Phase Diagram Evaluations: Section II The Ge-Na (Germanium-Sodium) System J. Sangster* and A.D. Pelton Ecole Polytechnique de Montreal Equilibrium Diagram on a graph. These are plotted in Fig: 1. The line shown in Fig. 1 is from the following equation [72Hub]: The complete equilibrium diagram has been reported in only one study [82Dri]. Thermal analysis was used over the compo- sition range from 0 to 76 at.% Na. Two compounds were re- ported, GeNa melting congruently at 1160 ~ and GeNa 3 melting congruently at 820 ~ Eutectics were reported at 930 ~ (~2.5 at.% Na), 765 ~ (~75 at.% Na), and at -97.8 ~ The stoichiometry of GeNa was confirmed by x-ray analysis. The existence ofGeNa 3 was inferred solely from the thermal analy- sis. However, phase diagrams of the Ge-Rb and Ge-Cs sys- tems, reported in the same article, are at variance with other studies of these systems, often by several hundred degrees [97Sanl, 97San2]. The compound GeNa s has not been ob- served in any other study. In fact, none of the compounds re- ported by [82Dri] in the Ge-Na, Ge-K, Ge-Rb, and Ge-Cs systems at alkali contents above 50 at.% have been observed by other authors. The reported liquidus of GeNa descends much less steeply on the Ge-rich side (from l 160 ~ at 50 at.% Na to 930 ~ at 2.5 at.% Na) than it does on the Na-rich side (from 1160 ~ at 50 at.% Na to 765 ~ at 65 at.% Na). This could only be the case if the liquid exhibited very strong nega- tive deviations from ideal mixing behavior on the Na-rich side, with strong positive deviations on the Ge-rich side or if there were extensive solid solubility of Ge in GeNa. Evidence from the Ge-Rb and Ge-Cs systems [97Sanl, 97San2] suggests that deviations from ideal behavior are slightly to moderately posi- tive in alkali-rich and slightly to moderately negative in Ge- rich solutions, and there is no evidence of extensive nonstoichiometry in any alkali-Ge compounds. For these rea- sons, the results of [82Dri] are not accepted in the present as- sessment. The solubility of Ge in liquid Na has been measured by emf techniques [72Hub] and by electrical resistivity measurements and thermal analysis [74Hub]. In the earlier study [72Hub], Na of 99.9% purity and Ge of unspecified purity were used. Con- centration cells employed pure liquid Na reference electrodes and Pyrex (Coming Glass Works, Coming, NY) glass electro- lytes with the working electrode compartment containing a two-phase mixture of saturated liquid Na plus solid. The emf was measured as a function of temperature in order to measure the activity of Na at saturation. Under the assumption that Raoult's Law applies in these solutions, which are concen- trated in Na, the phase boundary was calculated. In the second study [74Hub], thermal analysis and resistivity measurements were performed simultaneously under argon as the tempera- ture was varied. Ge of 99.999% purity and Na of 99.9% purity were used. X-ray diffraction of the solid phase after cooling to room temperature showed that it was GeNa. The total temperature range covered is 230 to 525 ~ In both studies [72Hub, 74Hub], the data were reported only as points 2101 logl0(at.%Ge) = 3.1539 - ~ (Eq 1) T(K) By extrapolating Eq 1 to the melting point of Na (97.8 ~ a eutectic liquid composition of -3.1 • 10-3 at.% Ge is calcu- lated. Under the assumption of negligible solubility of Ge in solid Na, the eutectic temperature may be calculated thermo- dynamically, from the enthalpy of fusion of Na, to lie approxi- mately 0.01 ~ below the melting point of Na. From these solubility measurements, from evidence presented below concerning the compounds, and by analogy with the Ge-Rb and Ge-Cs systems, the following description of the Ge-K equilibrium diagram is tentatively proposed. There are two compounds, Ge136Nax(0 < x < 24) and GeNa. The former probably melts peritectically at a temperature between 550 ~ and about 800 ~ while the latter probably melts, either peritectically or congruently, in approximately the same tem- perature range. The liquidus above 50 at.% Na is probably *Permanent address:Sangster Research Laboratories, Suite 402, 3475 rue de la Montagne, Montrral, Qu6bec, Canada, H3G 2A4. o 0.5- 0- -0.5 - -1.0- 1.2 114 ;5 i'6 1000/T(K) x x ;8 ;9' 2.0 Fig. 1 Solubility of Ge in liquid Na. Composition of liquid in equilibrium with GeNa. Journal of Phase Equilibria Vol. 18 No. 3 1997 295