  We perform a comparative experimental and theoretical study of the
temperature dependence up to 700 K of the frequency and linewidths of the
graphite E1u and E2g optical phonons (~1590 and 1580 cm-1) by infra-red (IR)
and Raman spectroscopy. Despite their similar character, the temperature
dependence of the two modes is quite different, being, e.g., the frequency
shift of the IR-active E1u mode is almost twice as big as that of the Raman
active E2g mode. Ab initio calculations of the anharmonic properties are in
remarkable agreement with measurements and explain the observed behavior.
