Cuspidine-Like Compounds Ln4[Ga2(1-x)Ge2xO7+x(square)1-x]O2 (Ln = La, Nd, Gd; x <=0.4)

The Ln4[Ga2(1-x)Ge2xO7+x(square)1-x]O2 (Ln = La, Nd, Gd; x <=0.4) compounds have been prepared first by solid-state reaction and then by a new regenerative sol-gel method which allows their sintering. They exhibit a Cuspidine-like structure. Upon substituting Ge4+ for Ga3+, the Cuspidine structure is maintained up to x = 0.4. For x =>0.15 a 1D modulation of the structure was observed and analyzed as driven by composition. The Ge4+ substitution for Ga3+ with concomitant filling of oxygen vacancies is beneficial to the anionic conductivity which increases rather regularly with x to reach 10-3 S/cm at 800 °C for x = 0.4 and Ln = Nd. Conductivity measurements on Ln4[Ga2(1-x)Ge2xO7+x(square)1-x]O2 (Ln = La, Nd, Gd) compounds with a fixed Ge content (x = 0.2) show a clear increase of the activation energy when the ionic radii of the Ln3+ cation decreases. La and Nd compounds exhibit similar conductivities, significantly larger than that of the Gd counterpart.