Laurea Magistrale in Scienza dei Materiali. Materiali Inorganici Funzionali. Electrolytes: New materials

Transcription

1 Laurea Magistrale in Scienza dei Materiali Materiali Inorganici Funzionali Electrolytes: New materials Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

2 Bibliography 1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells Elsevier P. Berastegui et al. J. Solid State Chemistry 164 (2002) T. Shimura et al Solid State Ionics 175 (2004) A. Rolle et al. Solid State Ionics 176 (2005) C.A.J. Fisher et al. Solid State Ionics 118 (1999) T. Yao et al. Solid State Ionics 132 (2000) K. Kakinuma et al J. Thermal analysis and Calorimetry 57 (1999) N. Trofimenko et al Solid State Ionics 118 (1999) I.R. Evans et al Chem. Mater 17 (2005) S.S. Pramana et al Acta Crystallographica B 63 (2007) M.C. Martin-Sedeno et al Chem. Mater 16 (2004) E. Kendrick et al Solid State Ionics 179 (2008) P.J. Wilde et al Solid State Ionics 112 (1998) A.S. Nowick et al Solid State Ionics 125 (1999) A. Arulraj et al Chem. Mater. 14 (2002) 2492

3 LAMOX Lacorre (2000) Nature 404 p. 856: High oxide ion conductivity in La 2 Mo 2 O 9 (σ = 6 x 10-2 S cm -1 at 800 C) Solid-state reaction of a stoichiometric mixture of La 2 O 3 and MoO 3 fired at 500 C, then at around C. Can also be obtained by direct ball-milling synthesis of the same mixture

4 La 2 Mo 2 O 9 : a good ionic conductor above 500 C: the phase transition is accompanied by an abrupt increase of the conductivity (almost two orders of magnitude). Conductivity DTA XRD Thermodiffractograms: abrupt narrowing above 580 C: structural phase transition towards a hightemperature cubic phase, the RT phase being most probably slightly distorted, DTA: thermal peaks upon heating and cooling around the same temperature as that of the transition determined by XRD, with a large hysteresis (35±40 K): first-order transition the increase in conductivity is associated with a phase transition from the RT α-form to a high-temperature β-form at 580 C

5 Arrhenius plot of the conductivity of La 2 Mo 2 O 9 compared to that of two typical stabilized zirconias. Filled triangles show the evolution of conductivity of La 2 Mo 2 O 9 on heating, and open triangles show the evolution on cooling. For comparison: data for two typical stabilized zirconias: line A, (ZrO 2 ) 0.87 (CaO) 0.13 ; line B, (ZrO 2 ) 0.9 (Y 2 O 3 ) 0.1 conductivity is mostly ionic in nature, since the electronic part is lower than 1% of the total conductivity at these temperatures. ( measurements in various atmospheres )

6 β-la 2 Mo 2 O 9 vs β-snwo 4 Divalent tin is a 5s2 lone-pair element (lone pair occupies a volume equivalent to that of an oxide ion O 2- ) La 2 Mo 2 O 9 : tin has been replaced by La (identical size but without a lone pair), and tungsten by iso-element molybdenum. As lanthanum is trivalent, an extra oxygen atom is necessary the formal substitution: from Sn 2 W 2 O 8 E 2 to La 2 Mo 2 O 8+1 A (E = lone pair, A = vacancy). Two lone pairs are replaced by one oxygen atom and one vacancy, through which oxygen diffusion can progress which suggests the origin of oxide ion conduction

7 A polycrystalline sample of La 2 Mo 2 O 9 was prepared from stoichiometric amounts of La 2 O 3 and MoO 3. The reactants were intimately ground, heated at a rate of 10 /min to 900 C, and held at this temperature for 3 days with intermediate grinding. A small amount of the La 2 Mo 2 O 9 obtained was melted in an alumina crucible, cooled at a rate of 3 /min to 300 C, and then furnace-cooled to RT. Very small clear crystals were isolated from the cooled melt. one of the most complex oxide structures reported to date. The structure of α -La 2 Mo 2 O 9 is far from that of a typical inorganic oxide. The crystal structure of α-la 2 Mo 2 O 9 contains 312 crystallographically independent atoms: 48 La, 48 Mo, 216 O

