Lattice Constants of Graphite at Low Temperatures

The lattice constants of a graphite single crystal, natural graphite powder, and artificial graphite powder have been measured at 297°K, 78°K, and 4.2°K. The $a$-spacing does not change in this temperature range within the error of ±0.0005 A. The material with the greatest $c$-spacing showed between 297° and 78° the greatest change of $c$-spacing and the smallest change below 78°. The $c$-spacing of the artificial powder did not return to its original value at 297° after a thermal cycle, demonstrating irreversible deformation.