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 be ta for the two compounds. This work establishes which are the isomorph ous forms. Quenching the molten mixtures into Liquid nitrogen produced molecular alloys (TeCB)(1-x)(TeBB)(x) at 295 K with any value of x, e ven 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 phenome na. Powder diffraction analysis on the whale range of molecular alloys proved that beta TeCB is isomorphous in the strictest sense (i.e. cap able of giving a continuous series of mixed crystals) with the gamma f orm of TeBB.