Apatite silicates are attracting significant interest as potential SOFC electrolyte materials. They are non-conventional oxide ion conductors in the sense that oxide ion interstitials, rather than vacancies, are the key defects. In this work we compare the structures of La9.6Si6O26.4 and La8Sr2Si6O26, both before and after hydration in order to gather information about the location of the interstitial oxide ion site. Neutron diffraction structural studies suggest that in the as-prepared La8Sr2Si6O26 and hydrated La8Sr2Si6O26, the interstitial oxide ion sites are close to the apatite channel centre. For La9.6Si6O26.4, a similar site close to the channel centre is observed, but on hydration of this particular sample, the interstitial site is shown to be significantly displaced away from the channel centre towards the SiO4 units. This can be explained by the need for additional displacement from the channel centre to accommodate the large amount of interstitial anions in this hydrated phase. The solid state 29Si MAS NMR spectra are shown to be very sensitive to the different speciation exhibited by the La8Sr2Si6O26 and La9.6Si6O26.4 systems, with the former being dominated by regular SiO4 framework species and the latter being dominated by interruptions to this network caused by cation vacancies and interstitials. The corresponding 17O MAS NMR study identifies a strong signal from the O atoms of the SiO4 groups, thus demonstrating that all of the O species in these systems are exchangeable O under heterogeneous gas phase conditions. In addition, interstitial O species attributed to pendant OH linkages on the Si positions are clearly identified and resolved, and these are removed on dehydration. This observation and assignment is corroborated by corresponding 1H MAS NMR measurements. Overall the neutron diffraction work indicates that the interstitial site location in these apatite silicates depends on the anion content with progressive displacement towards the SiO4 tetrahedra on increasing anion content, while the observation of exchangeable O on the SiO4 groups is consistent with prior modelling predictions as to the importance on the silicate units in the conduction process.

•The successful synthesis of Na2M(SO4)2·2H2O (M=transition metal) phases doped with selenate and fluorophosphate.•A change in structure is observed on selenate doping for M=Fe, Co, Cu.•This change in structure is also observed on fluorophosphate doping for M=Co, Cu.•The work highlights how isovalent doping can be exploited to alter the structures of Na2M(SO4)2·2H2O systems.In this paper an investigation into the effect of transition metal ion and selenate/fluorophosphate doping on the structures of Na2M(SO4)2·2H2O (M=transition metal) materials is reported. In agreement with previous reports, the monoclinic (Kröhnkite) structure is adopted for M=Mn, Fe, Co, Cu, while for the smallest first row divalent transition metal ion, M=Ni, the triclinic (Fairfieldite structure) is adopted. On selenate doping there is a changeover in structure from monoclinic to triclinic for M=Fe, Co, Cu, with the larger Fe2+ system requiring the highest level of selenate to complete the changeover. Thus the results suggest that the relative stability of the two structure types is influenced by the relative size of the transition metal: oxoanion group, with the triclinic structure favoured for small transition metals/large oxoanions.The successful synthesis of fluorophosphate doped samples, Na2M(SO4)2−x(PO3F)x·2H2O was also obtained for M=Fe, Co, Cu, with the results showing a changeover in structure from monoclinic to triclinic for M=Co, Cu for very low levels (x=0.1) of fluorophosphate. In the case of M=Fe, the successful synthesis of fluorophosphates samples was achieved for x≤0.3, although no change in cell symmetry was observed. Rather in this particular case, the X-ray diffraction patterns showed evidence for selective peak broadening, attributed to local disorder as a result of the fluorophosphate group disrupting the H-bonding network. Overall the work highlights how isovalent doping can be exploited to alter the structures of Na2M(SO4)2·2H2O systems.Partial substitution of sulfate in Na2M(SO4)20.2H2O (M=Co, Cu) by selenate or fluorophosphate leads to a structural change from the monoclinic Kröhnkite to the triclinic Fairfieldite structure.