8 (a) Structure of β-snwo 4 ; (b) structure of β-la 2 Mo 2 O 9 ; Sn/La atoms in yellow, W/Mo atoms in blue, O atoms in red. β-la 2 Mo 2 O 9 and β-snwo 4 β-snwo 4 : cubic structure - Sn and W lie on threefold axes and form a metal sublattice (each cation surrounded by 7 cations of the other type) O create a distorted octahedral coordination around Sn, while W atoms are found in regular, isolated WO 4 tetrahedral groups. β-la 2 Mo 2 O 9 Metal arrangement = the same, Rearrangement of oxygen atoms which accompanies the change in oxygen stoichiometry. β-snwo 4 : two fully occupied oxygen sites, one on the threefold axis and one on a general position. β-la 2 Mo 2 O 9 : three unique oxygen atoms: a fully occupied O1 position on the threefold axis and two disordered partially occupied sites: both sites have large atomic displacement parameters

9 La atoms: coordination shell containing between 6 and 12 oxygen atoms, with 30 out of the 48 independent La atoms being ninecoordinate. Mo atoms: three different basic coordination types: there are 15 tetrahedral, 15 trigonal bipyramidal, and 18 octahedral Small displacements from the basic cation arrangement of SnWO 4 ; great displacements within the oxygen sublattice (a) Three Mo coordination geometry types observed in α- La 2 Mo 2 O 9. (b) Polyhedral representation of α-la 2 Mo 2 O 9 : tetrahedral groups shown in pink, trigonal bipyramidal in purple, octahedral in green; yellow spheres = La.

10 The material contains a mixture of 4, 5, and 6 coordinated Mo sites: variable Mo coordination number is a key factor in providing a lowenergy O 2- conduction pathway. dynamically disordered oxygen distribution in β-la 2 Mo 2 O 9 (two perpendicular views) Comparison of B site coordination in β-snwo 4, β-la 2 Mo 2 O 9, and α -La 2 Mo 2 O 9 ; For α-la 2 Mo 2 O 9, the picture represents a superposition of all independent Mo atoms and their coordination spheres obtained by transformation of the monoclinic superstructure into the underlying cubic subcell.

11 partial substitution of La by cations with larger mean ionic radii, like Sr(II) and K(I), = increase in the pseudo cubic cell parameter at RT, partial substitution of Mo by cations with smaller mean ionic radii, like S(VI), Cr(VI), and V(V), resulted in the lowering of the RT lattice parameter of the resulting phases. All these substitutions resulted in the stabilization of the high-temperature cubic structure of La 2- Mo 2 O 9 at RT

12 Most substitutions on either the La (e.g., other rare earths, alkaline earths, alkali metals, Bi) or Mo (e.g., W, V, S, Cr,Nb, Ta) sites will stabilize the cubic β form, and enhance the low temperature conductivity. Optimum dopants? RE and W doping: La 1.7 Gd 0.3 Mo 0.8 W 1.2 O 9 = the most promising Nb doping = very high conductivities (higher than in the parent high temperature (β) form (0.11 S cm -1 at 800 C for La 2 Mo 1.94 Nb 0.06 O 8.97 )) Open questions: Stability in reducing conditions Mo volatility High reactivity with conventional SOFC materials Mo diffusion into the cathode

13 Fluorine can be partially substituted for oxygen Fluoride ions, owing to their size and charge (-1), could favor anionic conduction The mean ionic size of O 2- (ca Å) and F - (ca Å) are comparable. For the charge neutrality considerations, on partial substitution each oxide (O 2- ) ion will be replaced by two fluoride (F - ) ions. A progressive substitution of oxygen by fluorine would lead to fluoride ions occupying the vacancy sites, thereby leading to a decrease in the number of vacancies in this model.

