The effect of pressure to several hundred kilobars has been measured on the lattice parameters of diamond, graphite, and hexagonal boron nitride. The linear compressibility of diamond is independent of pressure. Theaaxis of graphite (characteristic distance in the plane) is considerably more compressible than diamond at low pressure but above about 160 kbar is much less compressible than diamond. Thecaxis of graphite is much more compressible than theaaxis. Assuming van der Waals forces between the graphite planes and using the lattice sums of Girifalco and Lad, it is possible to describe the compressibility of graphite to very high pressure from lowhyphen;pressure data. Hexagonal BN is much like graphite, with bothaandcaxes somewhat more compressible than the corresponding parameters of graphite.
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