We have determined the Gibbs free energy, enthalpy, and entropy of formation for yttrium germanides by measuring the emf of high-temperature galvanic cells at 820-920 K.The phase diagram of the Y-Ge system indicates the existence of eight intermediate phases, of which only YsGe3 melts congruently at a temperature of 1965 °C [ 1 ]. The compounds YsGe4, Yi IGl0, YGe, YGe 1.5(YGe2_y), YGel.7(YGe 2_x), YGe 2, and YGe3.5(YGe3+c~ ) are formed according to peritectic reactions. The last two germanides in the list exist in narrow temperature intervals and undergo eutectoid decomposition with a reduction in temperature down to 710°C and 760°C, respectively. All the compounds are phases of constant composition. The solubility of germanium in yttrium is about 0.3%; the ~olubility of yttrium in germanium is significantly less [1]. In the literature, information is available about the thermodynamic properties of liquid alloys of the system Y-Ge [2-6], while there is practically no data on the thermodynamic properties of the compounds.The goal of this work was to study the thermodynamic characteristics of yttrium germanides by measuring the emf's relative to the electrodes of the galvanic cells [YSn2+ (Sn) lllKCI-NaCi-BaCi 2-YCia[YxGe)_ , (x'<0,40), [YGet.7 + GeI[KCI-NaCI-BaCl 2-YCIaIYrGe,-), (y>0,40).(1)(2)The thermodynamic characteristics of YGel. 7 and YGel. 5 were determined in the temperature interval 820-920 K when measuring the emf of cell 1, and the thermodynamic characteristics of the germanides YG-e, YltGel0, YsG4, and Y5Ge 3 were determined when measuring the emf of cell 2. As in studying the thermodynamic properties of compound., of other rare-earth metals with silicon and germanium [7], as the reference electrode we did not use pure yttrium but rather its alloy with tin (cell 1) or germanium (cell 2), the thermodynamic characteristics of which have been reliably determined.The alloys were prepared from crystalline germanium (grade GDG-1) of purity 99.99% and yttrium (It.M-l) of purity 99.98%. The melt was produced in an electric-arc furnace on a copper water-cooled hearth in argon medium. All the alloys were subjected to homogenizing annealing. The alloys containing less than 40 at. % Y were annealed at 750°C for 250 h, and samples containing a larger amount of yttrium were subjected to two-step annealing (100 h at 1300°C and 250 h at 750°C) in a medium of high-purity helium. The phase composition of the alloys was monitored using metallographic analysis.As the electrolyte, we used mixtures (premelted under a vacuum of -10 -2 Pa) of potassium, sodium, and barium chlorides (Trap = 815 K [8]). The potential-determining ions were. introduced into the electrolyte, holding the pure yttrium in the eutectic KC1-NaC1 melt for several hours in helium medium at a temperature of 750-800°C. The charge on the yttrium ions was assumed to be equal to +3 [9, 10]. We carefully monitored for adherence to the conditions required for reversible operation of the galvanic cells: constancy of the emf at a stable temperature, agreement o...