Structure, defect chemistry, and proton conductivity in nominally Sr-doped Ba3La(PO4)3

The AC conductivity of eulytite-structured Ba3La(PO4)3 with 2 mol% Sr substituted for La has been investigated in the range 300–1300 °C at different oxygen and water vapor (H2O or D2O) partial pressures. The overall sample impedance has contributions from bulk (grain interior) and grain boundaries. It is concluded from the H/D isotope effect that the bulk conductivity is mainly protonic. The defect chemistry leading to a content of protons is discussed.