Interpretation of Diamond and Graphite Compressibility Data Using Molecular Force Constants

Abstract

The diamond and in‐plane graphite compressibility data of Lynch and Drickamer are analyzed in terms of simple bond‐compression and bond‐compression, buckling, and puckering models, respectively, using carbon–carbon stretching and out‐of‐plane displacement force constants from molecular studies. To the highest experimental pressures of over 300 kbar it is found that the quotient of potential energy of bond compression and macroscopic work of compression, for the same values of the lattice parameter ratio ${\mathrm{(a/a}}_0)$ , remains essentially constant for diamond at 0.7 indicating that bond compression is the dominant mechanism for storing energy of compression. Similar quotients for graphite at corresponding values of ${\mathrm{(a/a}}_0)$ vary from 1.4 at the lowest pressures to 0.6 at the highest for the in‐plane bond compression, from 100–4 for the out‐of‐plane buckling, and from 300–13 for the out‐of‐plane puckering with the last two modes assuming a fixed C–C in‐plane bond distance. It is suggested that in graphite bond compression is the primary mechanism for absorbing intraplanar energy of compression with out‐of‐plane buckling and puckering much less important until quite high pressures are reached, in contrast to earlier views. This would seem to lead to a partial understanding of the difficulties present in synthesizing diamond from graphite.