Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series

The La 10 - x (GeO 4 ) 6 O 3 - 1.5 x (9.33 ≤ 10 − x ≤ 10) apatite series has been synthesized and single phases have been obtained in a narrow compositional range (9.52 ≤ 10 − x ≤ 9.75). The apatites' phases are hexagonal (space group (s.g.) P 6 3 / m ) for 9.52 ≤ 10 − x ≤ 9.60 and triclinic (s.g. P1̄) for 9.66≤ 10 − x ≤ 9.75. The room-temperature crystal structures have been determined from joint Rietveld refinements of neutron and synchrotron X-ray powder diffraction data. La 9.60 (GeO 4 ) 6 O 2.40 is hexagonal ( a = 9.9374(1) Å, c = 7.2835(1) Å and V = 622.90(2) Å 3 ) and the Rietveld disagreement factors were low. La 9.75 (GeO 4 ) 6 O 2.62 is triclinic ( a = 9.9368(4) Å, b = 9.9220(3) Å, c = 7.2925(2) Å, α = 90.566(3)°, β = 88.992(4)°, γ = 120.334(3)°, and V = 620.46(3) Å 3 ) and the fits were satisfactory for such complex pseudohexagonal structure. This structure contains 72 variable positional parameters and 24 thermal factors. High-temperature neutron powder diffraction (NPD) data were also collected at 773 and 1173 K for hexagonal La 9.60 (GeO 4 ) 6 O 2.40 . The electrical results suggest that the samples are bulk oxide ion conductors. The plots of the imaginary parts of the impedance, Z‘ ‘, and the electric modulus, M‘ ‘, vs log(frequency), possess maxima for both curves separated by less than a half decade in frequency with associated capacities of ≈2 pF. The curvatures observed in the Arrhenius plots are not due to a phase transition. The conductivities are almost independent of the oxygen partial pressure under oxidizing conditions, which suggests pure oxide-ion conduction with negligible electronic contribution.