A new graphical representation for a systematic study of the defect structure in ternary oxides, with a specific application to forsterite Mg 2SiO 4

Abstract

The defect structure of a ternary oxide such as forsterite that is responsible for departures from the ideal composition Mg 2SiO 4 depends on three thermodynamic parameters under a constant total pressure according to the Gibbs phase rule. These are the temperature T and two activities that may be chosen as the oxygen partial pressure P o 2 and the silica activity a SiO 2 Therefore the precise state of the compound can be described by a point in a 3 dimensional space, the coordinates of which are related to T, p o 2 , a SiO 2 . In a given region characterized by a majority defect pair, the overall electroneutrality equation simplifies and, provided that interaction between defects remains negligible, the use of mass action laws leads to a simple functional dependence of the defect concentrations on the three thermodynamic parameters. These analytical expressions define a range of existence for every selected majority defect pair, that can be easily depicted in isothermal sections of the 3 dimensional space. It is shown that the boundaries between these domains in Ln p o 2 - Ln a SiO 2 plots are straight lines intersecting at points where several majority defects coexist. Such representations permit a logical deduction of the different possible configurations and may be quite useful when analyzing experimental data. Their evolution with temperature is considered and finally the case of quaternary solid solutions is briefly discussed.