14 the La 2 Mo 2 O 9-0.5x F x (x = ) compositions can be related to two different structures, distorted to various extents. The electron diffraction study shows the presence of superlattice XRD pattern of the (a) α-la 2 Mo 2 O 9 and (b) La 2 Mo 2 O 8.90 F 0.20 phase. Evolution of the powder XRD pattern of the La 2 Mo 2 O 9-0.5x F x system as the function of the fluorine content, x Evolution of lattice parameters and volume as the function of the fluorine content, x

15 La 2 Mo 2 O 9-0.5x F x F doping order-disorder transition temperature decrement. Transition temperature vs F content σ vs 1/T σt vs 1/T for (a) x= 0.00, 0.20; (b) = 0.10, 0.30

16 Apatite-type Materials: Framework conductors Oxygen conduction at low temperatures remediation of radioactive waste, reconstructive medicine, non-toxic pigments Non-stoichiometric lanthanum silicates and germanates [A I 4 ][AII 6 ][(BO 4 ) 6 ][X 2 ] with A, B larger and smaller cations, respectively, X = anion or oxy anion The structure has a zeolitic character being constructed by an A I 4 (BO 4 ) 6 framework that circumscribes [001] channels The size o the channel is adjusted (A I O 6 mataprisms) to accommodate the A II 6 X 2 component The BO 4 tetrahedron remains essentially undistorted

17 Apatite-type Germanate Materials La 10 (GeO 4 ) 6 O 3 La 10 (GeO 4 ) 5 (GeO 5 )O 2 Face-sharing LaO 6 metaprisms (yellow) are corner connected to GeO 4 tetrahedra (brown) and GeO 5 trigonal bipyramids (grey) Extra anion oxygen located within the channels: Mobile interstitial oxygen The [001] channel contains, La (yellow) and O (red). Metaprism twisting adjusts the channel diameter as a function of stoichiometry. Conductivity of a) YSZ, b) La 10 (SiO 4 ) 6 O 3 c) Nd 10 (SiO 4 ) 6 O 3

18 Apatite-type Materials: Framework conductors Conduction along channels and inter-tunnel

19 framework interstitial O atoms provide a reservoir of ions that can migrate into the conducting channels of apatite, also via a mechanism of inter-tunnel oxygen diffusion that transiently converts GeO 4 tetrahedra to GeO 5 distorted trigonal bipyramids. Proposed inter-tunnel migration path for oxygen in La 10 (GeO 4 ) 5 (GeO 5 )O 2. The two configurations (A and B) of the GeO 5 trigonal bipyramids (grey) are emphasized on the left and right, where the complete framework is shown. In the central portion of the drawing all the statistically occupied germanium (Ge 3/Ge 3a) and oxygen (O12, O13, O14) sites are included to demonstrate the feasibility of creating an ion migration pathway (yellow band).

20 Silicate Apatites High conductivities Thermal expansion coefficients compatible with current electrode materials Silicate apatites are cheap systems in terms of raw materials (the germanates suffer from the high cost of GeO 2 ) These materials can be readily produced by both conventional solid state and sol gel methods 5La 2 O 3 + 6GeO 2 = La 10 (GeO 4 ) 5 (GeO 5 )O K for 16 h K for 16 h in a Pt crucible

21 The flexibility of the SiO 4 substructure plays a crucial role in facilitating oxide-ion migration: the presence of interstitial oxide ions creates pseudo- SiO 5 units, which can effectively pass along the c direction by oxygen transfer. La vacancy migration has a high energy barrier.

