Staging Transitions In Li-Graphite: Phonon Spectra of Dilute Stage 1

  • PDF / 315,784 Bytes
  • 5 Pages / 420.48 x 639 pts Page_size
  • 67 Downloads / 211 Views

DOWNLOAD

REPORT


: PHONON

K.C. WOO, H. MERTWOY AND J.E. FISCHER Moore School EE and LRSM, University of Pennsylvania Philadelphia, PA, USA W.A. KAMITAKAHARA Ames Lab and Physics Dept., Ames, IA, USA

Iowa State University,

D.S. ROBINSON Physics Dept. and Materials Research Lab, University of Illinois, Urbana, IL, USA

ABSTRACT First-order transitions to dilute stage one from stages 2,3 and 4 and from mixed stage 1+2 are observed in Li-graphite as predicted by Safran. The upper phase boundary is asymmetric in concentration and is sharply peaked about x - 0.4. The phonons in the dilute stage 1 have energies between those of LiC6 and graphite. Unlike the stage 2 compounds, the compressibility does not scale in a simple way with intercalant density.

INTRODUCTION staging is one of the most interesting and unique properties of graphite intercalation compounds. Stage n is a periodic sequence of n layers of graphite and one layer of intercalant. Recently, Safran [1] predicted that only stage 1 is stable above a certain temperature Tn. The transitions from high stages to dilute stage 1 are predicted to be first order except in a limited region near Tm where the transition from stage 2 to dilute stage 1 should be second order. The theory is based on a Hamiltonian including two-body attractive in-plane and replusive interplane interactions and the entropy of a lattice gas at finite temperature. Pure staging at T=O is a result of interlayer repulsion, the in-plane density being determined (consistent with stoichiometry) by maximizing the total attractive energy. At finite T the entropy of a 2D lattice gas with vacancies overrides the entropy due to "staging defects", such that pure stages (or mixtures thereof) remain stable. Thus staging transitions from high to low stage are expected to occur as a function of increasing T, the transition temperature depending on x, the overall intercalant concentration expressed as a fraction of the maximum value. The upper phase boundary between dilute stage 1 and high stages (or mixtures thereof) is symmetric about x=0.5 in Safran's model, a direct result of the two-body Hamiltonian. In recent extensions of the theory, Dahn et. al. [2] included an elastic guest-host interaction and Millman et. al. [3] accounted for the energy required to separate host layers during intercalation. The latter authors also studied the effect of screening the interlayer repulsion. Either type of modification removes the symmetry about x=0.5 at the expense of introducing more parameters. The high-T transition to dilute stage 1 is a general consequence of the entropy contribution to the free energy and is thus independent of the details of the model Hamiltonian.

Mat. Res. Soc. Symp. Proc. Vol. 20 (1983) @Elsevier Science Publishing Co.,

Inc.

266 We report here an experimental study of staging transitions to dilute stage 1 versus x, and the high-T phonon spectra of the dilute (disordered) stage 1 phase. Li-graphite was chosen for this work because it is the only graphite intercalation system which showed evidence from preli

Data Loading...