Direct synthesis calorimetry of some binary alloys in the systems SiAs, GeAs and SnAs

Abstract

Abstract The standard enthalpies of formation of SiAs, SiAs 2 , GeAs, GeAs 2 and SnAs have been determined by direct synthesis drop calorimetry in fused silica capsules at 1123 K (for SiAs), 1058 K (for GeAs) and 987 K (for SnAs). The following values of ΔH f o (298) (kJ mol atoms −1 ) are reported: SiAs, −5.4 ± 1.2; SiAs 2 , −3.7 ± 2.3; GeAs, −5.5 ± 2.6; GeAs 2 , −5.8 ± 3.2; SnAs, −9.5 ± 3.2. Using our measured values for the heats of reaction of the processes a As(s) + b Me(s) = c As x Me 1− x ( l ) at 1058 K for Ge and 987 K for Sn, we have calculated the liquid-liquid enthalpies of mixing for some alloys in these two systems. Adopting the sub-regular solution model, we have calculated the following approximate expressions for the enthalpies of mixing (kJ mol −1 ): ΔH 1−1 M (Ge-As, 1058 K) = (−9.86 – 2.77 x As ) x As x Ge ΔH 1−1 M ( Sn - As , 987 K ) = (−16.87 + 8.90 x As ) x As x Sn The calorimetric values for the solid and liquid alloys are compared with predicted values from Miedema's semi-empirical model. It is found that all of the predicted values are more than one order of magnitude more exothermic than our experimental results.