X-ray diffraction data for graphite to 20 GPa

X-ray diffraction data have been obtained on polycrystalline graphite at pressures up to 20 GPa. A phase transition is observed at \ensuremath{\sim}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 ${C}_{13}$ is proposed. The variation c(P) is compared to the recently proposed universal isotherm equation.