Raman Measurements of Folded Interlayer Shearing Phonons in KC_{12 n} (n=2–6)

Abstract

Graphite intercalation compounds (GIC’s) have been the subject of extensive research, not only because of practical applications [1],but because of fundamental reasons [2]. Their highly anisotropic structures, characterized by the strongly coplanar bonded graphite layers with relatively weak interlayer interactions, provide unique quasi-one and two dimensional systems to investigate. Along the c-axis, the intercalant layers usually form a well-defined periodic sequence, known as staging. A “pure” stage n compound consists of a single phase of one intercalent layer per every n graphite layers [3]. In alkali-metal GIC’s (AGIC’s), it is relatively easy to synthesize pure stage compounds up to stage 6 by controlling both alkali vapor pressure and sample temperature. In spite of numerous investigations, the origin of the long-range interlayer interactions responsible for these one-dimensional modulated systems is still not well understood. Between the graphite layers in GIC’s, intercalant atoms or molecules form disordered or ordered layers, depending on stage and temperature. Recent x-ray [4,5,6], neutron [7] and electron microscope [8] experiments revealed interesting quasi-2D order-disorder phase transitions and dimensional crossover phenomena. High pressure x-ray [9,10] and Raman [10] experiments demonstrated staging phase transitions with formation of a 2 × 2 superlattice in high stage AGIC’s (n ≥ 2), It is of particular importance to study the dynamics of inter- and intralayer interactions in GIC’s, since they play a crucial role in the staging mechanism and in-plane order-disorder phase transitions. In addition, information about the phonons should contribute to an understanding of their electronic structures, which result in such novel properties as superconductivity and extremely high conductivity [11].