Crystallographic and transport studies on AsF5 intercalated graphite from 4.2 to 295 K. I. Structural ordering and phase separation

Abstract

A study of the (00l), (hk0), and (hkl) reflections of stage 1 AsF5 intercalated graphite between 4.2 and 295 K has been done using synchrotron radiation for single-crystal samples and a linear detector setup for those based on highly oriented pyrographite (HOPG). The nature and temperature dependence of the structural ordering allow the materials to be classified into (at least) two types related to the degree to which the AsF5 has been converted into AsF−6 and AsF3. At 295 K a small amount of in-plane order is detected within the essentially two-dimensional (2-D) liquid intercalate attributed to small quantities of AsF5 and AsF−6 − AsF5-ordered phases. Lowering the temperature leads to increased phase separation through crystallization and to changes in the in-plane unit cells associated with each. The most marked structural change is an incommensurate-to-commensurate (I⇉C) transformation within the AsF5 phase, which starts at 215 ± 5 K. No new structural order is detected below 180 K. The (00l) studies give clear confirmation of the existence of separate phases with different values of interplanar distance. A smaller number of stage 2 compounds were examined. The most clearly different feature is that the I⇉C transformation is downshifted by ∼ 70 K. At room temperature, the stacking sequences are A/A/A … for stage 1 and A/AB/BC/CA … for stage 2.