 Solution

For three-dimensional acoustic modes, $\omega \propto q$ and the mode count in a spherical shell is proportional to $q^{2}dq$, giving phonon DOS proportional to ${\omega }^2$. Thermally occupied modes have $\hslash \omega$ of order $k_{B}T$, so $U_{la}-U_0\propto \int {\omega }^3{n}_B(\omega )d\omega \propto T^4$. Differentiation gives $C_{la}\propto T^3$. At $T\gg {\theta }_D$, all $3N$ harmonic modes are classically populated; each has mean energy $k_{B}T$ by equipartition. Thus the molar heat capacity tends to $3R$, the Dulong-Petit law. Anharmonic expansion and electronic contributions are not included in that lattice limit.