Vaporization thermodynamics and heat capacities of Cr3Ge and Cr5Ge3

Thermodynamic stabilities of two chromium germanides Cr3Ge and Cr5Ge3 have been studied by means of high-temperature Knudsen effusion mass spectrometr y (KEMS) and differential scanning calorimetry (DSC). Thermodynamic activit ies of Cr and Ge have been measured over the Cr3Ge single phase and over tw o-phase mixtures of (Cr) solid solution+Cr3Ge and Cr3Ge+Cr5Ge3 in a tempera ture range of approximately 1400-1600 K by direct comparison with solid Cr and liquid Ge metals. The standard free energies of formation of the chromi um germanides at high temperature were determined from the activity data. H eat capacities of Cr0.75Ge0.25 and Cr0.63Ge0.37 were measured by DSC in the temperature range of 300-700 K. Standard enthalpies of formation of Cr0.75 Ge0.25 and Cro(0.63)Ge(0.37) at 298 K were calculated by the third-law meth od with the use of literature absolute entropy and estimated high-temperatu re heat capacity data. The results obtained are: Cr0.75Ge0.25, -19.3 +/- 2. 5 kJ; Cr0.63Ge0.37 -12.3 +/- 2.5 kJ. The results are compared with previous ly reported measurements. (C) 2001 Elsevier Science B.V. All rights reserve d.