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

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)0.4H0.6Hf2(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)0.4H0.6Hf2(PO4)3 (2.0 × 10–7 and 1.2 × 10–6 S/cm at 400 and 500°C, respectively).