Synthesis of hexagonal lanthanum germanate apatites through site selective isovalent doping with yttrium

Kendrick, Emma; Slater, Peter R.

doi:10.1016/j.materresbull.2007.07.041

Cited by 34 publications

(32 citation statements)

References 19 publications

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“…A key feature of these apatite oxide ion conductors is that their conduction mechanism is driven by the presence of interstitial oxide ions, contrary to the traditional fluorite and perovskite systems, where oxide ion vacancies are responsible [13]. For the silicates it has been shown that oxygen excess in the range, 0 ≤ x ≤ 0.5 is possible, while higher oxygen content, 0 ≤ x ≤ 1.0 has been achieved for the germanates [13,18,20,21]. For the silicate series, the exact location of extrastoichiometric oxygen, x, has attracted some controversy, with reports of interstitial sites close to the SiO4 tetrahedra, while in other studies, mobile oxygen at the centre of the channels has been claimed [4,8,9,10,12,13,23,27,35,36].…”

Section: Introductionmentioning

confidence: 99%

“…In prior work, we have shown that Y can be selectively doped into the La sites within the La3.33+2x/3(GeO4)6 framework altering the diameter, and hence relieving the triclinic distortion [18,37]. In this way it is possible to prepare hexagonal samples with high oxygen content, i.e.…”

Section: Introductionmentioning

confidence: 99%

“…(0 x 1.0), which show enhanced conductivity at low temperatures compared with equivalent triclinic samples without Y doping [18]. We have also investigated the effect of doping the lower valent Co 3+ , Ga 3+ ions on the Ge site, La10Ge6-x(Co/Ga)xO27-x/2, although in these cases the dopant appears to enhance the triclinic distortion [13,19].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2011

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High oxygen content apatite germanates, La 10 Ge 6-x W x O 27+x , have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La 10 Ge 5.5 W 0.5 O 27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO 4 tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units.Raman spectroscopy studies suggest that in the case of the WO 5 units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO 5 units, thus not contributing significantly to the conduction process.2

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2011

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“…The triclinic apatites have reduced conductivity at low temperatures, attributed to enhanced defect trapping in this lower symmetry cell, which is a problem in terms of applications. Recently we have shown that Y can be selectively doped into the sites within the A(1) 4 (MO 4 ) 6 framework altering the size of this framework, and hence relieving the triclinic distortion [18]. Thus it is possible through Y doping to prepare hexagonal samples with high oxygen content, i.e.…”

Section: Introductionmentioning

confidence: 99%

Neutron diffraction structural study of the apatite-type oxide ion conductor, La8Y2Ge6O27: location of the interstitial oxide ion site

2009

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Apatite-type rare earth silicates/germanates have attracted considerable interest recently due to their high oxide ion conductivities. Despite evidence in support of a conduction mechanism involving interstitial oxide ions, the exact location of the interstitial oxide ion sites continues to attract controversy. In this paper we report a neutron diffraction structural study for the high oxygen excess compound, La 8 Y 2 Ge 6 O 27 . The structural model indicates that the oxide ions are located between the GeO 4 tetrahedra, leading to significant localised distortions. These results, coupled with recent modelling studies, hence, support the conclusion that oxide ion migration proceeds via these tetrahedra.3

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“…The change of symmetry (triclinichexagonal) for the high oxygen excess samples (figure 5) is interesting. Previous work has shown that the triclinic distortion arises from a size mismatch between the (La/A) 4-x (GeO 4 ) 6 (A=alkaline earth) framework and the (La/A) 6 O 2 units due to the presence of interstitial oxide ions within the framework [21,25,30,35,43]. The increase in symmetry after heat treatment in NH 3 gas is therefore consistent with nitridation leading to a reduction in anion content, and hence loss of these interstitial oxide ions.…”

Section: Resultsmentioning

confidence: 99%

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2−x(Si/Ge)6O26+x/2, and ammonia

Orera

Headspith

Apperley

et al. 2009

Journal of Solid State Chemistry

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Following growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs), we have investigated the possible reaction between the apatite silicate/germanate electrolytes, La 8+x Sr 2-x (Si/Ge) 6 O 26+x/2 , and NH 3 gas. We examine how the composition of the apatite phase affects the reaction with ammonia. For the silicate series, the results showed a small degree of N incorporation at 600 ○ C, while at higher temperatures (800 ○ C), substantial N incorporation was observed. For the germanate series, partial decomposition was observed after heating in ammonia at 800 ○ C, while at the lower temperature (600 ○ C), significant N incorporation was observed. For both series, the N content in the resulting apatite oxynitride was shown to increase with increasing interstitial oxide ion content (x) in the starting oxide. The results suggest that the driving force for the nitridation process is to remove the interstitial anion content, such that for the silicates the total anion (O+N) content in the oxynitrides approximates to 26.0, the value for an anion stoichiometric apatite. For the germanates, lower total anion contents are observed in some cases, consistent with the ability of the germanates to accommodate anion vacancies. The removal of the mobile interstitial oxide ions on nitridation suggests problems with the use of apatite-type electrolytes in SOFCs utilising NH 3 at elevated temperatures.

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Synthesis of hexagonal lanthanum germanate apatites through site selective isovalent doping with yttrium

Cited by 34 publications

References 19 publications

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

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

Neutron diffraction structural study of the apatite-type oxide ion conductor, La8Y2Ge6O27: location of the interstitial oxide ion site

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2−x(Si/Ge)6O26+x/2, and ammonia

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