M.S. Islam

Dopant and proton incorporation in perovskite-type zirconates

Abstract Atomistic simulation techniques are used to investigate the energetics of defect and dopants, of oxygen migration and of proton incorporation in AZrO 3 (A=Ca, Sr). In agreement with recent EXAFS studies, small lanthanide dopants are predicted to substitute on the Zr-site (with oxygen vacancy compensation necessary for proton incorporation), while larger lanthanide dopants substitute on the A-site. Proton incorporation in AZrO 3 is found to be exothermic, with increasing acceptor dopant levels causing an energetic stabilisation of the protonic defect. Our results suggest that trends in proton conductivity can be rationalised in terms of effective oxygen vacancy creation arising from dopant site selectivity, relative values of water incorporation energies, and possible proton–dopant association.

Protons and other defects in BaCeO3: a computational study

Abstract Atomistic simulation techniques have been used to investigate the energetics of defects and dopants, of oxygen migration and of proton incorporation in BaCeO 3 . The calculations suggest that the concentration of intrinsic atomic defects is quite low even at relatively high temperatures. Under reducing conditions, BaCeO 3 seems to contain oxygen vacancies and conduction band electrons and thus exhibits n-type conduction. Furthermore, trivalent dopants at the Ce-site have favourable solution energies, particularly Y, Yb and Gd. Oxygen ion migration is found to have an activation energy of about 0.85 eV, which is consistent with experimental values. We have calculated the energy of incorporation of water in BaCe 1− x Y x O 3− δ and found it to be increasingly exothermic as we increase the Y 3+ concentration.

Defect, protons and conductivity in brownmillerite-structured Ba2In2O5

Abstract Defect energetics of Ba 2 In 2 O 5 have been investigated by atomistic modelling techniques with emphasis on different modes of conductivity. Oxygen Frenkel pairs were found to be the most energetically favourable intrinsic defects and are responsible for oxide ion conductivity in the orthorhombic structure. Formation energies of electronic species suggest that Ba 2 In 2 O 5 will oxidize readily to produce positive holes. Energies of proton incorporation reveal that Ba 2 In 2 O 5 should also exhibit reasonable proton conductivity in moist atmospheres.

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.

Computational studies of proton migration in perovskite oxides

Abstract The mechanism and energetics of proton migration in perovskite oxides are investigated by ab-initio quantum mechanical cluster calculations. We calculate the energy barrier to proton transfer between two adjacent oxygen ions. The charge distribution and relaxation effects are also discussed.

Atomistic simulation of defects and ion migration in LaYO3

Abstract Atomistic simulation techniques have been used to investigate the energetics of defects, oxygen migration and proton incorporation in the high temperature proton conductor LaYO 3 . The interatomic potentials have reproduced the observed unit cell parameters. Defect calculations indicate that Schottky-type defects have unfavourable energies of formation. The calculation of the energies of solution for alkaline-earth dopants revealed that Sr has the highest solubility in accord with experiment. It was estimated that oxygen migration takes place via a curved route with an activation energy of 1.22 eV. The enthalpies for the incorporation of water into alkaline earth doped LaYO 3 were also calculated.

Cation dopant sites in the CaZrO3 proton conductor : a combined EXAFS and computer simulation study

Ceramics of pure, 5% Yb- and 5% Nd-doped CaZrO3 were prepared and investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy and computer simulation techniques. The EXAFS results reveal that Yb3+ behaves as an acceptor-dopant, substituting for Zr4+, while the larger Nd3+ lanthanide dopant behaves as a donor-dopant, substituting for Ca2+. The EXAFS results are borne out by atomistic modelling where small lanthanide dopants are predicted to substitute on the Zr4+ site (with oxygen vacancy compensation that is vital for proton incorporation), while larger lanthanide dopants substitute on the Ca2+ site. The drop in proton conductivity exhibited for the larger lanthanides may be related to the reduced oxygen vacancy concentration arising from dopant site-selectivity.

Defects and protons in the CaZrO3 perovskite and Ba2In2O5 brownmillerite: computer modelling and EXAFS studies

Abstract Atomistic computer modelling techniques are used to investigate the substitution of dopants in the CaZrO 3 perovskite and the incorporation of protons in the Ba 2 In 2 O 5 brownmillerite. The defect calculations on CaZrO 3 predict that small trivalent cations show Zr-site selectivity with oxygen vacancy compensation, while larger cations show Ca-site selectivity with Ca vacancy compensation. EXAFS experiments on doped CaZrO 3 demonstrate Yb and Nd substitution at Zr and Ca sites, respectively, in agreement with the simulations. Finally, several water incorporation reactions in Ba 2 In 2 O 5 have been examined, including mechanisms involving oxygen interstitial sites.

Protons in LaMO3: atomistic modelling and ab initio studies

Abstract A combination of atomistic simulation and quantum mechanical techniques have been used to investigate proton transport phenomena in LaMO 3 perovskite-structured oxides. We examine the energetics of dopant substitution and of incorporation of water to yield hydroxyl groups. The energy barrier to proton transfer between neighbouring oxygen ions is evaluated by ab initio cluster calculations. The simulation results suggest that a key step for proton migration is the relaxation energy required for the two adjacent oxygen ions to acquire equivalent lattice environments preceding proton transfer.

Synthesis and characterisation of the perovskite-related cuprate phases YSr2Cu2MO7+y (M = Co, Fe) for potential use as solid oxide fuel cell cathode materials

In this paper we report the synthesis and characterisation of the perovskite cuprate phases YSr2Cu2MO7+y (M = Co, Fe) in order to examine their potential for use as cathode materials in solid oxide fuel cells (SOFCs). Both samples showed conductivities of ≈10 S cm−1 at 900 °C and were also shown to be stable at this temperature in N2. For YSr2Cu2FeO7+x, semiconducting behaviour was observed up to ≈550 °C, with a decrease in conductivity at higher temperatures, attributed to oxygen loss reducing the charge carrier concentration. In the case of YSr2Cu2CoO7+y, semiconducting behaviour was observed over the range of temperatures studied, although a small but significant steep increase in conductivity was observed above 800 °C. High temperature X-ray diffraction studies of this particular phase showed that this increase in conductivity coincided with an orthorhombic–tetragonal structural transition, accompanied by a significant reduction in cell volume. In addition to measurements in air, conductivities were also measured with varying p(O2) (0.2–10−5 atm) at 900 °C, and these data showed significant hysteresis between measurements on reducing and re-oxidising, suggesting poor oxide ion transport, poor oxygen surface exchange kinetics, or significant structural changes on varying p(O2). Chemical compatibility studies of these phases with SOFC electrolytes at temperatures between 900 and 1000 °C showed reaction in all cases. In the case of CeO2 based electrolytes, the reaction led to the formation of the “fluorite-block” phases, (Y/Ce)2Sr2Cu3−xMxO9+y (M = Co, Fe), and samples of these were subsequently prepared and the conductivities measured. Similar hysteresis between conductivity measurements on reducing and re-oxidising were also observed for these samples.