Crystalchemistry and Oxide Ion Conductivity in the Lanthanum Oxygermanate Apatite Series

Abstract

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.

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

“…No other band is found at higher frequencies, up to at least 1200 cm -1 . According to Hardcastle et al [42], this frequency (830 cm -1 ) is much lower than that expected for the stretching of a W-O bond in an ideal WO 4 tetrahedron, suggesting a higher coordination of tungsten in the form of WO 5 or even WO 6 . These results imply the distribution of the interstitial oxygen between both germanium and tungsten.…”

Strategies for the Optimisation of the Oxide Ion Conductivities of Apatite‐Type Germanates

“…In all these systems, the ionic conduction is mediated by oxide ion vacancy defects, introduced by aliovalent doping. More recently there has been growing interest in materials, whose conduction process is mediated by oxide ion interstitials [2], an important example being apatitetype silicates/germanates, Ln 9.33+x (Si/Ge) 6 O 26+3x/2 (Ln = rare earth) (Figure 1). The vast majority of the work has so far focused on the silicate systems, although the germanates show some benefits in terms of higher conductivities at elevated temperatures, lower synthesis/sintering temperatures and higher oxygen interstitial contents achievable.…”

“…However, one complexity of these high oxygen excess germanates is a change in symmetry from hexagonal to triclinic as x increases. Thus it has been shown that single phase samples of La 9.33+x Ge 6 O 26+3x/2 with hexagonal symmetry can be prepared for x ≤ 0.42, while samples with x > 0.42 exhibit a triclinic cell [4,6,11,14]. A consequence of the lower symmetry cell is a reduction in the conductivity at temperatures below ≈650°C, attributed to enhanced defect trapping, which is a problem in terms of applications [4].…”

Neutron diffraction and atomistic simulation studies of Mg doped apatite-type oxide ion conductors

“…Research interest in this area has grown following the initial reports by Nakayama et al of high oxide conductivities (>10 -3 Scm -1 at 500°C) in rare earth silicates, Ln 10-x Si 6 O 26+y , with the apatite structure [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The structure of these materials is shown in figure 1.…”