Phase transitions and aging effects of the graphite intercalation compoundα-C5n-HNO3

Abstract

Detailed x-ray studies of nitric acid intercalated in graphite have been carried out, notably on the α-${\mathrm{C}}_{5\mathrm{n}}$-${\mathrm{HNO}}_3$ modification. Single-crystal samples were prepared with staging indices n=8, 2, and 3. The material shows a rather complex behavior with pronounced aging effects, which have been identified. The normal, aging-independent phases were found to be the following: Above ∼250 K a disordered liquidlike phase exists, which by a first-order transition goes into an ordered phase upon cooling. The transition shows a clear hysteresis ($T_{\mathrm{ch}}$=249±1 K, $T_{\mathrm{cl}}$=244±1 K). The ordered phase is incommensurate with the graphite lattice in one unit cell-direction (A→=1.43a${\to }_G$), but commensurate in the other direction (B→=2a${\to }_G$+9b${\to }_G$). A tendency for a lock-in transition along A→ by a supercell 12 times larger was seen from the appearance of super-reflections for the fresh stage-2 and the stage-3 samples, but full commensuration (A→≃17a${\to }_G$) was not achieved. A unique feature of the ordering phase transition in this material is a pronounced sliding motion of the graphite layers out of the normal hexagonal stacking below $T_c$. The intercalant layers act effectively as ball bearings for this motion. The motion is probably restricted by elastic deformations at the domain walls. Aging effects over a time span of a few months involve a change of staging, the disappearance of an intermediate phase of the low-stage samples, and, maybe most importantly, the growth of an extra ordered phase. The latter phase is commensurate with the graphite lattice and has a separate transition temperature, $T_{\mathrm{cx}}$=259±1 K. This phase may be the cause of a second peak previously observed in calorimetric studies.