Enhancement of Hf4+ Ion Conductivity in a NASICON-Type Solid

Tetravalent hafnium ion conducting solid electrolytes with a NASICON-type structure, HfNbP3-xVxO12 and Hf1-y/4NbP3-yWyO12, were developed by partially replacing the P5+ site in a HfNb(PO4)(3) solid with larger V5+ or W6+ ions. Although the Hf4+ ion conductivity monotonically increased with increasing V content in the HfNbP3-xVxO12 solids, the electronic conduction appeared to be due to the valence change of V5+. In contrast, the Hf1-y/4NbP3-yWyO12 series maintained a high ionic transference number, above 0.995 for single-phase NASICON-type samples (y <= 0.15). The optimum lattice volume for Hf4+ conduction was determined to be 1.482 nm(3), which was obtained for samples with x = 0.1 or y = 0.05. The Hf395/400NbP2.95W0.O-05(12), solid, which contains the higher valence W6+ ion rather than the V5+ ion, showed the highest conductivity of 2.8 x 10(-4)S cm(-1) at 600 degrees C, approximately 2.5 times that of HfNb(PO4)(3).