  The temperature dependence of the first-order phonon mode of single crystal
of Silicon (Si) is determined by Raman scattering in a broad temperature range
of 4-623 K. Our studies reveal the anomalous red-shift of the Raman active
phonon mode at temperature (~ 50 K) attributed to the anomalous expansion of Si
in the low temperature region. Silicon shows negative thermal expansion below
120 K, however, odd behaviour is also observed at very low temperatures i.e.,
softening of the Si crystal is detected below 40 K. This peculiar behaviour of
Si is described by the anomalous phonon anharmonicity observed at low
temperature.
