Fusion curves and polymorphic transitions of the group III elements—aluminum, gallium, indium and thallium—at high pressures

Abstract

The fusion curves of aluminum and thallium to 50 kbars and of gallium to 70 kbars, were investigated by differential thermal analysis. Comparison is made with previous work and with the fusion curve of indium. A break in slope of the melting curve of gallium near 45 °C and 30 kbars led to the discovery of a new polymorph. This phase—Ga III—is slightly less dense than Ga II, both persisting to 75 kbars. Solid-solid phase boundaries for gallium and thallium were determined by changes in volume and resistance as well as with DTA. The Ga II to Ga III transition is rapid and characterized by a substantial heat as well as a drop in resistance. From the investigation of the metastability phenomena, it is tentatively concluded that Ga III is identical with Bridgman's Ga II' and that the metastable phase studied by Defrain at 1 atm. is closely related to Ga II. The existence of three polymorphs of thallium is definitely established, although the undercooling noted by Ponyatovskii for the b.c.c.-h.c.p. transition was verified. The triple point for the solid thallium phases is near 115°C and 39 kbars, with melting of the high pressure polymorph expected above 650°C and 90 kbars.