Structure evolution, ionic and proton conductivity of solid solutions based on Nd2Hf2O7

Abstract

Using mechanical activation of appropriate Nd2O3 + HfO2 mixtures, followed by single-step high-temperature (1600–1700 °C) annealing of green compacts, we have synthesized pyrochlore solid solutions in an isomorphous miscibility range around the nominally stoichiometric composition Nd2Hf2O7: (Nd2-xHfx)Hf2O7+x/2 (x = 0.2, 0.32, 0.39); Nd2(Hf2-xNdx)O7-x/2 (x = 0, 0.1).After annealing at 1700 °C for 5 h, only the Nd2(Hf2-xNdx)O7-x/2 (x = 0, 0.1) materials with the pyrochlore structure were single-phase, whereas after annealing at the lower temperature, 1600 °C, for 10 h only nominally stoichiometric Nd2Hf2O7 was free of impurities. The pyrochlore phases (Nd2-xHfx)Hf2O7+x/2 (x = 0.2), Nd2Hf2O7, Nd2(Hf2-xLnx)O7-x/2 (x = 0.1) have been characterized by Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and impedance spectroscopy. The order–disorder (pyrochlore–fluorite) transition in the Nd2Hf2O7–Nd2O3 isomorphous miscibility range has been studied by Raman spectroscopy and XRD. The two-phase (Nd2-xHfx)Hf2O7+x/2 (x = 0.2) material has been shown to have the brightest luminescence.A gas-tight Nd2(Hf2-xNdx)O7-x/2 (x = 0.1) ceramic (with a density of ~95.1%) has been produced by high-temperature annealing at 1700 °C. Neodymium oxide segregation has been detected by SEM on polished sections of the (Nd2(Hf2-xNdx)O7-x/2 (x = 0, 0.1) ceramics in air and attributed to Nd3+ diffusion from the hafnates at a thermal etching temperature of 1450 °C.For the first time, proton conductivity was found both in Nd2Hf2O7 (1.25 × 10−6 S/cm at 700 °C) and Nd2(Hf2-xNdx)O7-x/2 (x = 0.1) (~1 × 10−4 S/cm at 700 °C). The ionic conductivity maximum of Nd2Hf2O7 in dry air was ~1 × 10−6 S/cm at 700 °C, which is almost an order of magnitude higher than was reported in the literature before.