Solid solutions of isonicotinamide 2 :[fumaric acid (x) succinic acid (1-x) ] (IN 2 :[FA x SA (1-x) ] 1 ) co-crystals were prepared. The system is anchored by two isostructural bimolecular co-crystals with compositions IN 2 SA 1 and IN 2 FA 1 , respectively. The IN 2 :[FA x SA (1-x) ] 1 solid solutions of the co-crystals were formed using both solid-state grinding and solution crystallization. The solid solutions were found to be distinct from physical mixtures of IN 2 SA 1 and IN 2 FA 1 by analysis using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). DSC analysis of the solid solutions showed a linear interpolation of the melting point between the binary co-crystals whereas mixtures of the two separate co-crystals showed melting of the individual phases. PXRD was consistent with linear volume expansion of the lattice parameters without a concomitant change in the full width at half-maximum for the diffraction peaks for the solid solutions, but peak broadening and bimodality were observed for physical mixtures. Both techniques provided independent evidence that the solid solutions were a single phase and not physical mixtures of the two binary co-crystals.

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