Solid solutions KNaFeNb(PO) (0 ≤ x ≤ 1) are studied to determine the stability ranges of the NASICON and langbeinite structural types and to evaluate the mixing energetics. The materials under investigation may be used as cathodes for Na-ion batteries. Powder samples were obtained by a solid-state method and characterized by X-ray microanalysis, X-ray diffraction, and differential scanning calorimetry. Structural refinement was carried out by the Rietveld method using powder X-ray diffraction data at room temperature. It was established that phases isostructural to the mineral langbeinite (KMg(SO), space group P23) crystallize in the range 0 ≤ x ≤ 0.4, while phases isostructural to NASICON NaZr(PO) (space group R3-c) crystallize in the range 0.9 ≤ x ≤ 1. Standard enthalpies of formation were determined by high-temperature molten-salt calorimetry in a sodium molybdate melt (3NaO·4MoO) at 800°C using a Tian–Calvet-type isoperibolic differential calorimeter. The trends in the obtained energetic characteristics are discussed in relation to the structural evolutions across the indicated composition range of the solid solutions. This study expands current knowledge on the isomorphism of alkali cations in framework structures demonstrating possible pathways for changing properties within the investigated series.
A.K. Koryttseva (2025) studied this question.