  Monte-Carlo simulations and small-angle x-ray scattering experiments were
used to determine the phase diagram of aqueous dispersions of titratable
nano-colloids with a moderate size polydispersity over a broad range of
monovalent salt concentrations, 0.5 mM $\leq c_s \leq$ 50 mM and volume
fractions, $\phi$. Under slow and progressive increase in $\phi$, the
dispersions freeze into a face-centered-cubic (fcc) solid followed unexpectedly
by the formation of a body centered cubic (bcc) phase before to melt in a glass
forming liquid. The simulations are found to predict very well these
observations. They suggest that the stabilization of the bcc solid at the
expense of the fcc phase at high $\phi$ and $c_s$ results from the interaction
(charge) polydispersity and vibrational entropy.