In this article we review work on oxyanion (carbonate, borate, nitrate, phosphate, sulphate, silicate) doping in perovskite materials beginning with early work on doping studies in superconducting cuprates, and extending to more recent work on doping into perovskite-type solid oxide fuel cell materials. In this doping strategy, the central atom of the oxyanion group occupies the perovskite B cation site, with the associated oxide ions filling 3 (carbonate, nitrate, borate) or 4 (phosphate, sulphate, silicate) of the available 6 anion sites around this site, albeit displaced so as to achieve the required geometry for the oxyanion. We highlight the potential of this doping strategy to prepare new systems, stabilize phases that cannot be prepared under ambient pressure conditions, and lead to modifications to the electronic and ionic conductivity. We also highlight the need for further work in this area, in particular to evaluate the carbonate content of perovskite phases in general.

In this paper we report the successful incorporation of silicon into SrFeO3−δ perovskite materials for potential applications as electrode materials for solid oxide fuel cells. It is observed that Si doping leads to a change from a tetragonal cell (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). Annealing experiments in 5% H2/95% N2 (up to 800 °C) also showed the stabilization of the cubic form for the Si-doped samples under reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast to the cubic cell of the reduced Si doped system, reduction of undoped SrFeO3−δ leads to the formation of a brownmillerite structure with ordered oxide ion vacancies. SrFe0.90Si0.10O3−δ and SrFe0.85Si0.15O3−δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mössbauer spectroscopy data were also recorded for SrFe0.90Si0.10O3−δ, and indicated the presence of only Fe3+ and Fe5+ (i.e. disproportionation of Fe4+ to Fe3+ and Fe5+) for such doped samples. Conductivity measurements showed an improvement in the conductivity on Si doping. Composite electrodes with 50% Ce0.9Gd0.1O1.95 were therefore examined on dense Ce0.9Gd0.1O1.95 pellets in two different atmospheres: air and 5% H2/95% N2. In both atmospheres an improvement in the area specific resistance (ASR) values is observed for the Si-doped samples. Thus the results show that silicon can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFCs.

In this paper we report the synthesis, structure and Li ion conductivity of a new tetragonal garnet phase Nd3Zr2Li7O12. In line with other tetragonal garnet systems, the Li is shown to be ordered in the tetrahedral and distorted octahedral sites, and the Li ion conductivity is consequently low. In an effort to improve the ionic conductivity of the parent material, we have also investigated Al doping to reduce the Li content, Nd3Zr2Li5.5Al0.5O12, and hence introduce disorder on the Li sublattice. This was found to be successful leading to a change in the unit cell symmetry from tetragonal to cubic, and an enhanced Li ion conductivity. Neutron diffraction studies showed that the Al was introduced onto the ideal tetrahedral garnet site, a site preference also supported by the results of computer modelling studies. The effect of moisture on the conductivity of these systems was also examined, showing significant changes at low temperatures consistent with a protonic contribution in humid atmospheres. In line with these observations, computational modelling suggests favourable exchange energy for the Li+/H+ exchange process.

In this paper we report the successful incorporation of silicon into Sr1−yCayMnO3−δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a 29Si enriched Sr1−yCayMn1−xSixO3−δ perovskite system is confirmed by 29Si MAS NMR measurements at low B0 field. The very large paramagnetic shift (∼3000–3500 ppm) and anisotropy (span ∼4000 ppm) suggests that the Si4+ species experiences both Fermi contact and electron-nuclear dipolar contributions to the paramagnetic interaction with the Mn3+/4+ centres. An improvement in the conductivity is observed for low level Si doping, which can be attributed to two factors. The first of these is attributed to the tetrahedral coordination preference of Si leading to the introduction of oxide ion vacancies, and hence a partial reduction of Mn4+ to give mixed valence Mn. Secondly, for samples with high Sr levels, the undoped systems adopt a hexagonal perovskite structure containing face sharing of MnO6 octahedra, while Si doping is shown to help to stabilise the more highly conducting cubic perovskite containing corner linked octahedra. The level of Si, x, required to stabilise the cubic Sr1−yCayMn1−xSixO3−δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr0.5Ca0.5Mn1−xSixO3−δ series; x = 0.075 for Sr0.7Ca0.3Mn1−xSixO3−δ; x = 0.10 for Sr0.8Ca0.2Mn1−xSixO3−δ; and x = 0.15 for SrMn1−xSixO3−δ. Composites with 50% Ce0.9Gd0.1O1.95 were examined on dense Ce0.9Gd0.1O1.95 pellets. For all series an improvement in the area specific resistances (ASR) values is observed for the Si-doped samples. Thus these preliminary results show that silicon can be incorporated into perovskite cathode materials and can have a beneficial effect on the performance.

