Isomorphic relationship between 1,2,4,5-tetrachlorobenzene and 1,2,4,5-tetrabromobenzene

1,2,4,5-Tetrachlorabenzene (TeCB) exhibits a phase transition alpha-->beta at 185K whereas 1,2,4,5-tetrabromobenzene (TeBB) exhibits a phase transition beta-->gamma at 307K, the 'room temperature' form being beta for the two compounds. This work establishes which are the isomorphous forms. Quenching the molten mixtures into Liquid nitrogen produced molecular alloys (TeCB)(1-x)(TeBB)(x) at 295 K with any value of x, even in the 0.20 less than or equal to x less than or equal to 0.75 and 0.81 less than or equal to x less than or equal to 0.97 ranges where these molecular alloys are only metastable because of demixing phenomena. Powder diffraction analysis on the whale range of molecular alloys proved that beta TeCB is isomorphous in the strictest sense (i.e. capable of giving a continuous series of mixed crystals) with the gamma form of TeBB.