Mixed electronic and oxide ionic conduction and migration mechanisms in digermanate La 2- x Ca x Ge 2 O 7- x /2 .

Mixed electronic and oxide ionic conduction was enabled in digermanate-type La 2- x Ca x Ge 2 O 7- x /2 containing Ge 3 O 10 chains and isolated GeO 4 units by substitution of La 3+ with Ca 2+ . The structure and solid solution limit of Ca doped La 2 Ge 2 O 7 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 Ca 2+ and oxygen vacancies was investigated by Rietveld analysis of the NPD data. The obtained conducting material La 1.925 Ca 0.075 Ge 2 O 6.963 exhibits superior thermal stability and an order of magnitude improvement in conductivity compared to the parent La 2 Ge 2 O 7 (∼9 × 10 -5 S cm -1 at 1000 °C). BVEL calculations reveal that the oxygen vacancies were stabilized and transported within the framework of La 2 Ge 2 O 7 by sharing oxygen and oxygen exchange between the adjacent Ge 3 O 10 chains and GeO 4 units, exhibiting a three-dimensional oxide ion transport nature.