  A binary mixture of particles interacting with spherically-symmetric
potentials leading to microsegregation is studied by theory and molecular
dynamics (MD) simulations. We consider spherical particles with equal diameters
and volume fractions. Motivated by the mixture of oppositely charged particles
with different adsorption preferences immersed in near-critical binary solvent,
we assume short-range attraction long-range repulsion for the interaction
between like particles, and short range repulsion long-range attraction for the
interaction between different ones. In order to predict structural and
thermodynamic properties of such complex mixtures, we develop a theory
combining the density functional and field-theoretic methods. We show that
concentration fluctuations in mesoscopic regions lead to a qualitative change
of the phase diagram compared to mean-field predictions. Both theory and MD
simulations show coexistence of a low-density disordered phase with a
high-density phase with alternating layers rich in the first and the second
component. The density and the degree of order of the ordered phase decrease
with increasing temperature, up to a temperature where the theory predicts a
narrow two-phase region with increasing density of both phases for increasing
temperature. MD simulations show that monocrystals of the solid and liquid
crystals have a prolate shape with the axis parallel to the direction of
concentration oscillations, and the deviation from the spherical shape
increases with increasing periodic order.
