Ba1.7Ca2.4Y0.9Fe5O13 is a potential cathode material for solid oxide fuel cells (SOFC). In the bulk it has been reported to have a perovskite-based, cation ordered, layered structure.1 Some of the work in this chapter is published,2 and discusses the advantages of the thin film form over the bulk material. The Ba1.7Ca2.4Y0.9Fe5O13 material is related to an intergrowth between Ca2Fe2O5 3 and YBa2Fe3O8 4,5 building blocks leading to a complex superstructure displaying 20 times the unit-cell volume of a classic cubic perovskite (Figure 4.1).
Figure 4.1:Crystal structure showing the intergrowth between Ca2Fe2O5 and YBa2Fe3O8 building
Ba1.7Ca2.4Y0.9Fe5O13 belongs to the orthorhombic space group Imam, and has bulk lattice parameters a = 5.50Å, b = 5.54Å, and c = 38.20Å, with the c axis corresponding to a 10 layer (10ap) repeat of the basic cubic perovskite cell.
Hereafter Ba1.7Ca2.4Y0.9Fe5O13 shall be referred to as the 10 layer material. The c axis corresponds to the long axis and points out-of-plane as is conventional for describing thin films of layered materials.
Comparing the disordered structure of NdBaCo2O5 from Chapter 3 with the 10 layer structure of Ba1.7Ca2.4Y0.9Fe5O13 in Figure 4.2 it is clearly evident how the 10 layer is 20 times the unit-cell volume of a classic cubic perovskite.
Figure 4.2: Crystal structures of (a) disordered NdBaCo2O5 with random occupancy of the cation
site indicated by mixed colour central atom, and (b) Ba1.7Ca2.4Y0.9Fe5O13. The iron sites in the
There are six distinct cation sites (3 perovskite A sites and 3 B sites) describing this complex material. The eight-coordinated A site is predominantly occupied by Y3+, the nine-coordinated site by Ca2+ and the 12-coordinated site by Ba2+ and Fe is present in three unique B site co-ordination environments, namely tetrahedral, square based pyramidal and octahedral; the ordering of these cations leads to a layered structure in which partially oxygen deficient layers allow oxygen ion transport. The composition of the films deposited are described using the nominal stoichiometry of the target (Ba1.7Ca2.4Y0.9Fe5O13). A study on the experimentally determined cation ratios to obtain this layered and ordered structure free from impurities has been previously conducted by Demont et al.,1 where a nominal cation stoichiometry of Ba1.7Ca2.4Y0.9Fe5O13 was found to give a phase pure material. Bulk cathodes of Ba1.7Ca2.4Y0.9Fe5O13 have an area specific resistance (ASR) of 0.87Ωcm2 at 700°C,1 and with optimisation of the microstructure this has been lowered to 0.25Ωcm2 suggesting that they are candidates for intermediate temperature (IT) SOFC applications.6 This electrochemical behaviour is combined with excellent thermal stability in air and under a CO2 atmosphere, as well as chemical stability and a good thermal expansion match with the Ce0.8Sm0.2O(2-δ) (SDC) electrolyte. The electronic conductivity of the bulk ceramic is notably low compared with Co-containing materials such as La0.6Sr0.4Co0.2Fe0.8O(3-δ) (LSCF), La0.8Sr0.2CoO(3-δ) (LSC) and the LnBaCo2O5 systems.7 Density functional theory (DFT) calculations on the 10ap Fe-based material show that it is expected to have highly anisotropic conductivity behaviour as the 3d bandwidth and thus the electronic conductivity should be strongly enhanced in the ab plane.1 This suggests the growth of this SOFC
cathode in thin film form would be advantageous, in order to exploit the complex A cation ordering-based superstructure to enhance the properties over the bulk ceramic material. An epitaxial thin film would have a single crystallographic orientation enhancing anisotropic conductivity over a ceramic sample with randomly oriented crystallites.
There are several possible oxygen diffusion pathways, with different calculated activation energies6 in Ba1.7Ca2.4Y0.9Fe5O13 which arise as a consequence of having multiple iron environments and general structural anisotropy. The pathway with the lowest activation energy (0.45eV) was found to be for longitudinal oxygen diffusion, and this compares favorably with activation energies found for another commonly used perovskite-based IT-SOFC cathode, GdBaCo2O5. The rationale behind the low energy diffusion pathway is that the system is subject to an internal activation strain, which assists in the ionic bond breaking required as part of the diffusion process. This internal activation strain itself arises from the two sub-blocks of the structure inherently having different lattice parameters, and an intergrowth of the two generates an intrinsic lattice parameter compromise, and therefore strain. Surfaces that terminate in planes perpendicular to the layers, such as the (001) and (100) planes, form reactive surfaces from cleaving strong bonds across the layer. These reactive surfaces allow fast longitudinal oxygen ion transport away from the surface.6
4.2 Growth Optimisation
10 layer thin films were deposited on (001) SrTiO3 (STO) substrates by pulsed laser deposition (PLD) utilising a Nano PVD chamber, with a KrF COMPeX Pro Lambda Physik excimer laser with λ = 248nm. A dense pellet (90% of the theoretical density) of the 10ap compound was prepared by Dr. Ruth Sayers at the
University of Liverpool, following the procedure described by Demont et al.1 The growth conditions were optimised by investigating several parameters; growth temperature (varying the growth temperature from 750-950°C), partial pressure of oxygen (1mTorr and 10mTorr), laser energy (240-300mJ)/ fluence (0.30-0.24Jcm-2) and finally the cooling procedure.
Standard /2 XRD patterns were collected on the films prepared under the different growth conditions to determine the most favourable for the formation of the 10ap phase, with details on the two diffractometers in Chapter 2.
Dr. Ruth Sayers initially investigated the effect of growth temperature on the formation of the 10 layer film, at 1mTorr pO2. XRD patterns of films grown at 750°C, 850°C and 950°C are shown in Figure 4.3. From this Figure it can be seen that the temperature played a crucial role in the formation of impurities, namely BaCaFe4O8 and a 3ap phase. The 3ap phase is based upon the ABO3 perovskite structure from YBa2Fe3O8,5 that is extended to form a three-fold super structure.
Figure 4.3: XRD patterns of the optimization of the 10ap growth showing the effect the growth
temperature has on the film. The film deposited at 750°C is shown in blue, 850°C in green and 950°C in red. The radiation used to collect the patterns was contaminated by Cu K and W components marked with asterisks. Impurities from BaCaFe4O8 are highlighted with blue triangles
and peaks attributed to the 3ap phase are shown by red circles. The desired 10ap phase is indicated
by green squares.2
From the temperature study performed by Dr. Sayers at 1mTorr pO2, it was found that the 10 layer phase only formed at 850°C, and so this temperature was used as the basis for this work. Here, the temperature was also varied at 10mTorr from 850°C to 900°C to attempt to grow an impurity-free 10 layer film. Table 4.1 summarises the growth conditions used in this study.
Natasha Louise Olenka Flack Thin Film Components for SOFCs
Table 4.1 : Summary of the growth parameters varied.
Film Substrate
Temperature (°C)
pO2 (mTorr) Laser Fluence