Composition Change of Binary Compounds Under Conditions of Equilibrium and Vacuum Vaporization

The composition of binary compounds changes as a function of temperature when in equilibrium with the vapor. This change, for the IIB—VIA and IA—VIIA compounds is small, within the limits of solubility. The equilibrium vacancy concentrations are controlled by their free energies of formation. The composition changes in a different manner when the solid is heated in vacuum. Vacuum evaporation rates are controlled by the activation free energy of a surface reaction which, under proper conditions, may also control the bulk concentration of vacancies. Experimental verification of this effect is proposed. It has been shown that closed system vapor-pressure measurements are extremely sensitive to minute changes of the solid composition. The measured apparent vapor pressure is a function of the reaction-chamber volume for composition changes ≥2×10−8 mole/cm3. In turn, the sensitivity of closed-system vapor-pressure measurements may be used for accurate determination of off stoichiometry.