Phase-transformation-induced microstructures in perovskites

• Main Authors: Shun-Yu Cheng, 鄭舜宇
• Other Authors: Hong-Yang Lu
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/jbvj9v

博士 === 國立中山大學 === 材料科學研究所 === 96 === Phase-transformation-induced microstructures, including twin domains, anti-phase domains and inversion domains have been analyzed using the scanning and transmission electron microscopy for BaTiO3, BaCeO3 and CaTiO3 of the perovskite structure. Differential etching rate was taken to identify the ferroelectric domains in tetragonal (t-) BaTiO3. Space group Pbnm (No. 62) usually adopted for the orthorhombic crystals by materials scientists is assumed throughout this research to avoid confusion of the plane and direction indices. Traditional contrast analysis was adopted for determining dislocation Burgers vectors (b) and fault vectors (R) in deformed and phase-transformed perovskites, synthetic ceramics (BaTiO3, BaCeO3 and CaTiO3) as well as natural minerals (CaTiO3), polycrystalline (BaTiO3, BaCeO3 and CaTiO3) as well as single crystal (CaTiO3). Atomic images for the structures of twin boundaries and anti-phase boundaries were taken by high resolution technique and image contrast enhancement was performed using fast Fourier transform. Failure of Friedel’s law is adopted for determining if the crystal belongs to non-centrosymmetric point groups. Whether the twins are δ-, α- or π-type (i.e. anti-phase domain boundaries) is analysed from the contrast of extreme fringe patterns. Tilting experiments were performed on selected area diffraction patterns containing un-split row of reflections to ensure that the twin boundaries are the reflection or rotation type. Transformation twinning in all perovskites studied here follows the prediction by the relation of point group symmetries between the high- and low-symmetry phases, assuming continuous, diffusionless, second-order transitions that obey the restrictions imposed by the Landau theory of phase transition. Although such predictions of transformation-induced twinning are only permitted when crystallographic group-subgroup relationship exists and structural coherence retains between the high- and low-symmetry phases, experimental observations for r (rhombohedral) → o-BaCeO3 and t → o in CaTiO3 that are not related by group-subgroup, c (cubic) → t (tetragonal) in CaTiO3 and and c (cubic) → t (tetragonal) in BaTiO3 that are related by group-subgroup, are all consistent with the predictions from loss of point group symmetry elements and change of unit cell volume. In order that the Landau theory is conformed, however, an intermediate phase of either the lowest common supergroup (cubic Pm m) or highest common subgroup (monoclinic C2/c), with phase transition experiencing multistage pathways suggested by Christy and assumption of non-disruption conditions proposed by Guymont, was identified to bridge between two structures, such as rhombohedral and orthorhombic that are not group-subgroup related. Both the 90o and 180o ferroelectric twin domains in t-BaTiO3 are the reflection type and have been identified in pressureless-sintered ceramics. Further, fault vectors (R = ε<110]) for such domain boundaries were determined, boundary planes of {110) for the former, {100) and {220) for the latter deduced accordingly. The polar c-direction between adjacent domains was determined by differential etching rate across domain boundaries, convergent beam electron diffraction was also adopted for identification and confirmation of the c-axis for two types of domains in t-BaTiO3. Plastic deformation resulting from the thermodynamic driving force for sintering (?p) intensified by a multiplication factor φ) was evidenced microstructurally from analysing dislocations in pressureless-sintered BaTiO3 where a Frank-Read source was observed. Slip systems are activated for the effective stress acting on the slip plane along the slip direction has exceeded the critical value of resolved shear stress (τCRSS) and yielding occurs. It has contributed to densification, i.e. the overall system shrinkage of a green powder compact, although if such contribution is at all significant requires studies of sintering kinetics to ascertain. Dislocation dissociation into the scallop-shaped half partials according to the following reactions is determined from analysing corresponding Burgers vectors. [010] + [001] → [011] [001] + [10 ] → [100] [001] + [110] → [111] Both transformation twins lying in {110) and {112) and anti-phase domain boundaries with R = 1/2<111> are detected in o-BaCeO3. For orthorhombic (o-) BaCeO3, fault vectors of the latter R = 1/2<111> determined by contrast analysis was confirmed by high-resolution imaging, but on the contrary, fault vectors the former R = ε<110] and ε<021], respectively, could not be determined from such images. Utilizing the technique of large-angle convergent beam electron diffraction, such fault vectors and dislocation Burgers vectors determined by traditional contrast analysis have been confirmed. Both twinning and dislocations were observed in hot-pressed CaTiO3 prepared in a multi-anvil apparatus. Such twins are deformation twins since hot-pressing was conducted in the orthorhombic stable phase field at 1000oC under 8 GPa. Since fault vectors R = ε<110] determined for {112) and {110) twins are different from the transformation-induced twins in o-CaTiO3, R = ε<021] determined for the {112) twinning in natural perovskite may serve as a diagnostic feature for the deformation twins. Plastic deformation in hot-pressured sample was contributed by both slip and twinning. Slip occurred via slip systems with dislocations of b = [110] gliding in (110) is therefore {110}o <1 0>o (equivalent to {100}pc <001>pc, where pc for pseudo-cubic) often found in perovskites deformed at high temperatures. Another set of dislocations with b = [001] in screw orientation was also determined. APB with R = 1/2<111> detected in natural minerals suggests that the phase transition sequence in CaTiO3 is better described by: (c) → t (I4/mcm) → o (Pbnm) and such APB are generated from loss of the lattice point at I-centre (1/2,1/2,1/2) in the absence of a second orthorhombic Cmcm between t-I4/mcm and o-Pbnm reported before from neutron and X-ray powder diffraction studies.