22 Doping to introduce oxide ion excess Doping on the La sites: La 8+x A 2-x (MO 4 ) 6 O 2+x/2 (A=Ca, Sr, Ba; M=Si, Ge), results in increasing oxide ion conductivity with values >1 x 10-3 S cm -1 at 500 C for samples with high oxide excess (x > 0.5) Doping on the Si site with lower valent cations, e.g. Mg, Ga, Al, Zn. Mg doping on the Si sites has been shown to be particularly beneficial, with conductivities as high as S cm -1 (at 800 C) reported for La 9.8 (SiO 4 ) 5.7 (MgO 4 ) 0.3 O 2.4 Arrhenius plots of ionic conductivity.

23 Cuspidine group minerals Cuspidine group minerals: general stoichiometry: M 4 (Si 2 O 7 )X 2 (M = divalent cation; X = OH, F, O) Ca 4 (Si 2 O 7 )(OH,F) 2 Chains of edge-sharing MO 7 /MO 8 polyhedra with tetrahedral disilicates groups, Si 2 O 7, interconnecting them through the vertexes.

24 Oxy-cuspidine group minerals The oxy-cuspidine structure is tolerant to cation substitution: It is possible to mix trivalent (Al 3+, Ga 3+ ) / tetravalent cations (Ge 4+, Ti 4+ ) and the charges are balanced by extra oxygen Eu 4 (Al 2 O 7 )O 2, Y 4 (Al 2 O 7 )O 2, Pr 4 (Ga 2 O 7 )O 2, Y 4 (Al 2 O 7 )O 2 Nd 4 (Ga 2-x M x O 7+x/2 )O 2 (M = Ti, Ge)

25 Oxy-cuspidine group minerals: RE 4 (Ga 2 O 7 )O 2 (RE =La, Nd, Sm) La 4 (Ga 2-x Ge x O 7+x/2 )O 2 ceramic method using high purity oxides RE 2 O 3 [RE = La, Nd, Sm], GeO 2, Ga 2 O 3 Rare-earth oxides precalcined at 1273 K for 2 h in order to achieve decarbonation. The starting mixtures were ground in an agate mortar for 10 min, pelletized, and heated at 1473 K (1373 K for Ge-doped samples) for 12 h in Pt crucibles. After cooling, the samples were ground to powder and milled for 3 h The resulting powders were pelletized again and a second thermal treatment was carried out at 1673 K for 48 h La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 needed a last thermal treatment at 1773 K for 2 h. Pellet weight losses, linked to Ge volatilization, at these temperatures and times, were found negligible.

26 Oxy-cuspidine group minerals La 4 (Ga 1.6 Ge 0.4 O 7.3 )O 2 Bulk- La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 La 4 (Ga 1.8 Ge 0.2 O 7.3 )O 2 La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 Compaction = 99% La 4 (Ga 2 O 7 )O 2 La 4 (Ga 1.4 Ge 0.6 O 7.3 )O 2 Compaction = 80% Sm 4 (Ga 2 O 7 )O 2 Complex impedance plane plots for La4(Ga1.4Ge0.6O7.3)O2 [compactions 80% (crossed open square) and 99% (open square)] at 673 K and at 998 K in the inset (99% Ge0.6). Grain boundary contribution

27 The most important structural change in the La 4 (Ga 2-x Ge x O 7+x/2 )O 2 series is the insertion of extra oxide anions at the vacant site, O(10). The isolate tetrahedral digallate groups are converted to infinite bipyramid chains with some interruptions due to the partial occupancy of both oxygens.

28 Oxide ion vs Proton Conduction In complex oxides: Cuspidines, apatites, Doped-LAMOX

29 Cuspidine-based systems:la 4 (Ti 2 O 8 )O 2 Oxide ion conduction chains of TiO 5 bridged by oxygen (O3), with channels of La and O sandwiched between the chains Most favourable intrinsic defect = oxygen Frenkel defect (1.42 ev/defect). The lowest energy for an oxygen vacancy occurs at the O3 bridging

