Molecular interactions and packing in molecular alloys between nonisomorphous plastic phases

The experimental two-component phase diagram for the plastic crystals tris(hydroxymethyl)aminomethane (TRIS) and pentaerythritol (PE) has been established from 293 K to the liquid state using thermal analysis and X-ray powder diffraction at several temperatures, The miscibility and the intermolecular interactions of the mentioned system and other related binary systems formed from compounds which present also nonisomorphous plastic phases are discussed on the basis of the steric conditions (similarity of the unit-cell dimensions and of the size and shape of the molecules) and of the other factors governing the structural arrangements such as the effects of the orientational disorder. The evolution of the packing coefficient as a function of the composition in each two-component system is analyzed to attain further understanding of the intermolecular interactions in orientationally disordered pure and mixed crystals. (C) 1996 Academic Press, Inc.