  Lattice mismatch can substantially impact the spatial organization of
heterogeneous materials. We examine a simple model for lattice-mismatched
solids over a broad range of temperature and composition, revealing both
uniform and spatially modulated phases. Scenarios for coexistence among them
are unconventional due to the extensive mechanical cost of segregation.
Together with an adapted Maxwell construction for elastic phase separation,
mean field theory predicts a phase diagram that captures key low-temperature
features of Monte Carlo simulations.
