Thermodynamic properties of scandium, lanthanum, neodymium and gadolinium silicides and germanides

Abstract The Gibbs free energy, enthalpy and entropy of formation of the compounds ScSi, Sc 5 Si 3 , ScGe 2 , ScGe, Sc 11 Ge 10 , Sc 5 Ge 4 , Sc 5 Ge 3 , LaSi 2- x , LaGe 2- x , LaGe, La 5 Ge 4 , NdSi 2- x , NdGe 2- x , GdSi 2- x , GdSi 1.5 , GdSi, Gd 5 Si 4 , Gd 5 Si 3 , GdGe 2- x , GdGe 1.5 , GdGe, Gd 5 Ge 4 , Gd 5 Ge 3 were determined by measuring the e.m.f. of high-temperature galvanic cells. All the compounds investigated are thermodynamically stable. Their formation is followed by significant exothermic effects and a decrease in entropy. Absolute values of ΔG f o and ΔH f o increase in the sequence of silicides and germanides with increasing content of the rare earth element and reach a maximum for the compound R 5 X 3 . The thermodynamic stability of rare earth silicides and germanides is much greater compared with the appropriate compounds of 3d transition metals. Rare earth metal germanides are somewhat more stable than the corresponding silicides, whereas for 3d metals the inverse relationship is observed, silicides being more stable than germanides.