Hydrothermal preparation of the low-expansion NZP family of materials

This paper reports on the synthesis of low thermal expansion NaZr₂(PO₄)₃ (NZP) and CaZr₄(PO₄)₆ (CZP) families of materials and their analogues for each of the alkali and alkaline earth metals by the hydrothermal method. Starting gels were prepared by mixing stoichiometric proportions of alkali and alkaline earth metal chlorides of nitrates with a 0·5 m ZrOCl₂ solution and by titrating this mixture with a stoichiometric amount of 1·0 m H₃PO₄ while the solution was constantly stirred. The gels were dried to remove the HCl produced during the reaction between ZrOCl₂ and H₃PO₄, and were then heated with water in teflon-lined Parr bombs or gold capsules. In general, the hydrothermal method yielded 100-300 nm sized rhombohedral single crystals of different NZP materials at 175-200°C under saturated steam pressure conditions. The initial pH and the temperature of treatment are the most critical factors in the hydrothermal synthesis of these technologically important low expansion materials.