High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 2

Chapter Surface Layer in Relaxor Ferroelectric PZN-PT Single Crystals 5.1 Introduction An extremely broad lower 2θ peak adjacent to the major (002) R peak has been frequently reported in standard and high-resolution XRD profiles of mechanically polished bulk PZN-PT single crystals Lim et al [63] attributed this to the presence of trapped metastable phases in the crystal Meanwhile, Ohwada et al [56] and Xu et al [57-59] reported a dual structure in PZN-PT single crystals According to the latter authors, the lower 2θ diffraction peak indicates a rhombohedrally distorted structure of 10 to 50 µm thick surface layer The bulk structure, in contrast, was found to have an average C lattice, yet the true symmetry is undetermined Hence, the undetermined bulk structure is termed the “X”-phase by these authors Recently, a detailed study in search of the “X”-phase has been performed by Kisi and Forrester [60-62] The “X”-phase, however, was not detected by the high-resolution neutron powder diffractometer Not only confirming the R phase as the bulk structure, these authors also concluded that the structural difference between the surface and the bulk are due to inter-domain strains, leading to distorted R structure in the surface layer The origin of the lower 2θ peak, however, remains unanswered 53 In addition, x-ray penetration depth in high lead content materials remains controversial Normal x-ray diffraction with energy 8.048 keV performed on PZN-8%PT sample has a penetration depth of µm [16] Synchrotron x-ray diffraction of higher energy, being 18 keV, revealed similar penetration depth in PZN-8%PT sample [32] Meanwhile, using synchrotron x-ray diffraction at 10.7 keV, 32 keV, and 67 keV, the respective penetration depths are reported to be 13 µm, 59 µm, and 412 µm of a PZN sample [57] This suggests a nonlinear relation between the x-ray energy and the penetration depth despite similar test samples were used Whether the x-ray diffraction patterns pertain to bulk structure or a mere surface layer remains controversial although the general belief is that it reveals more the structure of the thin surface layer The experimental results pertaining to the origin of the broad lower 2θ peak adjacent to the major (002)R peak and the effects of surface layer on x-ray diffraction study of PZN-PT single crystals are presented and discussed in this chapter 5.2 5.2.1 Polished surface vs fractured surface at room temperature Effect of polished surface on x-ray diffraction results Typical (002) XRD profile of an as-polished sample of PZN-4.5%PT is given in Figure 5.1(a) The XRD result, which relates to the surface layer of the as-polished 54 sample, revealed two (002) diffraction peaks: A major peak at 2θ ≈ 44.65º and a minor but very broad peak in the range of 2θ ≈ 43.0-44.0º The major peak relates to the (002) R phase, and will be represented by (002)R in our further discussion The origin of the minor broad peak lying at the lower side of the major (002)R peak is still not well understood at present Since the x-ray used can only penetrate a thin surface layer (of