Discovery

Using the standard solid state synthesis route, samples with compositions YCa4Sr2Fe7O18, YCa2Sr4Fe7O18 and YCa3Sr3Fe7O18 were produced. 0.5 g was synthesised by hand grinding

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dried powders of Y2O3 (99.999 %), SrCO3 (99.995 %), CaCO3 (99.995 %) and Fe2O3 (99.945 %), all sourced from Alpha Aesar, in the required ratios in a pestle and mortar. The mixed powder was heated in a furnace to 1200 °C at a rate of 5 °C per minute in an Al2O3 crucible in static air. After dwelling at 1200 °C for 12 hours, the furnace was cooled at 3 °C per minute down to room temperature. The black crystalline material obtained was then hand ground using a pestle and mortar and a room temperature PXRD pattern was collected. The sample was then pressed into 13 mm pellets using a uniaxial press and the heat treatment was repeated, with the resultant sample hand ground in a pestle and mortar and analysed by PXRD.

This synthesis afforded a 10ap phase (at YCa4Sr2Fe7O18), a cubic phase (at YCa2Sr4Fe7O18) and a mix of these two phases (at YCa3Sr3Fe7O18) as identified in PXRD patterns collected for each composition. The PXRD patterns of these samples are given in Figure 4.1, together with an insert showing a zoomed in region, which clearly displays reflections at low angle indicative of a long axis phase.

Figure 4.1 PXRD patterns collected for YCa4Sr2Fe7O18 (black), YCa3Sr3Fe7O18 (red) and YCa2Sr4Fe7O18 (blue) samples.

The Insert shoes a zoomed in region more clearly displaying long d-space reflections and gradual pattern changes with composition.

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As was also described for the 16ap phase in section 3.2.2, the samples appeared to react with

the Al2O3 crucibles that they were synthesised in, as staining of the crucibles was observed. EDX composition analysis was carried out on the phase pure sample, YCa4Sr2Fe7O18, in order to determine the cation ratio and to establish if Al incorporation from the crucible into the 10ap phase occurred. This analysis showed that although no Al was incorporated, there

was a significantly different A-site cation molar ratio compared to the starting values, whilst the standard perovskite the A/B ratio of 1:1 was retained. Normalising the composition to A5B5Ox, the EDX attained composition based on an average of four particles was Y1.0(1)Ca2.2(1)Sr1.8(1)Fe5Ox, compared to the Y0.7Ca2.9Sr1.4Fe5Ox (in A5B5Ox form) nominal composition.

From this point in the chapter, the 10ap phase that forms with this YCa2.2Sr1.8Fe5Ox composition will now be referred to as YCSFO.

4.2.2 Isolation using the phase diagram approach

In order to carry out physical property measurements on YCSFO, large sample batches were needed. However, the problem of reactivity with the crucible had to be addressed prior the measurements in order to synthesise reproducibly large amounts of a phase pure 10ap sample.

New syntheses at various compositions were carried out in Pt foil lined Al2O3 or Pt metal crucibles, using the conditions described in section 4.2.1. A ternary phase diagram of A-Site cation ratios was constructed to guide synthesis, using the EDX given composition (YCa2.2Sr1.8Fe5Ox) as the starting point, and selecting compositions around this in order to map the phase space.

PXRD patterns collected for each of the compositions were carefully analysed so that the main phases formed could be mapped onto the ternary phase diagram shown in Figure 4.2. Within the phase space explored, the formation of four readily identifiable main phases was observed. Two long period phases were observed, the YCSFO target 10ap (a = 5.43424(6),

b = 38.0147(4), c = 5.56989(5)), and a second 10ap phase with smaller lattice parameters

(a = 5.4277(2), b = 37.783(2), c = 5.5649(2)) found in a region poorer in Sr and richer in Ca. As there was no gradual change in lattice parameters to the smaller cell 10ap that would be

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expected from a solid solution, it appears to be a different phase. Full identification of this phase has not been carried out for this thesis however. The other two major phases observed in the PXRD patterns of samples rich in Sr and poor in Ca were a 3ap phase and a cubic

phase.

Within the region labelled as 10ap formation in Figure 4.2, impurities were observed that

could not be identified. The impurity peaks were observed partially overlapping with the highest intensity 10ap perovskite reflections, the [1,10,1], [2,0,0] and [0,0,2]. It is likely that

these impurities were perovskite related, possibly the 3ap or cubic perovskites already

identified previously, which have reflections in similar regions.

Figure 4.2 Ternary phase diagram showing variation in A-site cation ratios (Y, Ca and Sr) and displaying regions of phase formation of two 10ap phases, a 3ap phase and a cubic phase. Pure phase at Y0.9Ca2.4Sr1.7 (red) and original composition

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Isolation of a phase pure sample was finally achieved through varying the A-site ratio and arriving at the required composition. No further variations to the reaction conditions were necessary and the final composition found was Y0.9Ca2.4Sr1.7Fe5O13-δ. Whilst this sample was

being isolated, SXRD and ND data were collected on the original YCa2Sr4Fe7O18 sample (YSr1.8Ca2.2Fe5Ox EDX composition), which will be referred to as YCSFO(I). Crystal structure characterisation was carried out for this sample, which is described in section 4.3, together with TEC a measurement based on VT-SXRD data (section 4.4.1). Samples with the composition Y0.9Ca2.4Sr1.7Fe5O13, appeared the same as the nominal YCa2Sr4Fe7O18 sample by PXRD. It was samples prepared at the composition Y0.9Ca2.4Sr1.7Fe5O13 that were subsequently used for physical property measurements described in section 4.4. They will be referred to as YCSFO(II) when distinction is required from YCSFO(I).

4.3 Crystal Structure Characterisation