Cation miscibility in KNO3RbNO3 binary system. A combined SEM, EDX and XRD Rietveld analysis

• Phase equilibrium in the KNO3RbNO3 system was studied by XRD Rietveld method.
• SEM and EDX analysis were used to corroborate the XRD results.
• The QPA was used to quantify the amorphous and crystalline phases.
• The K and Rb miscibility in RbNO3 and KNO3 respectively was determined.
• The identified solid solutions domain boundaries were determined.

Phase relationships in the KNO3RbNO3 binary system at normal conditions were investigated by powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray analysis (EDX) techniques. The solubility limits in the solid state was determined. In the high potassium concentration domain, the solid solution K(1–x)RbxNO3 (denoted κ) crystallizes in the Pmcn orthorhombic symmetry were Rb+ may substitute K+ up to 52 mol %. For the solid solution Rb(1–y)KyNO3 (denoted ρ) in the rubidium rich zone the P31 trigonal symmetry is preferred and K+ may substitute Rb+ up to 24 mol %. Furthermore, the quantitative phase analysis (QPA) was performed for every sample leading to the amorphous and crystalline proportions. Refining the occupancy rates of every alkali cationic site in the solid solutions allows, not only the knowledge of the total cationic substitution rate in κ and in ρ phases but also the preferred crystallographic sites in every crystalline phase. These results were confirmed by microstructure investigations.