30 Cuspidine-based systems:la 4 (Ti 2 O 8 )O 2 Oxide ion conduction Calculated low energy vacancy hopping migration via rotation of the O2, and O3 oxygen, Interstitials and Vacancies cause the high temperature oxide ion conduction in La 4 (Ti 2 O 8 )O 2 Interstitial oxide ion conduction migration mechanism (calculated most favourable interstitial position)

31 Cuspidine-based systems:la 4 (Ti 2 O 8 )O 2 Proton conduction water incorporation and proton conduction in cuspidine systems at low temperatures The defect energy for an OH defect has been calculated for a proton attached to different oxygen atoms within the structure and at the most favourable oxygen interstitial position The most favourable site for proton incorporation is at the O3 bridging oxygen The higher water incorporation energies at interstitial and mixed interstitial and vacant sites suggest that oxygen vacancies are required for water incorporation to occur.

32 Protonic conductors: Doped BaCeO 3 significant proton conduction at high temperatures (2 x 10-3 Ω -1 cm -1 at 900 C; activation energy = 52 kj/mol) due to water vapour incorporation from atmosphere during preparation. p = K 1 [V O] 1/2 P 1/4 O2 [H + ] = K 4 [V O] 1/2 P 1/2 H2O In hydrogen atmosphere: ½ H 2 + h = H + In wet atmosphere: H 2 O + 2h = ½ O 2 + 2H + H 2 O + V O = O x O + 2H+ H 2 O + O X O + V O = 2 (OH) O p = K 1 [V O] 1/2 P 1/4 O2 [H + ] = K 4 [V O] 1/2 P 1/2 H2O Protons are interstitials Hydroxide ions migrate between sites adjacent to oxygen ions or via vacancies

33 Mixed oxide ions/protonic conductors: doped BaCeO 3 Conductivity of BaCe 0.9 Gd 0.1 O 2.95 as a function of oxygen partial pressure and water partial pressure Protonic conduction is a minor component By substitution of Ba with Ca = < crystal symmetry < motion of oxygen-ions < oxygen-ion contribution to conductivity Conductivity of BaCe 1-x M x O 3-δ in hydrogen atmosphere

34 Protonic conductors: doped SrCeO 3 SrCe 1-x M x O 3-δ : significant proton conduction in hydrogen atmosphere M = Sc, Zn, Mn, Y, In, Nd, Sm, Dy, Yb and x = 0.05, 0.10 (SrCe 0.95 Yb 0.05 O 3-δ 2 x 10-3 Ω -1 cm -1 at 600 C in N 2 /H 2 5% with an activation energy of 57 kj/mol) Conductivity increases with the square root of water vapour pressure and is independent on oxygen partial pressure: strong contribution of protonic conduction Conductivity increases with oxygen-ion vacancy but is almost poorly dependent on dopant concentration Conductivity of doped SrCeO 3-δ in hydrogen

35 Pyrochlores A 2 B 2 O 7 = Pyrochlore oxides defective fluorite structure (A = Gd or Y, B = Zr or Ti) High intrinsic oxygen-ion conductivity (no doping required) Equilibrium pyrochlore phase (no long term aging effects expected) (A 3+ ) 2 (B 4+ ) 2 O 7 = Ln 2 Zr 2 O 7 (Ln = Gd, Y ): Gd 2 (Zr x Ti 1-x ) 2 O 7 (GZT), Y 2 (Zr x Ti 1-x ) 2 O 7 (YZT) x < 0.2 mixed ionic and electronic conduction x > 0.2 the electronic component of the conductivity decreases markedly x > 0.4 oxygen-ion conductivity dominates and exceeds Ω -1 cm -1 at 800 C

36 Pyrochlores The pyrochlore structure is essentially an ordered doped fluorite structure Fluorite structured compounds may be doped with low valence ions leading to vacancy compensation as in, e.g. yttria doped CeO 2. At low and intermediate concentrations there is no long-range order of the dopant ions Compared to the defective fluorite: in pyrochlore exactly half of the cations are of the trivalent type and there is ordering of the trivalent and tetravalent cations

