Effect of Ba and Bi doping on the synthesis and sintering of Ge-based apatite phases

Recently, high oxide ion conduction has been observed in the apatite-type systems La_9.33+x(Si/Ge)₆O_26+x/2, with conductivities approaching and even exceeding that of yttria-stabilized zirconia. The Ge-based phases have been reported to suffer from Ge loss and undergo irreversible structural changes on sintering at the high temperatures required to obtain dense pellets. In this paper we discuss doping studies (Ba, Bi for La) aimed at stabilizing the hexagonal apatite lattice to high temperature, and/or lowering the synthesis and sintering temperatures. The results show that doping with Ba helps to stabilize the hexagonal lattice at high temperatures, although Ge loss appears to still be a problem. Conductivity data show that, as previously reported for the Si-based systems, non-stoichiometry in the form of cation vacancies and/or oxygen excess is required to achieve high oxide ion conduction in these Ge-based systems. Neutron diffraction structural data for the fully stoichiometric phase La₈Ba₂Ge₆O₂₆ shows that the channel oxygen atoms show little anisotropy in their thermal displacement parameters, consistent with the low oxide ion conductivity of this phase. Bi doping is shown to lower the synthesis and sintering temperatures, although the presence of Bi means that these samples are not stable at high temperatures under reducing conditions.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, April 10–12, 2003     

Physicochemical compatibility of SrCeO3 with potential SOFC cathodes

The chemical and physical compatibility of SrCeO₃ is investigated with respect to LaMO₃ (M=Mn, Fe, Co) and La_2−xSr_xNiO₄ (x=0, 0.8), via the reaction of fine-grained powder compacts and solid-state diffusion couples. Compositions were chosen so as to give predictive insight into possible candidate materials for all-oxide electrochemical devices. Results show the primary reaction in these systems to be the dissolution of SrO from SrCeO₃ into the LaMO₃/La_2−xSr_xNiO₄, and corresponding formation of La-doped CeO₂. Reaction kinetics are observed to be relatively fast, with element profiles suggesting the diffusion of Sr²⁺ in ceria to be surprisingly rapid. It is demonstrated that perovskite starting materials represent poor candidates for use with SrCeO₃, reacting completely to form Ruddlesden-Popper/K₂NiF₄ type oxides. Reaction with La₂NiO₄ is less pronounced, and formation of secondary phases suppressed for the composition La_1.2Sr_0.8NiO₄. It is thus concluded that Ruddlesden-Popper type oxides represent good candidate materials for use with a SrCeO₃-based electrolytes when doped with appropriate levels of Sr.     

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.     

A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La9.67Si6O26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La_9.67Si₆O_26.5 are reported for temperatures ranging between 25 and 900 °C. The best fit to the data was obtained for space group P6₃ 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.     

A comparison of the effect of rare earth vs Si site doping on the conductivities of apatite-type rare earth silicates

Apatite-type lanthanum silicate (La_9.33Si₆O₂₆) has been attracting significant recent interest due to its high oxide ion conductivity. In this paper, synthesis and conductivity data for a range of doped samples (Mg, Ca, Sr, Ba, B, Ga and Zn) are reported, in particular, to compare the effect of rare earth vs Si site doping. The results show that Ga, B and Zn favour substitution on the Si site, while Ca, Sr and Ba favour La-site substitution. Mg is shown to be an ambi-site dopant, substituting on either site depending on the starting composition. The samples doped on the Si site show higher conductivities than comparable samples doped on the La site, providing further support for the importance of the silicate network in the conduction process, as initially predicted by atomistic modelling studies. For Ga doping on the Si site, the effect of varying the rare-earth size on the conductivities is also reported.     

Fabrication process parameters significantly affect the perovskite oxygen carriers materials (OCM) performance in chemical looping with oxygen uncoupling (CLOU)

Journal of Thermal Analysis and Calorimetry - The CLOU performance of the CaTixMn0.9?xMg0.1O3 (CMTM) perovskite-type system was investigated, comparing materials produced at laboratory scale...     

La(Ni2/3Nb1/3)O3 by neutron powder diffraction

Lanthanum nickel niobium trioxide has been synthesized and its structure refined for the first time. It was found to be a member of the family of technologically important `double perovskites', crystallizing in the monoclinic space group P2₁/n. The structure is characterized by a strong orthorhombic pseudosymmetry and a concurrent exhibition of both 1:1 B‐cation ordering and a⁻a⁻c⁺‐type tilting of the (Ni/Nb)O₆ structural units. Trivalent lanthanum resides on the perovskite A site, which is strongly distorted owing to the tilting of the (Ni/Nb)O₆ sublattice. Ordering of divalent nickel and pentavalent niobium on the B sublattice is described in terms of two twofold special positions (2c and 2d), with nickel taking almost complete occupancy of the 2d site and the 2c position being occupied by a statistical distribution of nickel and niobium.     

Synthesis and electrical characterisation of the apatite-type oxide ion conductors Nd9.33+xSi6-yGayO26+z

Apatite-type oxides have been attracting interest as a new class of oxide ion conductors. In this paper we examine the effect of Ga doping on the conductivity of the apatite silicate system, Nd_9.33+xSi₆O_26+3x/2 and compare the results to those reported for similar doping studies in La_9.33+xSi₆O_26+3x/2. The highest conductivities are observed for samples containing oxygen excess, which is in agreement with previous reports that interstitial oxide ions are important for high oxide ion conduction in these materials. For oxygen stoichiometric materials, i.e. Nd_9.33+x/3Si_6-xGa_xO₂₆, the Ga doping results in a significant increase in activation energy and a consequent lowering of the low temperature conductivity. This is contrary to results previously reported for the La containing analogues, which showed an enhancement of conductivity on Ga doping up to x=1.5. Possible explanations for the differences between the two systems are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properites, Patras, Greece, Sept. 14 – 18, 2004.     

Doping strategies to optimise the oxide ion conductivity in apatite-type ionic conductors

The apatite-type phases, La(9.33+x)(Si/Ge)(6)O(26+3x/2), have recently been attracting considerable interest as potential electrolytes for solid oxide fuel cells. In this paper we report results from a range of doping studies in the Si based systems, aimed at determining the key features required for the optimisation of the conductivities. Systems examined have included alkaline earth doping on the rare earth site, and P, B, Ga, V doping on the Si site. By suitable doping strategies, factors such as the level of cation vacancies and oxygen excess have been investigated. The results show that the oxide ion conductivities of these apatite systems are maximised by the incorporation of either oxygen excess or cation vacancies, with the former producing the best oxide ion conductors. In terms of samples containing cation vacancies, conductivities are enhanced by doping lower valent ions, Ga, B, on the Si site. The presence of higher valent ions on these sites, e.g. P, appears to inhibit the incorporation of excess oxygen within the channels, and so limits the maximum conductivity that can be obtained. Overall the results suggest that the tetrahedral sites play a key role in the conduction properties of these materials, supporting recent modelling studies, which have suggested that these tetrahedra aid in the motion of the oxide ions down the conduction channels by co-operative displacements.