Synthesis and ionic conductivity of (NH4)1–xHxHf2(PO4)3 (x = 0–1) NASICON-type materials

Abstract

Methods have been proposed for the preparation of NASICON-type hafnium hydrogen phosphate- based materials. The pH of the starting solution for the hydrothermal synthesis of NH₄Hf₂(PO₄)₃ has been shown to determine whether rhombohedral or cubic NH₄Hf₂(PO₄)₃ will be obtained. The thermal decomposition of rhombohedral NH₄Hf₂(PO₄)₃ leads to the formation of the triclinic phosphate HHf₂(PO₄)₃, whereas the decomposition of cubic NH₄Hf₂(PO₄)₃ yields a cubic phosphate with the composition (NH₄)_0.4H_0.6Hf₂(PO₄)₃. HHf₂(PO₄)₃ cannot be prepared from cubic NH₄Hf₂(PO₄)₃, because the temperature of water elimination coincides with that of the elimination of the last portions of ammonia. We have studied the morphology, thermal stability, and ionic conductivity of the synthesized materials. The electrical conductivity of cubic NH₄Hf₂(PO₄)₃ has been shown to exceed that of the rhombohedral phase, and the conductivity of the hydrogen forms slightly exceeds that of individual ammonium forms of hafnium phosphate. The highest conductivity among the materials studied here is offered by cubic (NH₄)_0.4H_0.6Hf₂(PO₄)₃ (2.0 × 10^–7 and 1.2 × 10^–6 S/cm at 400 and 500°C, respectively).