Powder X-ray Diffraction Data Across Wide 2 θ Range

In this section, the x-ray diffraction was used to probe the structure of the BFO- KBT system as a function of temperature. This was to determine the point at which the ratio between the rhombohedral and cubic phase components of the overall structure had shifted to such a point as the structure could be considered cubic. The measurements included here were chosen to best represent the system, the lowest two temperatures showing the stable region from room temperature up to the temperature at which this stable region was found to destabilise and larger

Table 4.1: Compositions and the temperatures at which changes were noticed from visual inspection of the data in Figure 4.1, Figure 4.2, Figure 4.3, and Figure 4.4. The blank cubic entries indicate that the sample did not appear to form a single cubic phase at high temperature.

Composition (mol% KBT) Rapid Cubic Phase Increase (◦C) Cubic After (◦C)

15 350 - 20 650 850 30 500 800 40 650 800 50 600 800 60 300 - 70 500 - 80 300 - 90 300 - 100 270 500

changes in the ratio between the cubic and rhombohedral phases were found to begin, with all the measurements above this temperature included. In all cases, the graphs are presented as a stacked plot.

Figure 4.1 and Figure 4.2 show a representative sample of the changes observed when heating a BFOKBT powder sample as stacked plots. The largest changes observed were often found to be in the{110}peak, which are then expanded in Figure 4.3 and Figure 4.4. It was found that few changes were observed until some given temperature, at which point the patterns would begin to look more cubic, and then at some higher temperature would often become indistinguishable from cubic; these temperatures are summarised in Table 4.1 for all samples.

The BFO-KBT powders were found to become more cubic with increasing temperature. Both BFO and KBT are known to become cubic at high temperatures (931◦C [1] and 410-450◦C [2, 3, 4] respectively), and so it was thought that the mixed state would also do so. In the case of BFO, it has been reported that the material undergoes an initial transition to orthorhombic between 820◦C and 830◦C [5]. In the case of KBT, the transition is reported to begin at 280◦, with a mixed cubic and tetragonal phase existing between 280◦C and 450◦C [2]. In this respect, it seems that the BFO-KBT material experiences a high temperature transition more like that found in KBT than that found in BFO, given that from Chapter 3 it was found that the material exists in a mixed phase at room temperature, and in addition there is no evidence to support the transition from this mixed phase to an orthorhombic phase before the transition to a cubic phase occurs.

At low mol% KBT compositions, the high temperature state visually resembled a single cubic phase, whereas at higher mol% KBT compositions a single cubic phase would not be able to represent the data, despite both end members

(a)15%mol KBT (b)20%mol KBT

(c)30%mol KBT (d)40%mol KBT

(e)50%mol KBT

Figure 4.1: Stacked plots of the different temperature measurements for the different compositions of BFO-KBT investigated with respect to temperature. Of particular note are the{110} peaks around 31-32◦ 2θ, which show the clearest change from the stable lower temperature measurements and the higher temperature measurements. The values included have been square rooted to allow for easier comparison between the different plots without losing the relevant details.

(a)60%mol KBT (b)70%mol KBT

(c)80%mol KBT (d)90%mol KBT

(e)100%mol KBT

Figure 4.2: Stacked plots of the different temperature measurements for the different compositions of BFO-KBT investigated with respect to temperature. Of particular note are the{110} peaks around 31-32◦ 2θ for lower mol% KBT, and the{200} peaks around 45-48 ◦ 2θ for higher mol% KBT. The values included have been square rooted to allow for easier comparison between the different plots without losing the relevant details. Some minor mechanical issues resulted in some scans being cut short.

(a)15%mol KBT{110}peaks (b)20%mol KBT{110}peaks

(c)30%mol KBT{110}peaks (d)40%mol KBT{110}peaks

(e)50%mol KBT{110}peaks

Figure 4.3: Stacked plots of the {110} peaks at different temperatures for the different compositions of BFO-KBT investigated with respect to temperature, presented with square rooted data. These peaks show the greatest difference with respect to temperature.

(a)60%mol KBT{110}peaks (b)70%mol KBT{110}peaks

(c)80%mol KBT{110}peaks (d)90%mol KBT{110}peaks

(e)100%mol KBT{110}peaks

Figure 4.4: Stacked plots of the {110} peaks at different temperatures for the different compositions of BFO-KBT investigated with respect to temperature, presented with square rooted data. These peaks show the greatest difference with respect to temperature.

forming a cubic phase at high temperature.In addition, given the lower temperature of the cubic transition in KBT, it was expected that the higher KBT % material would become cubic at lower temperatures than the lower KBT % material, and even before Rietveld refinement in Section 4.4 it appears that this is the case. It was also observed that at high temperature, the secondary phases within the material changed, even in 40% KBT samples which originally had no impurity phase which formed a Bi2O3 phase when returned to room temperature, the same phase present in other low mol% KBT samples. This is discussed in more detail in section 4.6.