New oxygen ion conductors have been prepared by substituting Ga3+ for Ge4+ in Nd4GeO8, Zn2+ and Mg2+ for Ga3+ in Nd3GaO6, and Ca2+ and Sr2+ for Nd3+ in Nd3GaO6.
A combustion technique using ethylenediamine tetraacetic acid has been
developed to synthesize these materials at ~900 °C, leading to powders
with spherical particles of about 100-200 nm. The green pellets obtained
through the combustion- synthesized powders could be sintered to
~98% at 1250 °C. It was found that Nd4Ge1-xGaxO8-x/2 are formed up to x = 0.10, Nd3Ga1-xMxO6-x/2 (M = Zn, Mg) up to x = 0.03, and Nd3(1-x) M′3xGaO6-3x/2
up to x = 0.03 and 0.015 for M′ = Ca and Sr, respectively. These
relatively small substitution rates induce a significant increase in
oxygen ion conductivity (σ800°C = 0.210-2 S cm-1 for Nd4Ge0.9Ga0.1O7.95; σ800°C = 0.5 10-2 and 0.4 10-2 S cm–1 for Nd3Ga0.97M0.03O5.985 with M) Zn and Mg, respectively, and σ800°C = 0.6 10-2 and 0.7 10-2 S cm–1 for Nd 2.91Ca0.09GaO5.955 and Nd2.955 Sr0.045 GaO5.9775, respectively) with respect to pure Nd4GeO8 (σ800°C = 2.8 10-4 S cm-1 ) and pure Nd3GaO6 (σ800°C = 2.7 10-4 Scm-1).