  We perform a rigorous study of the identical sphere packing problem in
$\mathbb{Z}^3$ and of phase transitions in the corresponding hard-core model.
The sphere diameter $D>0$ and the fugacity $u\gg 1$ are the varying parameters
of the model. We solve the sphere packing problem for values $D^2= 2, 3, 4, 5,
6, 8, 9, 10, 11, 12, 2\ell^2$, $\ell\in\mathbb{N}$. For values $D^2=2, 3, 5, 8,
9, 10, 12, 2\ell^2$, $\ell\in\mathbb{N}$ and $u>u^0(D)$ we establish the
diagram of periodic pure phases, completely or partially. For the case
$D^2=2\ell^2$, $\ell\in\mathbb{N}$ we use results from Hales' proof of Kepler's
conjecture.
