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Hydrogen zirconium phosphate (HZP) is a ceramic material with the formula HZr 2 (PO 4) 3 that can potentially offer anhydrous protonic conductivity, with thermostability for the intrinsic protons up to 650 °C, and good chemical stability in aqueous environments or acid gases. Despite the attraction of these features for electrolyte applications, such as gas separation, fuel cells or electrolysers, work on this material remains in its infancy. A possible reason for this lack of attention may be related to the complexity to obtain the HZP material as a pure phase. For this reason, the current article examines a new hydrothermal synthesis method to produce the ammonium precursor, NH 4 Zr 2 (PO 4) 3 , in its phase pure form, and consequently a refined overall route to produce the desired HZP target material. In the synthesis of the ammonium precursor, we underscore the precise control of reaction conditions that are necessary to obtain the pure NH 4 Zr 2 (PO 4) 3 material, both in its cubic and hexagonal structural polymorphs. For reaction times of 10 h, the cubic phase is obtained by employing a hydrothermal temperature of 150 °C and an aqueous ammonia volume of 2.1 mL, while the hexagonal phase is obtained for a hydrothermal temperature of 200 °C and an aqueous ammonia volume between 2.3 and 2.4 mL. Subsequently, we investigate the required conditions to convert hexagonal NH 4 Zr 2 (PO 4) 3 fully to hexagonal HZr 2 (PO 4) 3 via thermal treatment at 600 °C for 5 h, assessed by a combination of X-ray diffraction and infrared spectroscopy. In this way this article aims to open up further research on this interesting HZP material by documenting a robust route for its preparation. [ABSTRACT FROM AUTHOR]

Sodium zirconium phosphate (NaZr 2 (PO 4) 3 , NZP) type phosphate compounds have been considered as a candidate material for the immobilization of radionuclides. In this work, the highly densified NZP-type ceramic waste forms for immobilizing simulated radionuclides Sr and Cs, which were designed as the formula of Cs 1-2 x Sr x Zr 2 (PO 4) 3 (x = 0, 1/12, 2/12, 3/12, 4/12, 5/12, and 6/12), were prepared by microwave-assisted solid-state sintering method. The effects of Sr and Cs incorporation on the phase composition, microstructure, densification, and chemical durability of Cs 1-2x Sr x Zr 2 (PO 4) 3 ceramic waste forms were systematically discussed. It was shown that the single CsZr 2 (PO 4) 3 (CsZP) phase was generated in the samples when x ≤ 2/12, while two phases of CsZP and Sr 0.5 Zr 2 (PO 4) 3 (SrZP) were formed when 3/12 ≤ x ≤ 5/12. The Rietveld refinement results revealed that Sr/Cs could be incorporated in the NZP crystal structure. The as-prepared samples all presented a well dense microstructure, whose relative density reached up to approximately 98% with Sr incorporation. In addition, the Product Consistency Test (PCT) leaching results demonstrated that the ceramics waste forms simultaneously immobilizing Sr and Cs exhibited superior leaching resistance, and the leaching rates of Sr and Cs elements were in the order of 10−3-10−4 g m−2 d−1. The increase of Sr incorporation brought about the decreased leaching rates of ceramic samples. [ABSTRACT FROM AUTHOR]