  Silicon is indispensable in semiconductor industry. Understanding its
high-temperature thermodynamic properties is essential both for theory and
applications. However, first-principle description of high-temperature
thermodynamic properties of silicon (thermal expansion coefficient and specific
heat) is still incomplete. Strong deviation of its specific heat at high
temperatures from the Dulong-Petit law suggests substantial contribution of
anharmonicity effects. We demonstrate, that anharmonicity is mostly due to two
transverse phonon modes, propagating in (111) and (100) directions, and can be
quantitatively described with formation of the certain type of nanostructured
planar defects of the crystal structure. Calculation of these defects'
formation energy enabled us to determine their input into the specific heat and
thermal expansion coefficient. This contribution turns out to be significantly
greater than the one calculated in quasi-harmonic approximation.
