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\ifmmode^\circ\else\textdegree\fi{}K, 78\ifmmode^\circ\else\textdegree\fi{}K, and 4.2\ifmmode^\circ\else\textdegree\fi{}K. The $a$-spacing does not change in this temperature range within the error of \ifmmode\pm\else\textpm\fi{}0.0005 A. The material with the greatest $c$-spacing showed between 297\ifmmode^\circ\else\textdegree\fi{} and 78\ifmmode^\circ\else\textdegree\fi{} the greatest change of $c$-spacing and the smallest change below 78\ifmmode^\circ\else\textdegree\fi{}. The $c$-spacing of the artificial powder did not return to its original value at 297\ifmmode^\circ\else\textdegree\fi{} after a thermal cycle, demonstrating irreversible deformation.