Apatite-type Oxide Ion Conductors Research Articles

Dynamics in Bi(iii)-containing apatite-type oxide ion conductors: a combined computational and experimental study

Introduction of Bi(iii) into apatite-type germanate solid electrolytes can facilitate oxide ion diffusion.

Synthesis and characterisation of new Bi(iii)-containing apatite-type oxide ion conductors: the influence of lone pairs.

Lone-pair cations are known to enhance oxide ion conductivity in fluorite- and Aurivillius-type materials. Among the apatite-type phases, the opposite trend is found for the more widely studied silicate oxide ion conductors, which exhibit a dramatic decrease in conductivity on Bi(iii) incorporation. In this work, the influence of lone-pair cations on the properties of apatite-type germanate oxide ion conductors has been investigated by preparing and characterising seven related compositions with varying Bi(iii) content, by X-ray and neutron powder diffraction and impedance spectroscopy. All materials are very good oxide ion conductors (with conductivities of up to 1.29 × 10-2 S cm-1 at 775 °C). Increasing Bi(iii) content leads to increases in conductivity by up to an order of magnitude, suggesting significant differences in the oxide-ion conduction mechanisms between lone-pair-containing apatite-type germanate and silicate solid electrolytes.

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La 8+xSr 2−x(Si/Ge) 6O 26+x/2, and ammonia

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) 6O 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/2) 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.

Ambi-site substitution of Mn in lanthanum germanate apatites

A neutron diffraction study at 4 K of the Mn doped lanthanum germanate apatite-type oxide ion conductor of nominal starting composition “La 9.5Mn 0.5(GeO 4) 6O 2.75” is reported. The structure was refined in space group P6 3/ m, although high thermal displacement parameters were observed for the oxide ion sites (particularly O3, and O4). Reduced thermal displacement parameters were obtained by splitting the O3 site, and allowing the O4 oxygen to move off site, which may indicate local regions of lower symmetry within the structure. In addition, the data suggested ambi-site substitution of Mn, with it being present on both the Ge site and the La site. Assuming no change in La:Mn:Ge ratio, a composition of La 9.18Mn 0.28(GeO 4) 5.8(MnO 4) 0.2O 2 was determined. As such there are nominally no interstitial oxide ions, but rather cation vacancies on the La site. Therefore, the high conductivity for this sample is most likely related to the introduction of Frenkel-type defects at higher temperature, as previously proposed for other apatite-type systems containing vacancies on the La site.

An investigation of the high temperature reaction between the apatiteoxide ion conductor La9.33Si6O26and NH3

There is growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs). However, the possible reaction between the electrolyte and ammonia, and its potential effect on performance, has received little attention. In this paper, we report an investigation of the high temperature (950 °C) reaction of the apatite-type oxide ion conductor, La9.33Si6O26, and ammonia. The results show that such treatment leads to nitridation of the sample, with evidence for Si loss leading to an increased La:Si ratio in the final product. From neutron diffraction studies, the composition of the final product was determined to be La9.7(1)Si6O22.6(2)N2.7(2), with structural and 29Si NMR data suggesting the presence of N both within the apatite anion channels, and bonded to Si. An interesting feature of the structural studies are the relatively low atomic displacement parameters compared to the comparable apatite oxide systems, La9.33 + xSi6O26 + 3x/2, which can be related to the lack of interstitial anions in the oxynitride. Further studies on samples heated in ammonia at lower temperatures (600, 800 °C) suggest lower N incorporation, particularly for the 600 °C treatment. Considering the correlation of ionic conductivity, and interstitial oxide ion content in apatite systems, the data suggests the potential use of apatite-type electrolytes in SOFCs utilising NH3 as the fuel should be limited to temperatures <800 °C.

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

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, La8Y2Ge6O27. The structural model indicates that the oxide ions are located between the GeO4 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.

A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La 9.67Si 6O 26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La 9.67Si 6O 26.5 are reported for temperatures ranging between 25 and 900 °C. The best fit to the data was obtained for space group P6 3 and there was no evidence for any change in symmetry over the temperature range studied. Interstitial oxide ions are identified lying in sites similar to those predicted by previous computer modelling studies, and in agreement with structural studies on related materials. Furthermore, occupancy of these sites is enhanced by Frenkel-type disorder from neighbouring positions. The results thus add further weight to the interpretation that, in these apatite-type systems, the silicate substructure is important for the accommodation of interstitial oxide ions and their migration.

Atomic-scale mechanistic features of oxide ion conduction in apatite-type germanates

Atomistic modelling studies of the apatite-type oxide ion conductor La9.33(GeO4)6O2 show that a key role of the O4 channel oxygen atoms appears to be as a reservoir for the creation of interstitial oxide ion defects, while the migration of these defects proceeds via the GeO4 tetrahedra.

