Solid solubility and phase transitions in the system LaNb1−xTaxo4

Vullum, Fride; Nitsche, Fabian; Selbach, Sverre M.; Grande, Tor

doi:10.1016/j.jssc.2008.06.032

Cited by 79 publications

(87 citation statements)

References 15 publications

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“…3. Table 4 reports the phase transition temperature and the thermal expansion coefficients for each SLNT composition, which agree well with literature data for LaNb 1-x Ta x O 4 compositions [6]. [3].…”

Section: Resultssupporting

confidence: 85%

“…The structural changes from the low temperature monoclinic phase to the high temperature tetragonal structure affect LN thermo-mechanical and conduction properties; the expansion coefficient decreases from 17.3 × 10 −6°C−1 to 7.1 × 10 −6°C−1 and the activation energy for proton migration changes from 0.77 eV to 0.54 eV [3]. LaTaO 4 (LT) is another chemically stable HTPC electrolyte, showing a total conductivity of 8 × 10 −5 Scm −1 at 600°C in wet hydrogen, which also shows phase transitions from orthorhombic to monoclinic at around 220°C, and from monoclinic to tetragonal structure at about 1300°C [5,6]. Therefore, avoiding the phase transitions would represent a breakthrough for the development of stable electrolytes for intermediate-low temperature SOFCs.…”

Section: Introductionmentioning

confidence: 99%

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Tailoring phase stability and electrical conductivity of Sr0.02La0.98Nb1–xTaxO4 for intermediate temperature fuel cell proton conducting electrolytes

et al. 2012

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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.

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Section: Resultssupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

Tailoring phase stability and electrical conductivity of Sr0.02La0.98Nb1–xTaxO4 for intermediate temperature fuel cell proton conducting electrolytes

et al. 2012

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“…lanthanide vanadates, niobates, arsenates or antimonates) possess interesting functional properties, such as ferroelasticity [12][13][14] , photoluminescence [15][16][17][18][19][20] or ionic conductivity [21][22][23] . Out of this group lanthanum orthoniobate is considered to be among the best materials combining appreciable proton conductivity and chemical stability 22,[24][25][26][27] , reaching conductivity close to 10 -3 S/cm at 900 °C under wet conditions 22 . Doping and substitutions in lanthanum niobate, both alio-and isovalent, have a substantial influence on these materials properties and much effort to understand the influence of dopants on structural [24][25][26][28][29][30][31][32][33][34] and electrical [22][23][24][25][26][27][35][36][37][38][39][40][41][42][43] properties have been made.…”

mentioning

confidence: 99%

“…Doping and substitutions in lanthanum niobate, both alio-and isovalent, have a substantial influence on these materials properties and much effort to understand the influence of dopants on structural [24][25][26][28][29][30][31][32][33][34] and electrical [22][23][24][25][26][27][35][36][37][38][39][40][41][42][43] properties have been made. Lanthanum orthoniobate undergoes a structural phase transition at about 500 °C, which is accompanied by a decrease in the activation energy of the conductivity 37,39,43 and nearly a twofold change of the thermal expansion coefficient (TEC) [24][25][26]29 . Changes in TEC may cause challenges for the materials' applicability in e.g.…”

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Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

et al. 2016

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The results of ionic transport measurements for the lanthanum orthoniobate substituted with 10 and 30 mol% of antimony (LaNb0.9Sb0.1O4 and LaNb0.7Sb0.3O4) are presented and discussed. The influence of calcium co-doping on these properties has also been analysed. It has been shown that for the investigated material protonic conductivity predominates at temperatures up to 800 °C in oxidizing atmospheres, in wet conditions. Maximum observed protonic conductivity reaches ~10-4 S/cm at 800 °C (in humidified air); in the dry conditions the increasing influence of oxygen vacancies and holes is detected. Oxygen self-diffusivity has also been analysed by isotopic exchange to investigate the possible diffusion paths.

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“…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.…”

mentioning

confidence: 99%

Soft Chemistry Routes for the Synthesis of Sr0.02La0.98Nb0.6Ta0.4O4 Proton Conductor

Santibáñez-Mendieta

Fabbri

Licoccia

et al. 2011

J. Electrochem. Soc.

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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...

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Solid solubility and phase transitions in the system LaNb1−xTaxo4

Cited by 79 publications

References 15 publications

Tailoring phase stability and electrical conductivity of Sr0.02La0.98Nb1–xTaxO4 for intermediate temperature fuel cell proton conducting electrolytes

Tailoring phase stability and electrical conductivity of Sr0.02La0.98Nb1–xTaxO4 for intermediate temperature fuel cell proton conducting electrolytes

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

Soft Chemistry Routes for the Synthesis of Sr0.02La0.98Nb0.6Ta0.4O4 Proton Conductor

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