BISMUTH telluride, bismuth selenide and antimony telluride are being actively studied as semiconductors for thermoelectric cooling¹. After purification by zone melting these materials do not exhibit intrinsic properties². C. B. Satterthwaite and R. W. Ure³ have shown by electrical measurements that in the case of bismuth telluride there is a slight difference between the composition corresponding to the maximum of the liquidus curve and the stoichiometric composition. Electrical properties are sensitive to physical imperfections and their interpretation in terms of chemical composition is thus open to criticism⁴.