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

1,2,4,5‐Tetrachlorobenzene (TeCB) exhibits a phase transition α → β at 185 K whereas 1,2,4,5‐tetrabromobenzene (TeBB) exhibits a phase transition β → γ at 307 K, the `room temperature' form being β 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 ≤ x ≤ 0.75 and 0.81 ≤ x ≤ 0.97 ranges where these molecular alloys are only metastable because of demixing phenomena. Powder diffraction analysis on the whole range of molecular alloys proved that β TeCB is isomorphous in the strictest sense ( i.e. capable of giving a continuous series of mixed crystals) with the γ form of TeBB.