Microstructure and impedance spectroscopy of high density holmium hafnate (Ho2Hf2O7) from nanoparticulate compacts

Nanoparticles of holmium hafnate (Ho2Hf2O7) synthesised employing an environmentally sustainable Leeds Alginate Process (LAP) were compacted into pellets and sintered using conventional single step sintering (SSS) technique for 2 h over the temperature range 1100 °C to 1500 °C in the interval of 100 °C and with unconventional two-step sintering (TSS) technique at (I) −1500 °C for 5 min followed by (II) – 1300 °C for 96 h. The relative density of the pellets after sintering was calculated using the Archimedes’ principle and crystal structure data was used to determine the theoretical density. Grain size was measured using ImageJ from SEM micrographs. The pellets processed by the TSS process have been found to be denser (98%) with lesser grain growth (0.77 μm) in comparison with those processed using the SSS process. Ionic conductivities of Ho2Hf2O7 pellets sintered by the two techniques were measured using ac-impedance spectroscopy over the temperature range of 340 °C to 750 °C in the frequency range of 100 mHz to 100 MHz for both heating and cooling cycles. The bulk (1.0 × 10−4 S.cm−1) and grain boundary (4.66 × 10−5 S.cm−1) conductivities of Ho2Hf2O7 at 700 °C prepared by SSS process is slightly higher than those processed by TSS process. The value of grain boundary activation energy is lower in the case of TSS process than that of SSS process. The results of this study indicates that the ionic conductivity of holmium hafnate pellets is not much affected by the sintering conditions but the TSS process is the better route for processing the holmium hafnate since the pellet could be sintered to remarkably high densities at lower temperature and exhibited lesser grain growth.