to 500 mm3 depending on the pressure, which is sufficient for measurements using most physical methods. These techniques were used for the saturation of various forms of carbon with hydrogen, more specifically, nanofibers and nanotubes, graphite, and fullerite С60. New compounds with compositions of up to CH0.88 stable in vacuum at temperatures up to 450°С were prepared on the basis of carbon nanofibers and nanotubes [3]. A new compound with a composition close to CH was synthesized on the basis of graphite [4]. The compound is a crystal consisting of corrugated layers of graphane in chair conformation, which are perpendicular to the hexagonal axis of the initial graphite. The formation of this multilayer graphane is accompanied by an increase in the distance between graphite layers by 42% from 3.36 to 4.77 Å. In this work, we consider the most significant results obtained for hydrofullerites С60Hx. In particular, crystalline hydrofullerite C60H60 was synthesized

under high hydrogen pressure [5, 6], while other methods afford compositions of no higher than C60H36. Room-temperature ferromagnetism was discovered in hydrofullerite C60Hx with x = 24 [2]. Amorphous hydrofullerite C60Hx with x ≈ 90 has been synthesized. EXPERIMENTAL The initial material for hydrogenation was a powder of С60 with a purity of 99.99% with respect to С70