Diffusion coefficients of HNO3 and Br2 from near-saturated vapor into highly oriented pyrolytic graphite at 30C were respectively, 203 x 10-6cm2/min and 1.47 x 10-6cm2/min, and that of PdCl2 was 0.52 x 10-6cm2/min at 450C. Rapid diffusion of HNO3 was attributed to its intercalation as a two-dimensional liquid, whereas bromine and PdCl2 intercalate as two-dimensional solids. Estimated diffusion coefficients of alkali graphites, which also intercalate as two-dimensional solids, were smaller than those for Br2 or PdCl2, but transition metal chlorides which intercalate from the vapor above the bulk melting temperature have estimated diffusion coefficients comparable to those of HNO3. Rates of intercalation were linear functions of (time)-1/2 until intercalation by HNO3 was 80% complete and intercalation by Br2 was at least 23% complete, and indicate that a single diffusion process controlled each reaction even though gross compositions corresponding to the formation of discrete crystallographic "stages" were exceeded. This result accords well with structural models of intercalation compounds in which monolayer "rafts" of intercalant took up ordered positions throughout the bulk of a graphite crystal, but not with models, in which intercalant monolayers continuously cover basal planes. The apparent activation energy for diffusion of PdCl2 in graphite, 35kcal/mol, was attributed to a process which precedes diffusion.

Diffusion Coefficients of Br2, HNO3 and PdCl2 in Graphite. Dowell, M.B., Badorrek, D.S.: Carbon, 1978, 16[4], 241-9