Chemical vapor transport as a means of controlling the composition of condensed phases

Abstract

A new method is proposed for controlling the composition (nonstoichiometry) of low-volatile inorganic compounds. The basic principle of the method is the introduction (or removal) of one of the components into the low-volatile compound using reversible selective chemical vapor transport (CVT). Theoretical analysis is used to identify the process parameters determining the direction of mass transport: source temperature T1, source composition x1, and sample temperature T2. Using nonequilibrium thermodynamics concepts, steadystate conditions are found under which mass transport ceases. A new type of phase diagram, x2-T1-T2, is proposed, which describes the state of CVT systems under steady-state conditions without mass transport. The CVT process is used to prepare GaSe crystals with different deviations from stoichiometry. The crystals are characterized using x-ray diffraction and cathodoluminescence spectroscopy. The stability regions of two GaSe polytypes in the T-x phase diagram are located. CVT is used to control the compositions of phases in the In-S system. The advantages of the CVT method are analyzed with application to control over the composition of inorganic compounds.