Preparation and properties of Dion–Jacobson-type perovskite CsA2Nb3O10 (A = Ca, Sr, Ba) crystals and ceramics by the molten salt method

Abstract

Dion–Jacobson-type (DJ-type) perovskite CsA₂Nb₃O₁₀ (A = Ca, Sr, Ba) is a compound that can simultaneously accommodate Cs⁺ and A²⁺ ions and thus considered as a potential immobilization matrix for radionuclides ¹³⁷Cs and ⁹⁰Sr. In this work, using A²⁺ to simulate bivalent alkaline earth metals, CsA₂Nb₃O₁₀ single crystals were synthesized by the molten salt method. CsA₂Nb₃O₁₀ has good thermal stability up to 900°C. At the same time, CsA₂Nb₃O₁₀ has strong radiation resistance, with most crystals maintain their initial morphology with slight color change. Cations with smaller radii, for example, Na⁺, K⁺, and Rb⁺, do not replace Cs⁺ in the interlayer of CsA₂Nb₃O₁₀ and A²⁺ in the NbO₆ polyhedron. CsA₂Nb₃O₁₀ demonstrates a wide range of pH durability, where its frame structure remains stable in the pH range from 2 to 12 and will not be decomposed, while Cs⁺ in CsA₂Nb₃O₁₀ loses more into the solution at the pH range from 2 to 4 and from 10 to 12, and the loss of Sr²⁺ and Ba²⁺ is much lower than that of Ca²⁺ due to slight differences in the framework structure. Further sintering of CsCa₂Nb₃O₁₀ and CsSr₂Nb₃O₁₀ result in high density ceramic with a relative density of 94%–97%, along with improved chemical stability. According to the investigation of the surface morphology and elemental chemical state, it can be confirmed that Cs, A, and Nb elements are uniformly distributed on the surface of ceramics. The normalized leaching rates of Cs, Ca, and Sr in the CsCa₂Nb₃O₁₀ and CsSr₂Nb₃O₁₀ ceramic waste forms are about 1, 1, and 10⁻⁴ to 10⁻³ g·m⁻²·d⁻¹ after 28 days, respectively.