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

Using mechanical activation of appropriate Nd2O3 + HfO2 mixtures, followed by single-step high-temperature (1600-1700 degrees 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); Nd-2(Hf2-xNdx)O7-x/2 (x( )= 0, 0.1). After annealing at 1700 degrees C for 5 h, only the Nd-2(Hf2-xNdx)O-7(x = 0, 0.1) materials with the pyrochlore structure were single-phase, whereas after annealing at the lower temperature, 1600 degrees C, for 10 h only nominally stoichiometric Nd2Hf2O7 was free of impurities. The pyrochlore phases (Nd2-xHfx)Hf2O7+x/2 (x = 0.2), Nd2Hf2O7, Nd-2(Hf(2-x)Ln(x))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 Nd-2(Hf2-xNdx)O7-x/2 (x = 0.1) ceramic (with a density of similar to 95.1%) has been produced by high- temperature annealing at 1700 degrees C. Neodymium oxide segregation has been detected by SEM on polished sections of the (Nd-2(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 degrees C. For the first time, proton conductivity was found both in Nd2Hf2O7 (1.25 x 10(-6) S/cm at 700 degrees C) and Nd-2(Hf2-xNdx)O7-x/2 (x = 0.1) (similar to 1 x 10(-4) S/cm at 700 degrees C). The ionic conductivity maximum of Nd2Hf2O7 in dry air was similar to 1 x 10(-6) S/cm at 700 degrees C, which is almost an order of magnitude higher than was reported in the literature before.