Composition Control of Low-Volatility Solids Through Chemical Vapor Transport Reactions. I. Theory of Selective Chemical Vapor Transport

A new method is proposed for controlling the composition (nonstoichiometry) of low-volatility inorganic compounds. It is based on the introduction/elimination of one of the components into/ from the low-volatility compound using reversible selective chemical vapor transport. The conditions for composition control through selective chemical vapor transport are deduced from the principles of nonequilibrium thermodynamics, in which the direction of the mass-transfer of a component is unambiguously defined by the temperature and composition of the source (T1, x1) and of the sample (T2, x2). This approach can be employed to control the sample composition in a closed vapor-transport system, in which composition x2 of the sample is defined by the fixed values of x1, T1, and T2, provided the steady-state (no mass-transfer) condition is achieved. The effect of steady-state conditions on sample composition is visualized using T2-T1-x2 diagram for the following system: sample (Ga-S phases)-vapor-charge (pure Ga).