A simple comparison of XRPD data from the high purity samples of BFTO-513, BFTO-623 and BFTO-733 reveals a systematic shifting of peaks as additional layers are added to the structure (Figure 5.6). The peaks can be fitted to an orthorhombic unit cell. The most commonly reported space group for the Aurivillius phases, F2mm159, 167, 169, 174, was used for this initial basic modelling. A comparison of fitted orthorhombic lattice parameters shows an extension of the long axis by ~ 8 Å (Figure 5.7) between the materials, consistent with the addition of extra perovskite layers in each case.
The lattice parameters of the BFTO-623 sample are on the higher side of the literature reports, suggesting the possibility of some ‘disorder’ in the layering of this sample. This is also suggested by appreciably broad peaks associated with planes perpendicular to the long axis (see Figure 5.6, inset, red curve ~ 17° 2θ). The grain size of the BFTO-623 and BFTO-733 samples is smaller than BFTO-513 which explains some systematic broadening, but not some of the localized broad peaks which are most severe in BFTO-623. This sample is also unique as the only compound with a nominally odd number of perovskite layers in the crystal structure, and thus can exhibit different symmetry features and strain distributions, which is likely what makes it more difficult to synthesize and may affect the resulting properties. In addition, as the number of layers is increased, the more likely it becomes for stacking faults and intergrowths to develop385, 386, which can make structure refinement challenging.
Figure 5.6. Comparative plot of XRPD patterns for high purity BFTO samples produced with the MOD-EG method. Inset shows shifting peaks on increasing number of layers, and peak broadening. Asymmetric broadening for BFTO-623 at ~ 17° is most notable (inset).
Figure 5.7. Plot of refined orthorhombic unit cell parameters (a) and volume (b) for the BFTO materials. There is a significant increase in the long axis c as the number of layers are increased.
In order to understand the average structure of these compounds, Rietveld refinement was performed in the space group F2mm. This space group captures some information about off centre displacements of Bi and Ti/Fe with respect to the polar axis of the unit cell (defined as the crystallographic ‘a’ axis in the refinements presented in this thesis) which are the main contributors to the ferroelectric character in these materials. It should be noted that this space group does not capture octahedral tilting modes which impact ferroelectric properties, and are known to exist in the host Bi4Ti3O12 material387. However, an idea of the relative metal ion coordinates and potential chemical ordering phenomena might be obtained from refinement in F2mm as a starting point for improved future refinements.
This first set of refinements were conducted on the collected lab XRPD data. Several models were developed to test different fixed site occupancies in the perovskite B-sites to probe for preferential chemical ordering (Figure 5.8). These were based on F2mm reported structures for each compound159, 167, 169, 174. The oxygen positions were fixed at those quoted in the literature reports, as the lab XRPD data could not provide good resolution for such light elements. The cation positions and their thermal parameters were refined. The March- Dollase388 function was used to account for preferred orientation in the prepared samples.
Figure 5.8. Graphical representation of (a) BFTO-513, (b) BFTO-623 and (c) BFTO-733 structures in the F2mm space group as reported by others, which do not allow for octahedral rotations. Models I-IX represent different chemical occupancy models which were used in the subsequent refinement.
Refinements tended to converge with both the transition metal cations in the B-site (Fe and Ti) shifted with respect to the bismuth A-site cations along the polar axis, suggesting exhibition of ferroelectricity in each case, though the error bars on the positions were high (Table 5.3). The refinement of the thermal parameters (Uiso) for each cation was somewhat informative in identifying sites in the structure that may be subject to more complex local structural and/or chemical order. In all three materials, the bismuth ions in the [Bi2O2]2+ layers have the highest and most variable thermal parameters between models, suggesting some structural disorder between layers. The thermal parameters of the transition metal ions in the B-sites all suggest some instability. In BFTO-513 the thermal parameters for the outer layers are consistently high but moderately alleviated by placing more titanium at this site. However, there are still indications of a possible need for an extra degree of freedom in the modelling of this position. In BFTO-623, the inner site shows a large thermal parameter, somewhat alleviated by placing more titanium in the centre, but again indicates an issue with the structural model. The low thermal parameters of the outer sites indicate a need for more intensity in these areas in the refinement, which may reflect a need for more iron in the outer sites in the model. In BFTO-733 the thermal parameters of each site are reasonable, but again higher in the inner sites and lower in the outer site, suggesting a moderate preference for iron either spread across all sites, or in the outer sites.
Table 5.3.Rietveld refined thermal parameters Uiso for the transition metal ions in F2mm BFTO materials.
Sample Model Outer site
Uiso (Å-2) Middle site Uiso (Å-2) Inner site Uiso (Å-2) Rwp (%)
BFTO-513 I 0.17(4) N/A 0.06(2) 5.98 II 0.12(4) N/A 0.10(3) 6.02 III 0.18(4) N/A 0.03(2) 6.04 BFTO-623 IV -0.003(20)* -0.01(2)* 0.2(1) 6.33 V 0.003(21) -0.006(25)* 0.1(1) 6.29 VI -0.0004(212)* 0.002(27) 0.1(1) 6.31 BFTO-733 VII 0.003(39) 0.02(5) 0.06(5) 6.29 VIII 0.003(41) 0.02(5) 0.08(6) 6.33 IX -0.005(36)* 0.02(5) 0.09(6) 6.14
*Unfeasible negative values suggest sites that require more in-depth structural and chemical descriptions.
In each refinement, the standard error associated with the refinement is quite high, and so a definitive choice of best model could not be made. A cautious interpretation of these results is also necessary as the EDXS analyses revealed potential stoichiometric deviation in BFTO-623 and BFTO-733, and the XRPD data has limitations to elemental resolution (as outlined in Chapter 3). In order to get a better idea about the elemental distribution, Mӧssbauer spectroscopy was utilized instead.