Manometric method for the study of P-T-X diagrams

A new null-manometric method has been elaborated and applied for investigations fo the systems with the low-volatile compounds. The essence of this method is in obtaining the phase diagram for low-volatile binary compounds using the temperature dependence of equilibrium nonsaturated vapor pressure, formed by a third (auxiliary) component, which is put in contact with the investigated system. The aim of applying the third component assumes its reversible chemical interaction with one of the components of a weakly volatile compound. This compound component is converted into vapor (in the form of a compound with the auxiliary component) and the composition of a condensed part of the system is changed. Controlling equilibrium with temperature adjustment makes possible control of the disappearance of some of the condensed phases and formation of the others. Changes in equilibrium when employing this process are clearly observed in the temperature dependence of the vapor pressure. From the data, it is possible to trace the correlation between the total vapor pressure and the composition of binary condensed phases, as well as the correlation between the total and partial pressures. The overall results allow determination of the phase diagram. In the present work, the phase diagrams of the Ga-Se and Ga-S systems have been investigated using iodine and chlorine as third components (in the initial form of GaHal₃). It has been shown that at temperatures up to ～1080 K the homogeneity region for GaSe is 0.12±0.04 at.%, and it is completely shifted toward an excess of gallium. The boundary of the phase homogeneity region from selenium is close to stoichiometric composition. Gallium monosulfide is characterized by a more narrow range of homogeneity commensurable with the error of determination of the composition in the performed experiments (0.04 at.%). The temperature dependencies of equilibrium constants K _p and the values of gallium partial vapor pressure over solid gallium sulfides and selenides were calculated. For the equilibrium involving solid GaSe, solid Ga₂Se₃, and their vapor, the calculated partial vapor of gallium was shown to be independent of the nature of the third component.