37 Pyrochlores: results of Tuller & al. ( ) Gd 2 Zr 2-x Ti x O 7 and Y 2 Zr 2-x TiO 7 solid solutions with small values of x (Zr predominant) the ionic conductivity is higher than for those with large values of x; the latter class of compound, approximating to titanates, exhibit mixed conduction Gd 2 Ti 2 O 7: there is less intrinsic anion disorder (i.e. low concentration of anion defects) leading to low ionic conductivity (< 10-7 S/cm) Gd 2 Ti 2 O 7 doped with a series of different dopants of different sizes: the preferred dopants are Ca (A-site) and Al (B-site) Conductivity measurements on doped gadolinium titanate for a range of doping levels show a maximum in conductivity for 5 mol %. For a site doping with Ca the activation energy for ionic conductivity drops from 0.93 to 0.63 ev and for Al to 0.74 ev

38 Pyrochlores Anionic defects in pyrochlore structure High Frenkel energies Higher Frenkel energies for titanates (experimentally observed: low disorder for titanates) Energies (ev) of vacancy and interstitial defects in pyrochlore structured oxides

39 Pyrochlores Migration activation energy (ev) calculation results Saddle point energy calculated by fixing an O 2- ion in the mid-point position and creating two 48f site O 2- vacancies on either side. The activation energy was calculated as the difference between this energy and that for a single vacancy. The mid point is indeed the point of highest energy and the energy profile is symmetrical about this point The migration E a is small for all cases, in line with the high measured ionic conductivity The difference between the E a of Gd and Y compounds is small but in line with the experimental values Titanates have le lower barrier but zirconates have higher conductivities

40 Pyrochlores The O 2- Frenkel energies suggest that there will be a very low concentration of such defects in stoichiometric pyrochlores but the aterials are good ionic conductors: Effect of anti-site disorder Frenkel energies for disordered pyrochlores (ev) Cation anti-site disorder increases the similarity between the environments 48f and 8b facilitating anion disorder and promoting Frenkel defect formation

41 Dopants: the role of extrinsic defects in creating vacancies Evaluation of the solution energy for the replacement of each of the pyrochlore cations by the dopant cations: Ca 2+, Mg 2+, Sr 2+, Al 3+, Sc 3+, K + Solution reactions for doping of pyrochlore structured oxides Solution reactions for doping of pyrochlore structured oxides In most cases substitution takes place at the trivalent A cation site For some dopants (Ca, Sr) solution energies are low less than 1 ev indicating high dopant solubility and suggesting that the eextrinsic production of vacancies by dopant solution is possible

42 Y 2 Ti 2 O 7 : Doping with Calcium Synthesis Y 2 O 3, CaO, TiO 2 ball-milled for 5 h. The ground mixture was calcined at 1000 C for 12 h. The calcined powders were pressed into 20 mm diameter pellets and sintered at C With x > 0.2 the CaTiO 3 forms phase The new phase influence the conductivity YCT-1 YCT-1 YCT-1 Solution reactions for doping of pyrochlore structured oxides YCT-1 XRD pattern of Ca-doped Y 2-x Ca x Ti 2 O 7 (x = 0.2 YCT-1, x = 0.4 YCT-2, x = 0.6 YCT-3, x = 0.8 YCT-4)

43 oxide conduction or proton conduction La 2 Zr 2 O 7 structure: Zr ions = centers of the distorted octahedra O ions = at octahedra corners. La ions with O constitute an interpenetrating chain-like network. The pure material cannot accommodate any appreciable amounts of protons. Proton incorporation is facilitated by first doping the material with lower valence ions to the La or the Zr sites. The dopants, acting as negatively charged defects, are charge compensated by oxygen vacancies formed on the O or O sites. Upon exposure to a humid atmosphere the vacancies may then be replaced by protonic defects.

