No abstract

Several oxy-apatite materials La 102x Sr x (TO 4 ) 6 O 320.5x (T = Ge, Si; 102x = 9.00, 8.80, 8.65 and 8.00) and La 9.33 (Si 12x Ge x O 4 ) 6 O 2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La 9.33 (SiO 4 ) 6 O 2 . Time-of-flight neutron data have been collected for La 9.

The La 10-x (GeO 4 ) 6 O 3-1.5x (9.33 e 10 -x e 10) apatite series has been synthesized and single phases have been obtained in a narrow compositional range (9.52 e 10 -x e 9.75). The apatites' phases are hexagonal (space group (s.g.) P6 3 /m) for 9.52 e 10 -x e 9.60 and triclinic (s.g. P1 h) for 9.66e 10 -x e 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, R ) 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.

The apatite La10 - x □ x (Ge5.5Al0.5O24)O2.75 - 1.5 x (10 − x = 9.80, 9.75, 9.67, 9.60, 9.50, and 9.40) series has been prepared and the single phase existence range has been established, 9.75 ≥ 10 − x ≥ 9.45. The hexagonal crystal structures of La9.5□0.5(Ge5.5Al0.5O24)O2 have been determined at room temperature, 500 °C, and 900 °C from neutron powder diffraction data using the Rietveld method. The room-temperature unit cell parameters were a = 9.9206(4) Å, c = 7.2893(3) Å, V = 621.29(6) Å3, and Z = 1, and this refinement converged to R WP = 3.03 and R F = 1.30%. The most important structural result is the presence of interstitial oxygen ion associated with vacancies at the apatite oxide anions channels. Oxide ion conductivities have been measured by impedance spectroscopy. La9.5□0.5(Ge5.5Al0.5O24)O2 shows very high oxide conductivity, 0.16(1) S·cm-1 at 800 °C, with negligible electronic contribution. The ionic transport number, obtained by combination of impedance and ion-blocking data, is higher than 0.99 in the studied oxygen partial pressure range, 0.21 to 10-20 atm.