Synthesis and characterization of complex phosphates of alkaline elements, magnesium and zirconium as potential ceramics for nuclear waste immobilization

• Published in: Solid state sciences Vol. 149; p. 107481
• Main Authors: Pet'kov, V.I., Alekseev, A.A., Asabina, Е.А., Zanozin, I.D., Kulikova, E.V., Borovikova, Е.Y.
• Format: Journal Article
• Language: English
• Published: Elsevier Masson SAS 01-03-2024
• Subjects: Alkali metals; Chemical durability; Complex phosphates; Kosnarite; Langbeinite; Phase formation; Radwaste; Thermal expansion; Thermal expansion; Complex phosphates; Langbeinite; Alkali metals; Kosnarite; Radwaste; Chemical durability; Phase formation
• DOI: 10.1016/j.solidstatesciences.2024.107481
• EISSN: 1873-3085
• ISSN: 1293-2558

Phases of variable composition A2−хА'хMg0.5Zr1.5(PO4)3 (АA' – KNa, KRb, KCs, 0 ≤ x ≤ 2) with langbeinite and kosnarite structures can serve as matrices immobilizing radioactive waste. They were synthesized by sol-gel method followed by heat treatment and investigated by X-ray diffraction at different temperatures, IR spectroscopy, scanning electron microscopy and electron probe microanalysis. A Rietveld refinement of the KCsMg0.5Zr1.5(PO4)3 structure showed that the framework is formed by octahedra (Mg/Zr)O6 and tetrahedra PO4 connected by vertices; potassium and cesium cations are located in cavities. When the temperature changes, phosphates with the langbeinite structure expand uniformly in all directions: αа = αb = αc = (4.2−6.6) ∙ 10−6 °C−1, without undergoing polymorphic transitions. Phosphate Na2Mg0.5Zr1.5(PO4)3, which crystallizes in the kosnarite structure type, expands anisotropically: αа = −2.6·10−6, αс = 8.2·10−6, αav = 1.0·10−6 °C−1. The chemical durability test shows that the leaching rate of Cs ions from the solid solution with langbeinite structure is of the order of 10−5 g cm−2 d−1.
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•A2−хА'хMg0.5Zr1.5(PO4)3 (АA' – KNa, KRb, KCs, 0 ≤ x ≤ 2) phosphates were synthesized.•KCsMg0.5Zr1.5(PO4)3 structure is based on framework from octahedra and tetrahedra.•K2−хRbхMg0.5Zr1.5(PO4)3 phosphates (langbeinite) have isotropic thermal expansion.•Na2Mg0.5Zr1.5(PO4)3 (NZP) is a low-expanding substance with a significant anisotropy.