Thermochemical and structural study of sodium, iron, and niobium phosphate NASICON solid solutions

• Published in: Journal of solid state chemistry 2024-08, Vol.336, p.124742, Article 124742
• Main Authors: Koryttseva, A.К., Navrotsky, A., Beskrovnyi, A.I., Smetanina, K.E.
• Format: Article
• Language: English
• Subjects: NASICON; NbOPO4; Oxide melt solution calorimetry; Thermodynamics
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2024.124742

Solid solutions NaxFe0.5+0.5xNb1.5-0.5x(PO4)3 (0 ≤ x ≤ 1; x = 2) of NASICON structure (sp.gr. R3-c) and NbOPO4 were prepared via solid-state reaction and characterized by electron microprobe analysis, powder X-ray and neutron diffraction, and differential thermal analysis. The decrease of the a- and b-unit cell parameter prevails over the increase in c-parameters, resulting in nonlinear decrease of the unit cell volume with Na content. Enthalpies of drop solution in molten sodium molybdate solvent at 700 °C were measured by high temperature oxide melt solution calorimetry and were used to derive the enthalpies of formation. Strongly negative enthalpies of formation from elements and from constituent oxides indicate high thermodynamic stability. The mixing enthalpies of solid solutions show negative deviation from the ideality. Negative mixing enthalpies and negative mixing volumes indicate short-range ordering of cations in the structure. [Display omitted] •Solid solutions NaxFe0.5+0.5xNb1.5-0.5x(PO4)3 (0 ≤ x ≤ 1; x = 2) have NASICON structure.•High temperature oxide melt solution calorimetry in molten sodium molybdate was performed.•Enthalpies of formation from elements and oxides indicate high thermodynamic stability.•Negative mixing enthalpies and volumes suggest local cation ordering in the structure.