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X-ray diffraction data have been obtained on polycrystalline graphite at pressures up to 20 GPa. A phase transition is observed at 11 GPa, as evidenced by softening in the interlayer spacing and the observation of new diffraction lines. Below this pressure the variation of the lattice parameters a and c are compared with elastic stiffnesses obtained from ultrasonic measurements. A new value for C₁₃ is proposed. The variation c (P) is compared to the recently proposed universal isotherm equation.
Zhao et al. (Sat,) studied this question.