In this paper we examine the effect of partial substitution of Ga for Sc in the oxyanion (phosphate, sulphate) containing perovskites, Ba2Sc2−xPxO5+x and Ba2Sc2−xSxO5+3x/2 with the samples analysed through a combination of X-ray diffraction, TGA, Raman spectroscopy and conductivity measurements. The results demonstrate that in both cases, Ga can be incorporated in place of Sc up to 40%. In order to accommodate the increasing Ga content, a reduction in the oxyanion content is required. Thus for the highest Ga content sample achieved, only 10% oxyanion incorporation was achieved giving endmember compositions of Ba2ScGa0.8P0.2O5.2 and Ba2ScGa0.8S0.2O5.3 for phosphate and sulphate doping respectively. While the Ga doping was shown to significantly improve the stability of the systems towards CO2 containing atmospheres, conductivity measurements showed a reduction in the conductivity with increasing Ga content.Graphical abstractPhosphate and sulphate doped Ba2Sc2−xGaxO5 perovskites have been successfully prepared, with the highest conductivities observed for samples with the lowest Ga content.Highlights► The successful synthesis of phosphate and sulphate doped Ba2Sc2−xGaxO5 perovskites. ► The demonstration of significant oxide ion and proton conduction in these perovskites. ► The demonstration of improved CO2 stability with increasing Ga content.

In this paper we report the successful synthesis of the cubic oxyanion containing perovskites, Ba2Sc2−xPxO5+x (x = 0.4, 0.5), with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. Conductivity measurements indicate a p-type contribution to the conductivity in oxidizing conditions at elevated temperatures, with evidence for proton conduction in wet atmospheres. For the latter, bulk conductivities of 5.9 × 10−3 and 1.3 × 10−3 S cm−1 at 500 °C were obtained for x = 0.4 and 0.5 respectively, comparable to other perovskite proton conductors, while the stability towards CO2 containing atmospheres was improved compared to BaCeO3 based systems. Related Si doped systems have also been prepared, although in this case small Ba2SiO4 impurities are observed. We also provide evidence to suggest that “undoped” Ba2Sc2O5 contains carbonate groups, which accounts for its thermal instability.

In this paper we examine the effect of Ga doping on the structure and conductivity of the high Li ion content garnet-related system, La3Zr2Li7O12. Without Ga doping, La3Zr2Li7O12 is tetragonal and has low Li ion conductivity. The introduction of Ga leads to a change to a cubic unit cell, and a large enhancement in the conductivity. Prior structural studies of La3Zr2Li7O12 have shown the presence of both tetrahedral and distorted octahedral sites for Li, and the low conductivity can be explained by the ordered nature of the Li distribution. The present structural study of La3Zr2Ga0.5Li5.5O12 shows that Ga substitutes onto the tetrahedral site. Despite the presence of non-mobile Ga3+ on the Li sites, the conductivity is enhanced as a result of the introduction of vacancies in the Li sites, and consequent disorder on the Li sublattice. Further work has suggested that over time in air, there is some H+/Li+ exchange, and consequently some variation in the conductivity.

Fluorination of the Ruddlesden Popper phase, Sr3Fe2O7−x by heat treatment with polyvinylidine fluoride (PVDF) gives a range of novel oxide fluoride compounds. Fluorination with 1 mol equivalent PVDF leads to a filling of the normal Ruddlesden Popper structure anion sites and a material of composition Sr3Fe2O5+xF2−x (x≈0.28(4)) which contains both Fe4+ and Fe3+. Increasing the amount of PVDF to 2 mol equivalent leads to an increase in anion content due to filling of half the interstitial sites within the structure, with iron being completely reduced to Fe3+ leading to a composition Sr3Fe2O4F4. An increase in the amount of PVDF to ≈3 mol equivalent leads to a further increase in unit cell volume, attributed to complete filling of the interstitial sites and a composition Sr3Fe2O3F6. 57Fe Mössbauer spectra in the temperature range 10–300 K demonstrated the complexity of the magnetic interactions in each of the three phases which reflect different local compositions of oxygen and fluorine around the iron ions thus influencing the superexchange pathways.Graphical abstractLow temperature (375 °C) fluorination of Sr3Fe2O7−x with poly(vinylidene fluoride) leads to the production of three new Ruddlesden Popper oxide fluorides with progressive filling of the anion sites within the structure.Highlights► The fluorination of Sr3Fe2O7−x using PVDF. ► The control of the fluorine content with amount of PVDF used. ► The synthesis of three new Fe based oxide fluorides. ► The identification of the structures of these oxide fluorides.

