New Apatite-Type Oxide Ion Conductor, Bi sub(2)La sub(8)[(GeO sub(4)) sub(6)]O sub(3): Structure, Properties, and Direct Imaging of Low-Level Interstitial Oxygen Atoms Using Aberration-Corrected Scanning Transmission Electron Microscopy

The new solid electrolyte Bi sub(2)La sub(8)[(GeO sub(4)) sub(6)]O sub(3) is prepared and characterized by variable-temperature synchrotron X-ray and neutron diffraction, aberration-corrected scanning transmission electron microscopy, and physical property measurements (impedance spectroscopy and second harmonic generation). The material is a triclinic variant of the apatite structure type and owes its ionic conductivity to the presence of oxide ion interstitials. A combination of annular bright-field scanning transmission electron microscopy experiments and frozen-phonon multislice simulations enables direct imaging of the crucial interstitial oxygen atoms present at a level of 8 out of 1030 electrons per formula unit of the material, and crystallographically disordered, in the unit cell. Scanning transmission electron microscopy also leads to a direct observation of the local departures from the centrosymmetric average structure determined by diffraction. As no second harmonic generation signal is observed, these displacements are non-cooperative on the longer length scales probed by optical methods. The new solid electrolyte Bi sub(2)La sub(8)[(GeO sub(4)) sub(6)]O sub(3) is prepared and characterized using variable-temperature synchrotron X-ray and neutron diffraction, aberration-corrected scanning transmission electron micro-scopy, and physical property measurements (impedance spectroscopy and second harmonic generation).