Mixed electronic and oxide ionic conduction and migration mechanisms in digermanate La_2-CaGe₂O_7-

Mixed electronic and oxide ionic conduction was enabled in digermanate-type La_2-CaGe₂O_7- containing Ge₃O₁₀ chains and isolated GeO₄ units by substitution of La³⁺ with Ca²⁺. The structure and solid solution limit of Ca doped La₂Ge₂O₇ were studied by systematic experiments, including rotation electron diffraction (RED), X-ray diffraction (XRD) and neutron powder diffraction (NPD) experiments, etc. The preferred occupation of Ca²⁺ and oxygen vacancies was investigated by Rietveld analysis of the NPD data. The obtained conducting material La_1.925Ca_0.075Ge₂O_6.963 exhibits superior thermal stability and an order of magnitude improvement in conductivity compared to the parent La₂Ge₂O₇ (∼9 × 10^-5 S cm^-1 at 1000 °C). BVEL calculations reveal that the oxygen vacancies were stabilized and transported within the framework of La₂Ge₂O₇ by sharing oxygen and oxygen exchange between the adjacent Ge₃O₁₀ chains and GeO₄ units, exhibiting a three-dimensional oxide ion transport nature.