The structure and symmetry of crystalline solid solutions: a general revision.

Mixed single crystals composed of host and guest organic molecules of similar structures and shapes are shown to comprise sectors with different host-guest distributions and to have symmetries lower than that of the host crystal. These properties are determined by the structure of the guest and the surface structures of the crystal faces through which the guest molecules are occluded. This general concept is illustrated by studies of three mixed crystal systems,(E)-cinnamamide-(E)-2-thienylacrylamide, (E)-cinnamamide-(E)-3-thienylacrylamide, and(S)-asparagine-(S)-aspartic acid, with x-ray and neutron diffraction and solid-state photochemistry.