The structure and Li + ion dynamics of a new class of ABO 3 perovskite with Li on both the A-and B-sites are described. La 3 Li 3 W 2 O 12 is synthesized by solid state reaction at 900 °C and shown by powder X-ray diffraction to adopt the structure of a monoclinic double perovskite (A 2 )BB′O 6 , (La 1.5 Li 0.5 )WLiO 6 , with rock salt order of W 6+ and Li + on the B-site. High resolution powder neutron diffraction locates A-site Li in a distorted tetrahedron displaced from the conventional perovskite A-site, which differs considerably from the sites occupied by Li in the well studied La 2/3−x Li 3x TiO 3 family. This is confirmed by the observation of a lower coordinated Li + ion in the 6 Li magic angle spinning nuclear magnetic resonance (NMR) spectra, in addition to the B-site LiO 6 , and supported computationally by density functional theory (DFT), which also suggests local order of A-site La 3+ and Li + . DFT shows that the vacancies necessary for transport can arise from Frenkel or La excess defects, with an energetic cost of ∼0.4 eV/vacancy in both cases. Ab initio molecular dynamics establishes that the Li + ion dynamics occur by a pathway involving a series of multiple localized Li hops between two neighboring A-sites with an overall energy barrier of ∼0.25 eV, with additional possible pathways involving Li exchange between the A-and B-sites. A similar activation energy for Li + ion mobility (∼0.3 eV) was obtained from variable temperature 6 Li and 7 Li line narrowing and relaxometry NMR experiments, suggesting that the barrier to Li hopping between sites in La 3 Li 3 W 2 O 12 is comparable to the best oxide Li + ion conductors. AC impedance-derived conductivities confirm that Li + ions are mobile but that the long-range Li + diffusion has a higher barrier (∼0.5 eV) which may be associated with blocking of transport by A-site La 3+ ions.

a b s t r a c t a r t i c l e i n f oO 4 (SLNT, with x = 0.1, 0.2, and 0.4) proton conducting oxides were synthesized by solid state reaction for application as electrolyte in solid oxide fuel cells operating below 600°C. Dense pellets were obtained after sintering at 1600°C for 5 h achieving a larger average grain size with increasing the tantalum content. Dilatometric measurements were used to obtain the SLNT expansion coefficient as a function of tantalum content (x), and it was found that the phase transition temperature increased with increasing the tantalum content, being T = 561, 634, and 802°C for x = 0.1, 0.2, and 0.4, respectively. The electrical conductivity of SLNT was measured by electrochemical impedance spectroscopy as a function of temperature and tantalum concentration under wet (p H2O of about 0.03 atm) Ar atmosphere. At each temperature, the conductivity decreased with increasing the tantalum content, at 600°C being 2.68 × 10 −4 , 3.14 × 10 −5 , and 5.41 × 10 −6 Scm −1 for the x = 0.1, 0.2, and 0.4 compositions, respectively. SLNT with x = 0.2 shows a good compromise between proton conductivity and the requirement of avoiding detrimental phase transitions for application as a thin-film electrolyte below 600°C.

Niobates and tantalates of rare-earth compounds are high temperature proton conductor (HTCP) oxides that are gaining attention as possible stable electrolyte materials for application in intermediate temperature solid oxide fuel cells (IT-SOFCs). Sr 0.02 La 0.98 Nb 0.6 Ta 0.4 O 4 was synthesized by auto-combustion and co-precipitation routes, and by solid state reaction for sake of comparison, expecting an improvement in conductivity for the wet chemistry routes over the conventional solid state reaction method. Single phase materials were obtained at 1100°C by autocombustion and by co-precipitation. The synthesized powders were characterized by X-ray diffraction (XRD) and dilatometric analyses. Dense electrolytes were obtained by pressing the calcined powders into cylindrical pellets and then sintering at 1600°C for 10 h. The pellets were observed by scanning electron microscopy (SEM). Electrical conductivity of the sintered pellets was measured as a function of the temperature by electrochemical impedance spectroscopy (EIS) measurements. Proton conductivity of 2.2×10 -4 S cm -1 was obtained in wet argon atmosphere at 800°C for the sample produced via auto-combustion.

High temperature proton conducting (HTPC) oxides allow lowering the solid oxide fuel cell (SOFC) operating temperature, reducing SOFC costs. Furthermore, in protonic SOFCs water is generated at the cathode side, without diluting the fuel and thus reducing the cell efficiency. SrCeO 3 and other perovskite-type oxides have been studied in detail by different research groups [1][2][3]. Alternative to these materials, the ortho-niobates and ortho-tantalates have been recognized as promising HTPCs since they show good chemical stability [4][5][6]. Sr-doped LaNbO 4 and Ca-doped LaTaO 4 show a total conductivity at 800 °C of 4×10 -4 and 1.5×10 -4 Scm -1 , respectively. At around 500 °C, LaNbO 4 phase changes from fergusonite to scheelite structure, which results into different conductivity behavior and different thermal expansion coefficients for each phase, deriving in problems for the cell design [5,6]. A way to avoid this phase transition is using the solid-solution containing 40 % of Ta, LaNb 0.6 Ta 0.4 O 4 , which shows the phase transformation at 800 ± 10 °C [7]. This will allow operating fuel cells based on Sr 0.02 La 0.98 Nb 0.6 Ta 0.4 O 4 electrolytes below this temperature. Good quality electrolyte materials can be attained from wet chemistry synthesized powders, co-precipitation and sol-gel based procedures. The latter methods had proven for niobates to be successful in yielding homogeneous morphology and a low sintering temperature, by using chelating agents such as oxalates, malates and citrates [8]. However, for all these synthesis techniques two main challenges arise; first, to acquire the tantalum cation solution, and second, to combine the niobium and tantalum solution with the rare earth compounds. On these grounds, the aim of this work is to prepare Sr 0.02 La 0.98 Nb 0.6 Ta 0.4 O 4 powders using auto-combustion and co-precipitation methods, and comparing with conventional solid-state reaction. Water solutions of all the cations are needed for the coprecipitation method. In this case, strontium nitrate, lanthanum nitrate and niobium oxalate are commercially available and already water soluble. Tantalum oxalate was prepared from as-purchased tantalum chloride, used to allow Ta 2 O 5 ·nH 2 O precipitating in ammonia solution, which was then filtrated and dissolved in ammonium oxalate solution. Once prepared, the four solutions were mixed and dripped in ammonia solution to form a precipitate, which was filtered and dried to form the precursor powder of Sr 0.02 La 0.98 Nb 0.6 Ta 0.4 O 4 . Regarding the auto-combustion method, commercial strontium and lanthanum nitrates were used. To prepare tantalum citrate solutions, Ta 2 O 5 ·nH 2 O was produced and then dissolved in citric acid. Niobium citrate was prepared from niobium oxalate precipitation in ammonia solution to form the Nb 2 O 5 ·nH 2 O, which was then filtered and later dissolved in citric acid. The four solutions were mixed and pH was adjusted to 7. The solvent was then evaporated and the obtained gel was ignited and burned until ashes were forme...