Crystal chemistry and optimization of conductivity in 2A, 2M and 2H alkaline earth lanthanum germanate oxyapatite electrolyte polymorphs

Calcium (La_{10 - x}Ca_xGe ₆O_{27 - x/2}, 0 ≤ x ≤ 2.57) and barium (La_{10 - x}Ba_xGe ₆O_{27 - x/2}, 0 ≤ nominal x ≤ 3) doped lanthanum germanate apatites were synthesized by high temperature solid-state sintering. The lattice parameters obtained from Rietveld refinement of powder X-ray diffraction data showed that pseudomorphic transformations from 2A to 2H occurred as the alkaline earth was introduced. The likely appearance of an intervening 2M pseudomorph was not confirmed directly in this study; however, compositionally dependent inflections in cell constants and abrupt changes in oxygen mobility as a function of temperature support its existence, particularly for low alkaline earth contents. Oxygen excess apatites La ₉AEGe₆O_26.5 (AE = Ca, Sr, Ba) prove most promising as solid oxide fuel cell (SOFC) electrolytes at intermediate temperatures, by delivering the optimal compromise between higher mobile oxygen concentration and lowering of activation energy. In contrast, stoichiometric La₈AE₂Ge₆O₂₆ showed inferior conductivity, because although the activation energy was reduced, the proportion of extrastoichiometric oxygen was negligible. Atomistic modeling suggests that interstitial oxygen is located near (0, 1/2, 1/2) and causes the O3 position to be split statistically into the (0.050, 0.278, 0.587) and (0.084, 0.335, 0.565) sites. These co-existing locations may facilitate inter-tunnel oxygen transport.