C 24 Cs samples containing oriented N 2 molecules

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C24Cs samples containing oriented N2 molecules Y. Finkelstein, R. Moreh, and D. Nemirovsky Physics Department, Ben-Gurion University, Beer-Sheva 84105, Israel

O. Shahal Physics Department, Nuclear Research Center-Negev, Beer-Sheva 84190 Israel

F. Beguin and L. Duclaux Centre de Recherche sur la Matiere Divisee, CNRS-Universite´, 1B, F-45071 Orle´ans Cedex 02, France (Received 18 September 1998; accepted 29 March 1999)

The adsorption of N2 molecules inside C24Cs has been studied in detail as a function of temperature and pressure using n-diffraction and nuclear resonance photon scattering (NRPS). Large differences were observed between samples derived from highly oriented pyrolytic graphite and those prepared from graphite powder. The tilt angle of the N2 molecular axis was determined by using NRPS and found to lay nearly parallel to the graphite planes. The amount of adsorbed N2 increased with decreasing temperatures, reaching saturation composition C24Cs(N2)1.5 at approximately 100 K with an initial N2 pressure of 2 bar. At higher pressure, 9 bar, two new phases were formed: a second stage C32Cs(N2)2.1 and a first stage C16Cs(N2)1.8 at T < 170 K and are discussed in detail.


In many problems in physics and chemistry there is a need to prepare oriented samples of molecular diatomic nitrogen in which the symmetry axes of the molecules lay parallel to a certain plane. Second-stage graphite intercalation compounds (GICs) of the form C24M (M ⳱ K, Rb, Cs) are layered compounds that are known to adsorb large amounts of nonpolar molecules1 and hydrocarbons,2,3 such as N2, H2, C2H2, C2H4, and C6H6. In this study our attention was focused on the physintercalation of N2 in C24Cs. The molecular N2 is characterized by two diameters (∼2.85 and ∼3.9 Å) corresponding to a standing and a laying N2 molecule.4 Because the small diameter of the N2 molecule is larger than that of a Cs atom, the intercalation process of N2 involves the swelling of the alkali layer to allow for the “accommodation” of the gas molecules. In this study we suggest a method for preparing oriented N2 samples in which the gas molecules are not in a pure form but reside in the alkali atomic planes of the graphite intercalation compound, C 24 Cs, forming C24Cs(N2)x. The extent of the swelling of the GIC during gas intercalation may be illustrated by considering C24Rb, which is a second-stage GIC; the identity period Ic, defined as the distance between two Rb layers sandwiching two graphite layers, is Ic ⳱ 9.01 Å at 120 K. The 3130


J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 14 Mar 2015

physisorption of N2 inside C24Rb at 120 K involves a swelling4 of the identity period by 0.45 Å, namely, increasing Ic from 9.01 to 9.46 Å. This may be understood by noting that the thickness of the Rb layer is 2.34 Å, whereas that of a lying N2 molecule (residing within the Rb layer) is 2.79 Å. It may be noted that the extent of swelling influences the kinetics of the physis