44 All dopants were found to form associates with the protons = proton diffusivity is reduced in their vicinity. Sr, Ca = Dopant-proton binding energy is quite small: reasonable conductors. Ba, Mg: negative effect on the proton mobility.

45 Complex perovskites High proton conductivity has also been reported for complex perovskites of the type A 2 (B B )O 6 and A 3 (B B 2 )O 9, where A is a divalent cation, B is a divalent or trivalent cation respectively and B is a pentavalent metal, usually Nb or Ta To attain this high conductivity, it is necessary to increase the B -ion content relative to that of B : A 2 (B 1+x B 1-x )O 6, A 3 (B 1+x B 2-x )O 9 Some examples: A 3 Ca 1+x Nb 2-x O 9-3x/2, Sr 3 CaZr 0.5 Ta 1.5 O 8.75,or Sr 2 Sc 1-x Nb 1-x O 6-x. A 2 (B B )O 6 : 1:1 type compounds, B and B have charges 3+ and 5+, respectively; A 3 (B B 2 )O 9 : 1:2 type; B and B have charges 2+ and 5+ In both cases, the mean B-site charge remains 4+.

46 An additional feature of these complex perovskites : the possibility of ordering on the B-sites. Elementary cubic cell of an ABO perovskite = A cation at the corner, B cation at the center and O anions at the face centers. The ordered structure of the 1:1 type compounds: alternate (111) planes of the elementary perovskite structure consist of B ions and of B ions. The net result is a larger cubic unit cell in which the B and B ions are in a NaCl like arrangement. Fully ordered 1:2 type compounds: B ions are distributed on three (111) planes (one plane containing B ions the other two B ions): a B -B -B stacking producing trigonal symmetry

47 off-stoichiometric compounds as A(B B )O x and A(B B 2 )O x become HTPCs after treatment in water vapor to undergo reaction: isotope effect and calculations: protonic conduction takes place by means of transfer of a proton from one O 2- inon to the next: No detectable water uptake (weight increment) for stoichiometric or substoichiometric samples and no proton conductivity. Both increase with increasing nonstoichiometry, x > 0

48 In both 1:1 and 1:2 type compounds higher protonic conductivity if favored by a higher degree of disorder: Also oxide ion conductivity increases with disorder: Arrhenius plots of conductivity of ordered and disordered (1000 C) Sr 2 (Nd 1.05 Nb 0.95 )O 6-δ for both vacuum and H 2 O pretreatments.

49 In the case of 1:2 compounds the type of order changes with increasing x (from 1:2 order to 1:1+random) The onset of 1:1 order occurs at higher x for Sr than for Ba Variation of mobility with nonstoichiometry, x, for the BaCaNb and SrCaNb compounds Higher conductivity and lower E a for Ba compounds Comparison of E a of proton conduction of Ba vs Sr in the A site for 1:1 and 1:2 type perovskites

50 Variation with ionic radius difference r(b )-r(b ) of the protonic conductivity at 500 K and of the activation energy for Sr 2 (B 1.1 Nb 0.9 )O 6 As the size of B is varied, for a fixed B ion, a higher conductivity and lower activation energy are found when r(b )-r(b ) is small Large radius difference gives rise to ordering, and a small difference to disordering

51 In the 1:2 ordered structure, half of the O ions sit between a B and a B ion (mean charge = +3.5) and half between two B ions (mean charge +5). Thus, the first site is a favourable one for protons, while the second is relatively unfavorable. Note that in perfect 1:1 order, all O sites fall between B and B. The occurrence of disorder creates some B B sites (charge +2)

52 The activated or transition state. While the equilibrium O 2- site is at a/2 the transition state comes closer to one B ion ( 2) the difference between hopping across a B ion and a B ion should affect the proton hopping energy: B-ion size and B-ion charge Compositional inhomogeneity will affect the local proton concentration (protons provide the charge compensation for nonstoichiometry) Interdispersed regions of relatively high and low conductivity Ba and Sr: influence on the O-O distance:. H-bonds