In this paper we report the successful incorporation of phosphate and sulfate groups into the ionic conductor, Ba2In2O5, with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. The results show that such oxyanion incorporation leads to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, and hence increases the conductivity at temperatures <800 °C. In wet atmospheres, there is evidence for a significant enhancement of the conductivity through a protonic contribution.

In the solid oxide fuel cell (SOFC) field, proton conducting perovskite electrolytes offer many potential benefits. However, an issue with these electrolytes is their stability at elevated temperatures in the presence of CO2. Recently we have reported enhanced oxide ion/proton conductivity in oxyanion (silicate, phosphate) doped Ba2In2O5, and in this paper we extend this work to examine the stability at elevated temperatures towards CO2. The results show improved CO2 stability compared to the undoped system, and moreover this can be further improved by co-doping on either the Ba site with La, or the In site with Zr. While this co-doping strategy does reduce the conductivity slightly, the greatly improved CO2 stability would suggest there is technological potential for these co-doped samples.Highlights► Synthesis of P/Si doped Ba2In2O5 samples co-doped with La, Zr. ► Co-doped samples show high conductivities. ► Co-doped samples show enhanced stability towards CO2.

High oxygen content apatite germanates, La10Ge6−xWxO27+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 La10Ge5.5W0.5O27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO4 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 WO5 units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO5 units, thus not contributing significantly to the conduction process.

In this paper we report the successful incorporation of silicon into SrMO3 (M = Co, Mn) leading to a structural change from a hexagonal to a cubic perovskite. For M = Co, the cubic phase was observed for low doping levels (3%), and these doped phases showed very high conductivities (up to ≈350 Scm−1 at room temperature). However, annealing studies at intermediate temperatures (700–800 °C), indicated that the cubic phase was metastable with a gradual transformation to a hexagonal cell on annealing. Further work showed that co-doping with Fe resulted in improved stability of the cubic phase; a composition SrCo0.85Fe0.1Si0.05O3−y displayed good stability at intermediate temperatures and a high conductivity (≈150 Scm−1 at room temperature). For M = Mn, the work showed that higher substitution levels were required to form the cubic perovskite (≈15% Si doping), although in these cases the phases were shown to be stable to annealing at intermediate temperatures. Conductivity measurements again showed an enhancement in the conductivity on Si doping, although the conductivities were lower (≈0.3–14 Scm−1 in the range 20–800 °C) than the cobalt containing systems. The conductivities of both systems suggest potential for use as cathode materials in solid oxide fuel cells.

In this paper we report the successful incorporation of sulfate and phosphate into SrCoO3 leading to a change from a 2H- to a 3C-perovskite polymorph. Structural characterization by neutron diffraction showed extra weak peaks related to oxygen vacancy ordering, and these could be indexed on an expanded tetragonal cell, containing two inequivalent Co sites, similar to previously reported for Sb doped SrCoO3. Conductivity measurements on the doped systems showed a large enhancement compared to the undoped hexagonal system, consistent with corner-sharing of CoO6 octahedra for the former. Further work on the doped samples shows, however, that they are metastable, transforming back to the hexagonal cell on annealing at intermediate temperatures. The incorporation of Fe was shown, however, to improve the stability at intermediate temperatures, and these co-doped phases also showed high conductivities.Graphical abstractPhosphate/sulfate doping in SrCoO3−y leads to a structural change to a 3C-perovskite framework, with an accompanying large increase in conductivity.Highlights► Sulfate and phosphate are successfully doped into SrCoO3. ► The doping stabilises the 3C-perovskite framework. ► The doped samples show high conductivities.

