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Bandgap State Mapping via Valence-Loss EELS at Grain Boundaries in Non-Stoichiometric Pr x Ce 1-x O 2-δ

We recently reported the enhancement of grain boundary electrical conductivity in polycrystalline GdxCe1xO2-δ solid solution resulting from the addition of Pr [1]. The Pr concentration at grain boundaries was measured via STEM EELS to be approximately three times greater than adjacent grains. Thus it is believed that the modification of grain boundary electrical properties is related to the population and depopulation of a bandgap electronic state associated with the Pr 4f level [2] localized to within 2 nm – 3 nm of the boundaries. To elucidate the nature of this bandgap state, here we employ monochromated valence-loss EELS using a Nion UltraSTEM to spatially map—and correlate with composition—the position and occupancy of the Pr 4f level at grain boundaries in PrxCe1-xO2-δ.