Decomposition of the 1212 phase

In the previous section only partial decomposition of the 1212 phase was covered; however, when the temperature is high enough to drive the decomposition rapidly then a number of events are seen to occur.

6.21. Formation of new phases as a result of decomposition.

The samples in this experiment were heated to the required temperature with no annealing stages and then rapidly cooled before XRD analysis was carried out. These

are not in situ measurements. Upon entering section III the sample shows some exsolution of the (Ca,Sr)CuOy impurity and individual peak intensities vary, as described in the previous section. Figure 6.19 shows a single phase PbCu:1212 sample with the important reflections indexed. Referring to the argon environment TGA, figure 6.1, increasing the target temperature by approximately 50°C to 850°C sees

more exsolution of "SrCuO" type impurities. The appearance of some other new peaks

as well as the near disappearance of the 5.9Â —15° 20 (002) 1212 peak is shown in figure 6.20. Instead of the (103) (110) doublet of the 1212 phase 6.10a there are clearly three peaks present, also worth noting is the appearance of a new peak at -13° 10.

Further increasing the temperature to 900°C, figures 6.21 and 6.22, shows the

appearance of peaks at low 20 angles, which represent d-spacings of -15.5 and -13.5Â. In figure 6.21, the -13° 20 peak intensity has increased and the triplet of peaks between 32-34° 20 has clearly changed. What could be said about the 900°C pattern compared to that seen at 850°C is that the XRD plot looks "clearer" and less peaks appear to be present. Above 900°C there is very little change in the diffraction patterns, except for the exsolution of more "SrCuO" impurity phases. However, around the 975°C region there is evidence that Pb and a number of elements are being lost from the sample as it turns a grey colour and red marks appear on the AI2O3 crucible.

This is what was seen when samples were heated to 900°C in a vacuum environment.

t_{c 800} 002 103 110

II

Figure 6.19. XRD pattern taken from a single phase FtCu: 1212 sample with the important reflections labelled.

1 002 (disappearing)

i

= positions of ideal 1212 peaks

lAj

25 30 35

2 TTiat* (d*gr«««)

Figure 6.20. Room temperature XRD pattern collected from PbCu; 1212 sample which had been heated to 850°C in low pC>2 environment. I 300 100 “13 (degrees)

\

UjuAyu-üiUjJU

2 TTiata (dagrees)

Figure 6.21. Room temperature XRD pattern collected from PbCu; 1212 sample which had been heated to 900°C in low pOz environment. Decomposition of the 1212 phase has occurred.

150 140 130 120 110 100 î ” S 80 I ,0 4 : « 50 40 30 H 20 10 -15.5 -1 3 .3 (Angstroms) 13 15 2 Thatm (dagraaa)

Figure 6.22. Low angle peaks present in diffraction pattern from the sample which had been heated to 900°C in a low pOi environment.

Examining the d-spacing values of some of the new peaks formed when the 1212 phase decomposes, indicates that possibly some sort of intergrowth phase(s) are formed. These new phases would appear to be quite dominant and there is no clear indication that the 1212 phase is present in any quantity. Looking at the largest peaks

in the XRD patterns, it would seem that the 1212 phase is present at all stages, but it is more likely that the new phases show their most intense peaks around this region and give the appearance that the 1212 phase is still present. This would not be surprising for hkl peaks with 1 = 0 in particular, as the a, b unit cell edges are likely to be similar in the product phases, or simple multiples of these.

The decomposition of the 1212 phase may also be achieved by selecting a temperature within section III and holding the temperature (say 875°C) for varying periods of time, figure 6.23. Using a tube furnace instead of the TGA apparatus shows the same results, the only differences being in the isothermal treatment where temperature and isotherm time vary due to slight changes in pOz This figure also confirms that during the early stages of section III it is possible to attain equilibrium,

although this process is much slower than in stage II which does not entail a phase decomposition process. Assuming that all the weight loss can be attributed to changes

in oxygen content, the final oxygen content of this

(Pb[i+x]/2Cu[i-x]/2)Sr2(Yi.xCax)Cu2 0 7-ô (x = 0-0.4) phase assembly is 6.0. This implies

that the new phases are nominally Pb^^ with perhaps some Cu\

Oxygen content 101 850 Celcius Isotherm 1 0 0- - 7.0 K

I

Figure 6.23. Fixed temperature variable isotherm periods in 1212 sample in a low pO? environment.

6.22. HREM analysis of the decomposition product

Annealing the 1212 phase at 875°C for 45 minutes in an argon environment produced the sample for electron microscopy. Initially looking at sample crystals under

a light microscope showed two distinct types to be present. Under the electron

microscope, one type of crystal was most probably "SrCuO" type of material and no further studies were carried out. Studying the other type, the emphasis was placed on finding crystals orientated to show the c-direction of the unit cell. Unsurprisingly the microscopy revealed quite a number of stacking faults to be present and the variable

nature of spot intensities within layers appeared to indicate disorder also. Figure 6.24 shows the diffraction pattern from a fragment orientated in the [101] direction. The main set of spots was calculated to have a repeating length of—13.5Â, while the weak

11.8Â) was also present in the particular fragment under investigation. Of course this may well have been an isolated fragment and not a true representation of the decomposed sample. The electron micrograph in figure 6.25 shows two different blocks present. One block is what appears to be 1212 and the other is -13.5Â. Another structure was found to be present, with a unit cell edge o f-16Â, figures 6.26-6.27. During the investigations, by far the most common structures found had cells of -13.5Â and -16Â. There was no sign of any regular intergrowth structures, nor was there any indication that supercells were present.

Figure 6.25. Micrograph corresponding to selected area diffraction pattern.

Figure 6.27. Micrograph corresponding to selected area diffraction pattern

6.23. Structures of decomposition products.

At 16Â a c-axis length would seem to suggest the presence of the 3212 phase in its orthorhombic Cmmm form, which is synthesised using low pOz conditions^"*’^’^^. A simulated XRD plot for the 3212 phase, figure 6.28, may be matched as one of the phases present in a decomposed 1212 phase, figure 6.29, although the low 2^ angle data shown in figure 6.22 shows the presence of a 3212 type phase particularly clearly.