In this review article, new systems being investigated for application in solid oxide fuel cells are discussed. For the electrode materials, materials with the perovskite or related structures continue to dominate the field, due to the need for high electronic conductivity. Research in this field is being directed toward compositions allowing high ionic conductivity in addition to their electronic contribution. In contrast, research on new electrolyte materials has shown a diverse range of structure-types, with an apparent tendency toward structures containing cations in lower coordination environments, particularly tetrahedral. In both the electrode and electrolyte area, materials allowing the incorporation of oxygen excess into interstitial sites have shown promising results, warranting further investigations of materials that will allow this type of defect chemistry.

In the solid oxide fuel cell (SOFC) field, the presence of silicon impurities is commonly considered to be detrimental to the performance due to segregation as silica at the grain boundaries. In this paper we demonstrate, however, that silicon doping into Ba2In2O5 leads to a significant enhancement in the oxide ion conductivity. The results indicate that silicon is incorporated into the structure leading to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, with the oxygen vacancy disordering leading to the conductivity enhancement. In wet atmospheres, the conductivity is further enhanced through a protonic contribution, leading to conductivities (2.4 × 10−3 Scm−1 at 400 °C) comparable to the best perovskite proton conductors. Thus, the results show that silicon can be incorporated into the perovskite structure, suggesting further studies in this area are warranted, particularly related to electrode materials.

In this paper, neutron diffraction studies are reported on the La1−xBa1+xGaO4−x/2 system in order to locate the proton sites. Difference Fourier maps suggested the presence of unfitted nuclear density (negative for H2O treated samples and positive for D2O treated samples) adjacent to the O3 and O4 sites, giving sensible O–H/D distances. The results therefore indicate more than one proton site, consistent with modelling studies which suggested that there was little difference between the proton defect energies for different oxygen sites. The results indicate a mixture of inter- and intra-tetrahedra H-bonding interactions, with the shortest interaction being of the order of 2 Å. Further modelling studies into dopant site selectivity are also reported which indicate that charge and size effects dominate the solution energies.

In this communication, we demonstrate the successful incorporation of phosphate into Ba2In2O5, which leads to the conversion from an orthorhombic to a cubic unit cell. The resulting increased oxygen vacancy disorder leads to an enhancement in the oxide ion conductivity at low temperatures. In addition, in wet atmospheres, significant proton conduction is observed.

In this paper we show that through Ti-doping it is possible to synthesise high-oxygen-content silicate-based apatites, La10Si6−xTixO27 (0.25 ≤ x ≤ 2.0), and report conductivity data for this series. The results show that Ti-doping leads to a general decrease in the conductivity, and an increase in activation energy, particularly for the range 1.0 ≤ x ≤ 2.0. With reference to computer modelling and 29Si NMR data, this decrease in conductivity is attributed to trapping of the interstitial oxide ions around the Ti dopant. Attempts to prepare similar compositions containing La cation vacancies, i.e. La10−ySi6−xTixO3−3y/2, have a tendency to result in La2Ti2O7 impurities, particularly for high values of y.

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.

In this paper we report synthesis, conductivity and structural data for the novel tetragonal garnet-related system, Li7La3Sn2O12. Neutron diffraction data shows that the tetragonal distortion is related to ordering of Li in three sites within the structure to ensure no short Li–Li interactions. Consistent with the ordered nature of the Li ions, the conductivity is low, with a high activation energy. The results are relevant to related highly conducting cubic garnets, Li5+xLn3−xAxM2O12 (Ln = rare earth, A = alkaline earth; M = Nb, Ta, Sb), showing how a high Li content can be accommodated by Li ordering within the garnet structure, supporting previous suggestions by Cussen for the cubic garnets, who proposed the presence of local ordering/clustering of Li in tetrahedral and “octahedral” sites to limit unfavourable short Li–Li interactions.

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, La8+xSr2−x(Si/Ge)6O26+x/2, and NH3 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 NH3 at elevated temperatures.In this paper we show that heating the apatite-type electrolytes La8+xSr2−x(Si/Ge)6O26+x/2 in NH3 at high temperatures leads to nitridation of the electrolyte, with the level of nitridation increasing with increasing x.