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

Apatite-type lanthanum silicates/germanates have been attracting considerable interest as a new class of oxide ion conductors, whose conductivity is mediated by oxide ion interstitials. For the germanates, it has been shown that, depending on composition, the cell can be either hexagonal or triclinic, with evidence for reduced low-temperature conductivities for the latter, attributed to increased defect trapping in this lower symmetry cell. In this paper we show that site selective doping of Y into the triclinic apatite-type oxide ion conductors, La 9.33+ z (GeO 4) 6O 2+3 z/2 (0.33 ≤ z ≤ 0.67) results in a hexagonal lattice for the complete series with correspondingly enhanced low-temperature conductivity.

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

In this paper, detailed studies of the effect of Mg doping in the apatite-type oxide ion conductor La9.33Si6O26 are reported. Mg is confirmed as an ambisite dopant, capable of substituting for both La and Si, depending on the starting composition. A large enhancement in the conductivity is observed for Si site substitution, with a reduction for substitution on the La site. Neutron powder diffraction studies show that in agreement with cation size expectations, an enlargement of the unit cell is observed on Mg substitution for Si, with a corresponding increase in the size of the tetrahedral sites. For Mg substitution on the La site, a contraction of the unit cell is observed, and the neutron diffraction results indicate that there is preferential occupancy of Mg on the La2 (1/3, 2/3, approximately 0.5) site. Atomistic simulation studies show significant local structural changes affecting the oxide ion channels in both cases. Mg doping on the Si site leads to a local expansion of the channels, while doping on the La site results in a large displacement of the silicate O4 site, such that it encroaches the oxide ion channels. The observed differences in conductivities are discussed with respect to these observations.

Investigation of the structural changes on Zn doping in the apatite-type oxide ion conductor La 9.33Si 6O 26: A combined neutron diffraction and atomistic simulation study

Partial substitution of lower valent dopant ions (e.g. Al, Ga, Mg, Zn) onto the Si site in the oxide ion conductor La 9.33Si 6O 26 has been reported to increase the conductivity. In this paper, we report combined atomistic simulation and neutron powder diffraction studies of Zn doping with a view to explaining this enhancement. The neutron diffraction studies show that, in agreement with cation size expectations, an enlargement of the unit cell is observed on Zn substitution for Si. In addition, the refinement suggests that a significant number of oxide ions are displaced into interstitial positions at the channel periphery. Atomistic simulation studies show significant local structural changes affecting the oxide ion channels on Zn doping. In particular, a local expansion of the channels is observed, while the tilting of the MO 4 tetrahedra is altered by replacing Si with Zn. The presence of significant interstitial oxide ions and both the above local structural changes are consistent with the observed increase in conductivity.

Synthesis and conductivities of the Ti doped apatite-type phases (La/Ba) 10− x(Si/Ge) 6− yTi yO 26+ z

In this paper we report further doping studies of the apatite-type phases, (La/Ba) 10− x (Si/Ge) 6O 26+ z , of interest as a new class of oxide ion conductors. We show that it is possible to substitute Ti onto the Si/Ge site, which is, to our knowledge, the first report of Ti substitution on the tetrahedral site in the apatite structure. The conductivities were shown to be highest for samples containing either cation vacancies or oxygen excess, consistent with previous studies of apatite-type oxide ion conductors. However, the Ti doping was shown to generally decrease the conductivity in comparison to equivalent samples with only Si/Ge in the tetrahedral sites, with the greatest decrease for Si containing samples. Further work is required to account for this decrease in conductivity, but one possible explanation could be trapping of oxide ions due to Ti increasing its coordination sphere.

Structural studies of apatite-type oxide ion conductors doped with cobalt

A series of Co doped lanthanum silicate apatite-type phases, La9.83Si4.5Co1.5O26, La9.66Si5CoO26, La10Si5CoO26.5 and La8BaCoSi6O26, have been synthesised, and neutron diffraction, EXAFS and XANES used to investigate their structures in detail. All compositions were shown to possess the hexagonal apatite structure, and the results confirmed that cobalt can be doped onto both the La and Si sites within the structure depending on the starting composition. The Co doping is shown to cause considerable local distortions within the apatite structure. In the case of Si site doping two compositions showed anisotropic peak broadening, which has been attributed to incommensurate ordering of oxygen within the apatite channels.

Development of apatite‐type oxide ion conductors

Research into materials displaying oxide ion conductivity is attracting considerable attention due to their potential technological applications in devices such as Solid Oxide Fuel Cells. In this paper, recent work on apatite-type oxide ion conductors is reviewed, showing that a wide range of cation substitutions are possible, due to the flexibility of the apatite structure in accommodating a range of ion sizes. The conductivity studies on these doped samples show that to achieve high oxide ion conduction, non-stoichiometry in terms of cation vacancies and/or oxygen excess is required, with the latter resulting in the highest conductivities. In contrast to most common oxide ion conductors, e.g. perovskite and fluorite in which oxide ion conduction proceeds via oxygen vacancies, the research on these apatite systems suggests that the conductivity involves interstitial oxide ions. With further optimization of these materials, particularly in terms of the Ge-containing systems, significant improvements in conductivity are likely, leading to the very real possibility of the application of apatite-type electrolytes in fuel cell and other applications.