Revisiting the Crystal Structure of BaCe_0.4Zr_0.4Y_0.2O_3−δ Proton Conducting Perovskite and Its Correlation with Transport Properties

Abstract

Oxides with proton conductivity have a great potential for applications in environmental energy technology. Despite the Ba-Ce 0.4 Zr 0.4 Y 0.2 O 3−δ (BCZY) perovskites being well-known proton conductors, it is a challenge to determine the optimal operating temperature range where the energy applications benefit most from this unique property. The protonic transport properties strongly depend on crystal structure and local distortions in the participating cation coordination sphere, according to related temperatures and gas feed. The transport and crystallographic properties of BCZY were simultaneously studied by impedance spectroscopy (IS) and synchrotron X-ray diffraction (S-XRD). A strong correlation between conductivity and the lattice parameter, corresponding in principle to a cubic symmetry, was observed, mainly between 400 and 700°C. The protonic conductivity range was analyzed by the H/D isotopic effect on the impedance spectra, which helped to identify protonic conduction as the governing transport mechanism below 600°C, while the transport via oxygen vacancies dominates above this temperature. In order to assess the real crystallographic structure, the simultaneous refinement of laboratory XRD and neutron diffraction (ND) patterns was performed. According to this, BCZY changes from rhombohedral symmetry below 400°C to cubic at 600°C in a second-order phase transition. Complementary quasielastic neutron scattering (QENS) enables us to determine a protonic jump length of 3.1 Å, which matches the O−O distances in the octahedral oxygen coordination sphere around the cations. These results support the protonic self-diffusion through proton hopping between intraoctahedral O sites as the main transport mechanism up to 600°C.

Structure–property correlation in oxide-ion and proton conductors for clean energy applications: recent experimental and computational advancements

“…This is in agreement with a recent structural and dynamical study of BaCe 0.4 Zr 0.4 Y 0.2 O 3Àd , using QENS, and simultaneous synchrotron XRD and impedance spectroscopy, which, in addition, revealed a strong correlation between the proton conductivity and the lattice parameter. 130 Possibly, the higher activation energy observed on the macroscopic length-scale is a result of trapping of the protons at the dopant sites. Such a picture was initially proposed by Hempelmann et al on the basis of QENS data on SrCe 0.95 Yb 0.05 O 2.975 and has since found support from various experimental and computational studies on similar materials.…”

In situ neutron diffraction study of BaCe0.4Zr0.4Y0.2O3−δ proton conducting perovskite: insight into the phase transition and proton transport mechanism

“…77 Raw data were corrected using the standard procedures with the ILL software LAMP and then fitted using a Chudley–Elliott model (CEM). 64,78–80 Some of the results obtained have already been presented in a previous publication, 64 however, in this work we have extended the analysis to temperatures previously not covered and used a different fitting engine based on a Monte Carlo approach 81 in order to obtain better estimates on the uncertainties of the fitted parameters. Only the Q -range below 1.2 Å −1 was used in the fitting, in order to avoid any influence from the Bragg peaks in the resulting parameters and to be able to extract an average intensity per temperature, which should be approximately proportional to the amount of water adsorbed in the sample.…”

“…These patterns were refined by the Rietveld method using Fullprof suite 72 with the rhombohedral and cubic structural models. 64,65 The background, peak profile, crystallite size, Debye–Waller factors, etc. , were fitted according to our previous work.…”

“…This previous work indicated a strong correlation between crystallographic and electrochemical properties, which was evidenced by conductivity reduction and lattice shrinkage between 490 and 530 °C. 64 It is suggested that the second order phase transition might be in this temperature range, however, it is not clear if this phase transition affects the protonic transport mechanism or if they are independent processes.…”

Solid oxide proton conductors beyond perovskites

“…1a). Depending on the composition and the level of hydration, the ideal cubic perovskite structure (space group Pm3 ̅ m) is usually stable at high temperatures, with rhombohedral, orthorhombic, and monoclinic distortions at lower temperatures due to tilting of the octahedral units 23,24,25,26 . Proton conduction in these systems is enabled by the introduction of protonic defects via the dissociative absorption of water according to the hydration reaction,…”