## Info

(n

Pressure, kbar

400 600

Pressure, kbar

### Fig. 5.5 The equilibrium phase diagram for carbon

Example: At what pressure does graphite transform to diamond at room temperature? Diamond has a higher density (and consequently a lower molar volume) than graphite, and Av^g = -2x10- m /mole. Assuming that this quantity is independent of pressure, which is equivalent to assuming that the compressibilities of the two allotropes are the same, the above equation can be integrated to obtain the pressure at which the free energies of the two forms are the same:

Agd-g = 0 = Agd-g(1 atm) + (pd-g - 105)Avd-g where pd-g is the transformation pressure (in Pa) at 300 K, which is the temperature at which

Agd-g(1 atm) = 2880 J/mole and Avd-g = above equation yields:

Agd (latm)

•2x10- m /mole are specified. Using these values in the

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