Phase equilibria and thermodynamics of solid state contact reactions in the ternary TaGaAs system

Abstract

The phase equilibria of the ternary TaGaAs system and the binary TaGa and TaAs systems were determined using X-ray diffraction of samples annealed at 800 °C and differential thermal analysis up to 1400 °C. In the binary TaGa system, peritectic melting of the compounds TaGa3 (520 °C) and Ta5Ga3 (1060 °C) was detected and a phase diagram is proposed which explains the conflicting literature data for phases in this system. The ternary isothermal section at 800 °C was determined and the two variant reactions L+TaAs⇋GaAs+TaGa3 at 455°C and L+Ga5Ga3⇋TaAs+TaGa2 at 908 °C were identified. The polythermal liquid+solid equilibria are developed schematically, comprising a total of six invariant reactions plus additional equilibria with the gas phase. Tantalum and the compounds TaGa2, Ta3Ga2, Ta5Ga3 and Ta3As are not in equilibrium with GaAs at any temperature. However, TaGa3, TaAs and TaAs2 are stable on GaAs at 1 bar up to 455 °C, 1150 °C and 950 °C respectively. These phases, especially TaAs, may be used to reduce the reactivity at a Ta-GaAs interface. Based on the ternary solid state equilibria the thermodynamic data are deduced and the thermodynamic driving forces of all possible competing reactions at the Ta-GaAs interface are calculated. These data support quantitatively the interpretation of the reactions observed in Ta/GaAs diffusion couples.