Structural phase transitions, oxygen vacancy ordering and protonation in doped BaCeO3: results from time-of-flight neutron powder diffraction investigations
Introduction
The ionic properties of doped barium cerate perovskites have been well-established for over a decade since the pioneering work of Iwahara et al., and the subject has been recently reviewed by Bonanos et al. [1 and references therein]. The characterisation of the compound in terms of the more basic physical properties of structure and dynamics has unfortunately lagged behind in the drive to make useful solid-state electrochemical devices. In this review, we aim to place the structural aspects of the materials that have been studied so far on to a firm footing. The heavy atoms in BaCeO3 occupy sites of high pseudosymmetry, thus making detailed, accurate structural refinements all but impossible with X-ray diffraction. By contrast, the sensitivity of neutron diffraction to both the light atom positions and site occupancies have made the technique invaluable in the structural characterisation of this compound. This article reviews the results from time-of-flight neutron diffraction measurements made on both the undoped and the doped phase as a function of doping level and temperature.
The article begins with a basic review of the aristotype structure and the distortions that are frequently observed in the perovskite family. Glazer's [5], [6] classification is introduced and related to the important zone boundary modes that give rise to the tilting of the octahedra. Superlattice reflections that arise from the condensation of one or more of these modes are then used to show the advantages of neutron diffraction over X-ray diffraction in the structural characterisation of BaCeO3. The ambient temperature crystal structure and the problem of determining the correct metric and space group are then reviewed, before the effects of doping and the possible phase transitions that doping may induce are considered. The high-temperature behaviour has long been regarded as contentious, and high- and medium-resolution neutron diffraction data are presented to show the correct phase transition sequence in both BaCeO3 and with Y and Nd dopants. Low temperature neutron diffraction measurements on undoped and Y-doped BaCeO3 have been used to derive a possible site for the incorporated structural proton, and improvements to this experimental determination are suggested. Finally, some of the remaining questions that neutron diffraction may be able to solve are outlined in the last section.
Although this article draws from results made using time-of-flight neutron powder diffraction instruments, most, if not all conclusions deduced from the data collected could have been as easily measured using reactor-based instrumentation.
Section snippets
Perovskite crystallography
The aristotype perovskite structure with stoichiometry ABC3 is cubic, with space group Pmm and consists of infinite, three-dimensional, corner-sharing BC6 octahedra with a dodecahedrally coordinated A-site in the centre of a cavity generated by eight surrounding octahedra. In the aristotype, the A-site forms the centre of a cube with the B-site cations at each of the eight corners as shown in Fig. 1. However, this ideal structure is rarely achieved at ambient temperature due to the strict
Ambient temperature crystal system, unit cell metric, space group and crystal structure
Common to many perovskites, the crystal system, unit cell metric and space group of BaCeO3 has ‘evolved’ in time as instrumentation has improved and alternative radiation probes to X-rays have been employed to study the material. Early work, employing X-ray powder photography, with insufficient resolution to determine the splitting of the fundamental reflections and insufficient flux to observe the superlattice reflections, found the compound to be cubic at room temperature [12], [13], [14],
The effects of doping on the crystal structure and dopant-induced phase transitions
The crystal structure refinements of BaCe0.9Gd0.1O2.95, BaCe0.9Y0.1O2.95 and BaCeO3 showed that the effect of doping on the crystal structure was minimal; however, Knight and Bonanos found that the extrinsic vacancy, introduced on Y-doping was localised only on the oxygen atom in general positions. Due to the extreme absorption cross-section of Gd to thermal neutrons, it was not possible to derive anything other than a very rough structural model for BaCe0.9Gd0.1O2.95 because at the time the
High-temperature structural phase transitions
Since the interest generated in the ionic conducting properties of doped BaCeO3, the number, the crystal symmetry, and thermodynamic order of the phase transitions in BaCeO3 has been contentious. In this review, we shall attempt to put the temperature variation of the crystal structure of BaCeO3 on a firm footing using the arguments that have been developed earlier.
The first report of a structural phase transition in BaCeO3 as a function of temperature was made by Preda and Dinescu [17] in a
Protonation site
The light atom sensitivity of neutron diffraction extends to the proton itself and a structural site for the proton in doped BaCeO3 was proposed by Knight [44] using high-resolution neutron powder data collected on both BaCe0.9Y0.1O2.95 and BaCeO3 at 4.2 K. Neutron scattering lengths are isotope-dependent, and Knight pointed out that the optimum experiment would utilise the change in sign of the scattering length for protium- and deuterium-stabilised samples. In the former case, a negative peak
Conclusions
Neutron diffraction has successfully characterised the doped and undoped perovskite BaCeO3 and the main conclusions of this work are summarised below.
(1) At room temperature, BaCeO3 is orthorhombic, with space group Pmcn and unit cell a=8.777 Å, b=6.236 Å, c=6.216 Å. The identical space group and axial setting has been found for BaCe0.9Y0.1O2.95 and BaCe1−xNdxO3−x/2 with 0<x≤02.
(2) Neutron powder diffraction has refuted the structural phase transitions as a function of Nd-doping inferred from
Acknowledgements
It is with great pleasure that I acknowledge the technical and scientific help I have received over the past 15 years working on doped BaCeO3 and other perovskites. John Dreyer, Duncan Francis and Chris Goodway are thanked for their help in maintaining the furnaces used in the ISIS experiments, Jimmy Chauhan, Richard Down and Jon Bones for their support in the cryogenic measurements. Steve Hull and Ron Smith aided the experiments on POLARIS as local contacts and Richard Ibberson helped on HRPD.
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