Cuspidine-Like Compounds Ln₄[Ga_2(1-x)Ge_2xO_7+x□_1-x]O₂ (Ln = La, Nd, Gd; x ≤ 0.4)

The Ln 4 [Ga 2(1 - x ) Ge 2 x O 7+ x □ 1 - x ]O 2 (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 Ge 4+ for Ga 3+, 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 Ge 4+ substitution for Ga 3+ 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 Ln 4 [Ga 2(1 - x ) Ge 2 x O 7+ x □ 1 - x ]O 2 (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 Ln 3+ cation decreases. La and Nd compounds exhibit similar conductivities, significantly larger than that of the Gd counterpart.