Thermodynamics of phase formation in the V-Ge system

Vaporization thermodynamics in the vanadium-rich portion of the vanadium-ge rmanium system has been studied by high temperature Knudsen effusion mass s pectrometry. Thermodynamic activities of V and Ge have been measured over t he two-phase mixtures of V+V3Ge and V3Ge+V5Ge3 in a temperature range of ca . 1750-1980 K, and Ge activity was measured over the V5Ge3+V11Ge8 mixture. The activities were measured by direct comparison with solid V and liquid G e metals. Thr standard free energies of formation of V3Ge and V5Ge3 at high temperature were determined from the activity data. The standard free ener gies of formation were then combined with published heat capacity and stand ard molar entropy data, and used in a third-law evaluation of the standard enthalpies of formation at 298 K of all the four intermediate compounds in this system. The results obtained are: 1/4V(3)Ge, -36.2+/-2.5 kJ: 1/8V(5)Ge (3), -45.9+/-2.5 kJ, 1/19V(11)Ge(8), -45.5+/-3.0 kJ; 1/48V(17)Ge(31), -35.0 +/-3.0 kJ. The results obtained in this work are compared with previous est imates and measurements. (C) 2000 Published by Elsevier Science S.A. All ri ghts reserved.