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Thermal evolution of the crystal structure of proton conducting BaCe0.8Y0.2O3-δ from high-resolution neutron diffraction in dry and humid atmosphere

A.K. Eriksson Andersson ; S.M. Selbach ; T. Grande ; Christopher S Knee (Institutionen för kemi och kemiteknik, Oorganisk miljökemi)
Dalton Transactions (1477-9226). Vol. 44 (2015), 23, p. 10834-10846.
[Artikel, refereegranskad vetenskaplig]

The crystal structure of the proton conducting perovskite BaCe0.8Y0.2O3-δ (BCY20) has been studied via high-resolution in situ neutron diffraction performed in controlled dry and humid (heavy water) oxygen flow. Two phase transitions, cubic Pm3m→R3c (775 °C)→Imma (250 °C) were observed on cooling from 1000 °C in dry O2. A significant shift of the phase stability fields was observed on cooling in wet oxygen (pD2O ≈ 0.2 atm) with the R3c structure stabilised at 900 °C, and the R3c→Imma transition occurring at 675 °C. On cooling below 400 °C a monoclinic, I2/m, phase started to appear. The structural dependence on hydration level is primarily due to the de-stabilisation of the correlated, octahedra tilts as a consequence of structural relaxation around the oxygen vacancies present in the non-hydrated phase. The tendency of hydrated BaCe0.8Y0.2O3-δ to show octahedral tilting is also found to be enhanced, indicating that the deuteronic (protonic) defects influence the crystal structure, possibly via hydrogen bonding. Stabilisation of the monoclinic I2/m phase is attributed to the structural effect of deuterons that is inferred to increase on cooling as deuterons localise to a greater extent. Changing from wet oxidising (O2 + D2O(g)) to wet reducing (5% H2 in Ar + D2O(g)) atmosphere did not influence the structure or the phase stability, indicating that Ce4+ was not reduced under the present conditions. Based on the observed cell volume expansion protonic defects are present in the material at 900 °C at a D2O partial pressure of ∼0.2 atm. The origin of the chemical expansion is explained by the effective size of the oxygen vacancy being significantly smaller than the [OD] defect. Rietveld analysis has been used to locate possible sites for the deuterons in the high temperature, R3c and Imma, phases that are most relevant for proton transport.



Denna post skapades 2015-06-23. Senast ändrad 2015-12-11.
CPL Pubid: 218731

 

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