In this article we review work on oxyanion (carbonate, borate, nitrate, phosphate, sulphate, silicate) doping in perovskite materials beginning with early work on doping studies in superconducting cuprates, and extending to more recent work on doping into perovskite-type solid oxide fuel cell materials. In this doping strategy, the central atom of the oxyanion group occupies the perovskite B cation site, with the associated oxide ions filling 3 (carbonate, nitrate, borate) or 4 (phosphate, sulphate, silicate) of the available 6 anion sites around this site, albeit displaced so as to achieve the required geometry for the oxyanion. We highlight the potential of this doping strategy to prepare new systems, stabilize phases that cannot be prepared under ambient pressure conditions, and lead to modifications to the electronic and ionic conductivity. We also highlight the need for further work in this area, in particular to evaluate the carbonate content of perovskite phases in general.

In this paper we report the successful incorporation of phosphate and sulfate groups into the ionic conductor, Ba2In2O5, with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. The results show that such oxyanion incorporation leads to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, and hence increases the conductivity at temperatures <800 °C. In wet atmospheres, there is evidence for a significant enhancement of the conductivity through a protonic contribution.

Apatite silicates are attracting significant interest as potential SOFC electrolyte materials. They are non-conventional oxide ion conductors in the sense that oxide ion interstitials, rather than vacancies, are the key defects. In this work we compare the structures of La9.6Si6O26.4 and La8Sr2Si6O26, both before and after hydration in order to gather information about the location of the interstitial oxide ion site. Neutron diffraction structural studies suggest that in the as-prepared La8Sr2Si6O26 and hydrated La8Sr2Si6O26, the interstitial oxide ion sites are close to the apatite channel centre. For La9.6Si6O26.4, a similar site close to the channel centre is observed, but on hydration of this particular sample, the interstitial site is shown to be significantly displaced away from the channel centre towards the SiO4 units. This can be explained by the need for additional displacement from the channel centre to accommodate the large amount of interstitial anions in this hydrated phase. The solid state 29Si MAS NMR spectra are shown to be very sensitive to the different speciation exhibited by the La8Sr2Si6O26 and La9.6Si6O26.4 systems, with the former being dominated by regular SiO4 framework species and the latter being dominated by interruptions to this network caused by cation vacancies and interstitials. The corresponding 17O MAS NMR study identifies a strong signal from the O atoms of the SiO4 groups, thus demonstrating that all of the O species in these systems are exchangeable O under heterogeneous gas phase conditions. In addition, interstitial O species attributed to pendant OH linkages on the Si positions are clearly identified and resolved, and these are removed on dehydration. This observation and assignment is corroborated by corresponding 1H MAS NMR measurements. Overall the neutron diffraction work indicates that the interstitial site location in these apatite silicates depends on the anion content with progressive displacement towards the SiO4 tetrahedra on increasing anion content, while the observation of exchangeable O on the SiO4 groups is consistent with prior modelling predictions as to the importance on the silicate units in the conduction process.

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.

In this paper we report the successful incorporation of silicon into Sr1−yCayMnO3−δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a 29Si enriched Sr1−yCayMn1−xSixO3−δ perovskite system is confirmed by 29Si MAS NMR measurements at low B0 field. The very large paramagnetic shift (∼3000–3500 ppm) and anisotropy (span ∼4000 ppm) suggests that the Si4+ species experiences both Fermi contact and electron-nuclear dipolar contributions to the paramagnetic interaction with the Mn3+/4+ centres. An improvement in the conductivity is observed for low level Si doping, which can be attributed to two factors. The first of these is attributed to the tetrahedral coordination preference of Si leading to the introduction of oxide ion vacancies, and hence a partial reduction of Mn4+ to give mixed valence Mn. Secondly, for samples with high Sr levels, the undoped systems adopt a hexagonal perovskite structure containing face sharing of MnO6 octahedra, while Si doping is shown to help to stabilise the more highly conducting cubic perovskite containing corner linked octahedra. The level of Si, x, required to stabilise the cubic Sr1−yCayMn1−xSixO3−δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr0.5Ca0.5Mn1−xSixO3−δ series; x = 0.075 for Sr0.7Ca0.3Mn1−xSixO3−δ; x = 0.10 for Sr0.8Ca0.2Mn1−xSixO3−δ; and x = 0.15 for SrMn1−xSixO3−δ. Composites with 50% Ce0.9Gd0.1O1.95 were examined on dense Ce0.9Gd0.1O1.95 pellets. For all series an improvement in the area specific resistances (ASR) values is observed for the Si-doped samples. Thus these preliminary results show that silicon can be incorporated into perovskite cathode materials and can have a beneficial effect on the performance.

