Valence and core excitation spectra in K, Rb, and Cs alkali-metal stage-1 intercalated graphite

Abstract

Excitation spectra of the alkali-metal stage-1 intercalated graphites KCS, RbCS, and CsC8 have been measured by high-resolution (0.1 eV) electron-energy -loss spectroscopy. The m intraband and interband plasmon excitations in the region below 10 eV are the same in aB three compounds. However, in the valence region between 15 and 30 eV, small peaks are observed in RbC8 and CsC8 which are weaker than those previously observed in KC8. In KC8, these structures were identified as both excitations to the backfolded graphite bands created by the introduction of the intercalant and also as characteristic metal-atom core excitations. Within this interpretation, the weakening of the structure due to the backfolded bands with intercalant atomic number is consistent with weakening the coupling far above the Fermi level between the graphite and intercalant atoms. The carbon 1s core shell excitation at -285 eV is also reported. %'e find that both the spectral shape and threshold energy are unaffected by the choice of alkali-metal intercalant (K, Rb, Cs), although the resulting spectrum differs from that of pristine graphite. Hence the graphite p states just above the Fermi level which are probed by the C 1s excitation are identically perturbed by the K, Rb, and Cs intercalations, suggesting that the degree of hybridization and charge transfer in these materials is the same.Intercalation of graphite with alkali metal to stage 1 yields a model layer compound in which the alternating metal and graphite atom planes interact, resulting in a compound exhibiting electronic properties different than those of pristine graphite. In particular, we expect the alkali-metal atoms to at least partially donate their weakly bound outermost s electron to the graphite planes. Such charge transfer shifts the Fermi level (FF} up from its value before intercalation, changing the nature of the highest occupied states. Also, since the alkali-metal valence s states are degenerate in energy with the graphite m bands, and since the metal-carbon (M-C} interatomic distances are quite small, we may expect to observe hybridization between the metal and the graphite orbitals. Electron-energy -loss spectroscopy (EELS) has been shown to be a sensitive probe of the electronic properties of intercalated graphite. ' By performing a KramersKronig analysis on the loss data, the real and imaginary parts of the dielectric function may be obtained, and the elementary excitation spectrum of the solid determined. This paper contributes to a growing body of literature and consists of a comparative study at high-energy resolution and statistics of the three alkali-metal graphite intercalated compounds (GIC's) which form an MCs stage-1 structure, KC8, RbCS and CSCS. In the valence portion of the spectrum, we observe features unresolved by previous measurements taken at poorer resolution and statistics. We shall find that those excitations involving states close to the Fermi level are virtually indistinguishable in KC8 versus RbCS versus CsC8, providing strong evidence that...

Comparative Study of Rubidium and Cesium as Promoters in Carbon-supported Ruthenium Catalysts for Ammonia Synthesis

“…This may be due to the insertion of cesium ions between carbon layers and/or the formation of intercalation compounds similar to CsC 8 . Indeed, similar C1s binding energy value (285 eV) has been reported earlier by Grunes and Ritsko [12] for the intercalated compounds of graphite with alkali metals. Such shifts are not observed for the samples promoted with rubidium compounds, which indicates the absence of substantial interaction between the rubidium promoter and the support (Table 3; Fig.…”

Low-energy plasmonic structure in CaC6

“…This feature can be classified as a conventional π plasmon also presented in pristine graphite at ω πp ≈ 7 eV 19,20,39,56 and in the GICs at lower energies. [19][20][21][22][23][24][25] The redshift of the πp plasma frequency in CaC 6 in comparison with the plasmon frequency in pure graphite can be explained by the presence of a broad peak centered at 4.6 eV at small Q's in the imaginary part of seen in Figs. 2(c) and 3(c).…”

“…[15][16][17][18] The collective electronic excitations, to the best of our knowledge, were neither investigated in YbC 6 nor in CaC 6 . At the same time, there exist a rich literature [19][20][21][22][23][24][25][26] on this topic for other GICs. Thus in a variety of these compounds, modifications of the low-energy π plasmon mode existing in pure graphite at the 7-12 eV energy range were observed [27][28][29] resulting in a plasmon frequency shift to lower energies.…”

Optical properties of calcium under pressure from first-principles calculations

“…If the origin of a plasmon resides in these interband transitions, it is sometimes referred to in the literature as an interband plasmon. 32,33 We will employ the expression "interband plasmon" with this in mind in what follows. Moreover, interband transitions are generally enhanced under pressure.…”