SOLID-STATE PHASE-EQUILIBRIA IN THE NI-AL-AS SYSTEM

Solid state phase equilibria in the ternary Ni-Al-As diagram were esta blished at 800 degrees C. The experimental techniques used to elaborat e the phase equilibria were X-ray diffraction (XRD), electron probe mi croanalysis (EPMA) and scanning electron microscopy (SEM). Three terna ry phases, which crystallize in hexagonal symmetry and are all structu rally derived from the NiAs type, were evidenced in the Ni-rich part o f the diagram. Among them, two ternaries labelled as A and D phases, b y comparison with the isostructural ternary phases in the Ni-Ga-As dia gram, reveal fully disordered structures; in contrast, the ternary B p hase shows a hexagonal superlattice (a root 3, 3c), which denotes an o rdered structure. Very limited solid solubilities were measured in the binary constituent Ni-Al and Ni-As compounds, with the exception of N iAs which showed a homogeneity range with an Al-rich limit correspondi ng to the formula NiAs0.7Al0.3. Neither nickel nor ternary phases are in thermodynamic equilibrium with AlAs, in contrast with the binaries NiAl, Ni2Al3 and NiAs which are. The bottom part of the experimental d iagram differs from the theoretical one deduced from estimated thermod ynamic data on Ni-Al and Ni-As binaries using Miedema's model, but is in agreement with interfacial reaction studies which concluded that Ni Al and substituted NiAs are the stable phases when Ni thin films are r eacted to completion on AlAs. Finally, the influence of the IIIa eleme nt (Ga or Al) in the Ni-IIIa-As systems was considered through a compa rative study between Ni-Ga-As and Ni-AI-As diagrams. The pseudoternary Ni-(Ga,AI)-As system was experimentally estimated for the atomic comp osition (Ga0.7Al0.3), which is the value usually chosen for the elabor ation of GaAs/(Ga,Al)As heterostructures with Ga1-xAlxAs exhibiting a direct gap of highest energy.