  We report on the phase behavior of an aqueous binary charged sphere
suspension under exhaustively deionized conditions as a function of number
fraction of small particles p and total number density n. The mixture of size
ratio 0.557 displays a complex phase diagram. Formation of bcc crystals with no
compositional order dominates. We observe a region of drastically decreased
crystal stability at 0.55 < p < 0.95 with the minimum located at p = 0.8 +/-
0.05 at densities above n = 26um-3. A peaked region of enhanced stability is
observed at p = 0.4. Further light scattering experiments were conducted to
characterize the crystallization time scales, the density profiles and the
composition of formed phases. For 0.82 > p > 0.95 crystal formation is
partially assisted by gravity, i.e. gravitational separation of the two species
precedes crystal formation samples in the coexistence range. In the composition
range corresponding to the decreased crystal stability only lower bounds of the
freezing and melting line are obtained, but the general shape of the phase
diagram is retained. At p = 0.93 and n = 43um-3 two different crystalline
phases coexist in the bulk, while at p = 0.4 additional Bragg peaks appear in
the static light scattering experiments. This strongly suggests that we observe
a eutectic in the region of decreased stability, while the enhanced stability
at p = 0.4 seems to correlate with compound formation.
