Intercalate Phonon Densities of States for Alkali-Graphite Compounds
- PDF / 402,974 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 71 Downloads / 246 Views
W. A. KAMITAKAHARA AND H. ZABEL *kAmes Laboratory-USDOE and Dept. of Physics,
Iowa State University,
Ames,
Iowa 50011, USA; **Dept. of Physics, University of Illinois at UrbanaChampaign and Materials Research Laboratory, Urbana, Illinois 61801, USA
ABSTRACT Neutron scattering methods have been used to determine the partial phonon densities of states qgM(v) for inplane vibrations of M atoms in MCx compounds, where M = K, Rb or Cs, and x = 8, 24, 36. Only modes with both phonon wave vectors and displacements in the basal plane are represented in gM(V). Detailed measurements of the temperature dependence were made for RbC 24 and KC 2 4 , in order to study the influence of the order-disorder transformations in these compounds. The results are interpreted in terms of different pictures of the disordered state.
INTRODUCTION Most of the lattice modes of an alkali-graphite intercalation compound (AGIC) can be described as either intercalate-derived or graphite-derived. Exceptions to this rule occur for [001] longitudinal modes, in cases where C and intercalate layers have comparable mass/unit area, and for layer bending modes [1,2] at certain wave vectors where the alkali and carbon motions are strongly coupled. In this article, we concentrate on intercalate-derived phonons with wave vectors and polarizations parallel to the basal plane, as studied by neutron scattering from pyro-graphite-based samples. Of particular interest in our work are (i) a systematic overview of how these modes depend on alkali species, stage, and temperature, and (ii) the effect of orderdisorder phase transformations which are a general feature of AGIC's. We have studied (ii) in some detail in the stage-two compounds RbC24 and KC 2 4 *. EXPERIMENT The samples were prepared by the usual two-bulb vapor transport method, using pyrolytic graphite starting material. Intercalation typically required 3 several days for these rather large samples (-10 cm volume). The neutron scattering technique for observing intercalate modes, which has been recently discussed elsewhere [2], consists essentially of measuring scattered neutron distributions for a number of values of the wave vector transfer ý, where ý lies in the basal plane. Because a pyro-graphitebased sample implies an orientational average over 6, it is possible to obtain a good, uniform sampling over all in-plane phonon wave vectors, sufficient to determine a phonon density of states, by choosing only a few fairly large values of the magnitude Q. In our case, we chose five values ranging from 3.7 to 4.3 A-'. The technique is very similar to methods [3] which have been used for years to determine the phonon density of states (PDOS) of three-dimensionally (3D) polycrystalline materials, except that in our case, we use a 2D polycrystal and determine only a partial PDOS. The meaning of "partial" in this context is that only phonon wave vectors and displacements parallel to the basal plane are sampled. Also, because our
Nat. Res. Soc. Symp. Proc. Vol. 20 (1983) Published by Elsevier Science Publishi
Data Loading...
