High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 4

Figure 5.6 Same as Figure 5.4(b) but after the fractured surface of PZN-7%PT crystal sample was polished with SiC papers of different particle sizes. 65 tweed-like patterns (Figure 5.7c). After repeated polishing with care to remove a sufficiently thick layer (>40 µm) from the surface of the sample, the surface domain patterns as shown in Figure 5.7(a) persisted; and so was the presence of the broad lower 2θ peak in the XRD profile. These observations led us to believe that the specific thin domain layer is related to the polished surface. To check for this, a PZN-4.5%PT sample, initially polished in the [100]pc direction (Figure 5.7a), was tilted by an angle of 45° and again polished back and forth in the new direction. After the second polishing, the earlier set of domains disappeared and was replaced by a new set of elongated domains aligned at an angle of about 30° towards the pc direction of polishing (Figure 5.7b). This confirms the observed domain patterns are related to the polished surface. We further confirmed that optically the polishing induced surface domains can be reliably detected only if polishing direction was roughly parallel to a certain crystallographic direction, e.g., [010]pc or [110]pc in our experiments. Otherwise, the domains are not evident and the sample is covered with areas exhibiting slight but varying birefringence when observed under the crossed polarizers (Figure 5.8). Despite the above, the lower 2θ peak is always present in the x-ray profile following any direction of polishing, suggesting that the surface phase is always present regardless of the direction and/or mode of polishing. This behavior can be explained in the following way. If the polishing 66 a b c [010] 50 µm [100] Figure 5.7 Surface domain patterns of the (001)-cut PZN-4.5%PT crystal plate (a) after polishing along the [010]pc direction and (b) after repolishing in the [110]pc direction. Arrows indicate the direction of polishing. Note the realignment from (a) to (b). (c) The domain patterns in the underlying material revealed by the focusing technique. 67 [010] 50 µm [100] Figure 5.8 No clear surface domain patterns of (001)-cut PZN-4.5%PT crystal plate as a result of none crystallographic polishing direction. 68 direction changes in a non-coordinated manner, the directions of induced surface stresses and domain directions are randomly distributed. As the size of domains is very small, they are not visible in the optical microscope. In this case, since the surface layer is composed of randomly oriented submicroscopic domains, the surface layer does not exhibit a clear birefringent domain pattern. In contrast, after directional polishing the domains (and thus their optical indicatrix) are aligned into macroscopic regions. These regions are birefringent and can be identified under a polarizing microscope. 5.3 5.3.1 Stability of the polishing-induced surface layer Thermal stability The stability of the deformed surface layer was investigated by subjecting the as-polished samples to different annealing treatments. The heating and cooling rates used were 1.5 ºC/min. Figure 5.9 shows the (002) XRD profiles of the differently annealed PZN-4.5%PT samples and the (002) XRD profile of the as-polished sample. After annealing for h at 300 °C, the position of the lower 2θ peak shifted to 2θ ≈ 43.95°. Subsequent annealing at 400 °C for h did not produce significant change to either the position or intensity of this peak. The lower 2θ peak persisted even after annealing at 600 °C for h and the position shifted further to 2θ ≈ 44.05° (Figure 5.9). 69 as-polished 300oC (1h) ~44.68 o R Intensity (arb. units) 400oC (1h) 600oC (5h) ~44.05 ~43.50 41 42 43 o o 44 45 46 47 2θ Figure 5.9 (002) XRD profiles of as-polished (solid curve) and differently annealed (dashed curves) PZN-4.5%PT showing the effects of the different annealing treatments on the lower 2θ peak. Sample thickness is 1mm. 70 . 2θ 41 42 43 44 45 46 47 Intensity (arb. units) as-polished 300 o C (1h) 40 0 o C (1h) 600 o C (5h) <002> R ~43 .50 o ~44 . 05 o ~44 .68 o Figure 5. 9 (002) XRD profiles of as-polished. treatments. The heating and cooling rates used were 1 .5 ºC/min. Figure 5. 9 shows the (002) XRD profiles of the differently annealed PZN- 4 .5% PT samples and the (002) XRD profile of the as-polished. Figure 5. 7 Surface domain patterns of the (001)-cut PZN- 4 .5% PT crystal plate (a) after polishing along the [010] pc direction and (b) after repolishing in the [110] pc direction. Arrows indicate