In this paper we examine the effect of Ga doping on the structure and conductivity of the high Li ion content garnet-related system, La3Zr2Li7O12. Without Ga doping, La3Zr2Li7O12 is tetragonal and has low Li ion conductivity. The introduction of Ga leads to a change to a cubic unit cell, and a large enhancement in the conductivity. Prior structural studies of La3Zr2Li7O12 have shown the presence of both tetrahedral and distorted octahedral sites for Li, and the low conductivity can be explained by the ordered nature of the Li distribution. The present structural study of La3Zr2Ga0.5Li5.5O12 shows that Ga substitutes onto the tetrahedral site. Despite the presence of non-mobile Ga3+ on the Li sites, the conductivity is enhanced as a result of the introduction of vacancies in the Li sites, and consequent disorder on the Li sublattice. Further work has suggested that over time in air, there is some H+/Li+ exchange, and consequently some variation in the conductivity.

In this paper we report the successful incorporation of silicon into SrFeO3−δ perovskite materials for potential applications as electrode materials for solid oxide fuel cells. It is observed that Si doping leads to a change from a tetragonal cell (with partial ordering of oxygen vacancies) to a cubic one (with the oxygen vacancies disordered). Annealing experiments in 5% H2/95% N2 (up to 800 °C) also showed the stabilization of the cubic form for the Si-doped samples under reducing conditions, suggesting that they may be suitable for both cathode and anode applications. In contrast to the cubic cell of the reduced Si doped system, reduction of undoped SrFeO3−δ leads to the formation of a brownmillerite structure with ordered oxide ion vacancies. SrFe0.90Si0.10O3−δ and SrFe0.85Si0.15O3−δ were analysed by neutron powder diffraction, and the data confirmed the cubic cell, with no long range oxygen vacancy ordering. Mössbauer spectroscopy data were also recorded for SrFe0.90Si0.10O3−δ, and indicated the presence of only Fe3+ and Fe5+ (i.e. disproportionation of Fe4+ to Fe3+ and Fe5+) for such doped samples. Conductivity measurements showed an improvement in the conductivity on Si doping. Composite electrodes with 50% Ce0.9Gd0.1O1.95 were therefore examined on dense Ce0.9Gd0.1O1.95 pellets in two different atmospheres: air and 5% H2/95% N2. In both atmospheres an improvement in the area specific resistance (ASR) values is observed for the Si-doped samples. Thus the results show that silicon can be incorporated into SrFeO3−δ-based materials and can have a beneficial effect on the performance, making them potentially suitable for use as cathode and anode materials in symmetrical SOFCs.

In this paper we report the synthesis, structure and Li ion conductivity of a new tetragonal garnet phase Nd3Zr2Li7O12. In line with other tetragonal garnet systems, the Li is shown to be ordered in the tetrahedral and distorted octahedral sites, and the Li ion conductivity is consequently low. In an effort to improve the ionic conductivity of the parent material, we have also investigated Al doping to reduce the Li content, Nd3Zr2Li5.5Al0.5O12, and hence introduce disorder on the Li sublattice. This was found to be successful leading to a change in the unit cell symmetry from tetragonal to cubic, and an enhanced Li ion conductivity. Neutron diffraction studies showed that the Al was introduced onto the ideal tetrahedral garnet site, a site preference also supported by the results of computer modelling studies. The effect of moisture on the conductivity of these systems was also examined, showing significant changes at low temperatures consistent with a protonic contribution in humid atmospheres. In line with these observations, computational modelling suggests favourable exchange energy for the Li+/H+ exchange process.

In this paper, neutron diffraction studies are reported on the La1−xBa1+xGaO4−x/2 system in order to locate the proton sites. Difference Fourier maps suggested the presence of unfitted nuclear density (negative for H2O treated samples and positive for D2O treated samples) adjacent to the O3 and O4 sites, giving sensible O–H/D distances. The results therefore indicate more than one proton site, consistent with modelling studies which suggested that there was little difference between the proton defect energies for different oxygen sites. The results indicate a mixture of inter- and intra-tetrahedra H-bonding interactions, with the shortest interaction being of the order of 2 Å. Further modelling studies into dopant site selectivity are also reported which indicate that charge and size effects dominate the solution energies.

In this paper we report the successful incorporation of silicon into SrMO3 (M = Co, Mn) leading to a structural change from a hexagonal to a cubic perovskite. For M = Co, the cubic phase was observed for low doping levels (3%), and these doped phases showed very high conductivities (up to ≈350 Scm−1 at room temperature). However, annealing studies at intermediate temperatures (700–800 °C), indicated that the cubic phase was metastable with a gradual transformation to a hexagonal cell on annealing. Further work showed that co-doping with Fe resulted in improved stability of the cubic phase; a composition SrCo0.85Fe0.1Si0.05O3−y displayed good stability at intermediate temperatures and a high conductivity (≈150 Scm−1 at room temperature). For M = Mn, the work showed that higher substitution levels were required to form the cubic perovskite (≈15% Si doping), although in these cases the phases were shown to be stable to annealing at intermediate temperatures. Conductivity measurements again showed an enhancement in the conductivity on Si doping, although the conductivities were lower (≈0.3–14 Scm−1 in the range 20–800 °C) than the cobalt containing systems. The conductivities of both systems suggest potential for use as cathode materials in solid oxide fuel cells.

High oxygen content apatite germanates, La10Ge6−xWxO27+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 La10Ge5.5W0.5O27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO4 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 WO5 units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO5 units, thus not contributing significantly to the conduction process.

In this communication, we demonstrate the successful incorporation of phosphate into Ba2In2O5, which leads to the conversion from an orthorhombic to a cubic unit cell. The resulting increased oxygen vacancy disorder leads to an enhancement in the oxide ion conductivity at low temperatures. In addition, in wet atmospheres, significant proton conduction is observed.

In this paper we report the successful synthesis of the cubic oxyanion containing perovskites, Ba2Sc2−xPxO5+x (x = 0.4, 0.5), with the samples analysed through a combination of X-ray diffraction, NMR, TGA, Raman spectroscopy and conductivity measurements. Conductivity measurements indicate a p-type contribution to the conductivity in oxidizing conditions at elevated temperatures, with evidence for proton conduction in wet atmospheres. For the latter, bulk conductivities of 5.9 × 10−3 and 1.3 × 10−3 S cm−1 at 500 °C were obtained for x = 0.4 and 0.5 respectively, comparable to other perovskite proton conductors, while the stability towards CO2 containing atmospheres was improved compared to BaCeO3 based systems. Related Si doped systems have also been prepared, although in this case small Ba2SiO4 impurities are observed. We also provide evidence to suggest that “undoped” Ba2Sc2O5 contains carbonate groups, which accounts for its thermal instability.

In this paper we report synthesis, conductivity and structural data for the novel tetragonal garnet-related system, Li7La3Sn2O12. Neutron diffraction data shows that the tetragonal distortion is related to ordering of Li in three sites within the structure to ensure no short Li–Li interactions. Consistent with the ordered nature of the Li ions, the conductivity is low, with a high activation energy. The results are relevant to related highly conducting cubic garnets, Li5+xLn3−xAxM2O12 (Ln = rare earth, A = alkaline earth; M = Nb, Ta, Sb), showing how a high Li content can be accommodated by Li ordering within the garnet structure, supporting previous suggestions by Cussen for the cubic garnets, who proposed the presence of local ordering/clustering of Li in tetrahedral and “octahedral” sites to limit unfavourable short Li–Li interactions.

In this paper we show that through Ti-doping it is possible to synthesise high-oxygen-content silicate-based apatites, La10Si6−xTixO27 (0.25 ≤ x ≤ 2.0), and report conductivity data for this series. The results show that Ti-doping leads to a general decrease in the conductivity, and an increase in activation energy, particularly for the range 1.0 ≤ x ≤ 2.0. With reference to computer modelling and 29Si NMR data, this decrease in conductivity is attributed to trapping of the interstitial oxide ions around the Ti dopant. Attempts to prepare similar compositions containing La cation vacancies, i.e. La10−ySi6−xTixO3−3y/2, have a tendency to result in La2Ti2O7 impurities, particularly for high values of y.