P. Skalicky

Crystallography of (111) twins in BaTiO3

Abstract A transmission electron microscopy study has revealed (111) twins in cubic/tetragonal BaTiO3 ceramics. The relation between the lattices in the bicrystal is described by a coincident site lattice cell with Σ = 3. By considering the structure of BaTiO3, the formation of a twin can be understood as a shear operation with (111) planes successively translated by ⅓[1ζ2] vectors. Because of the small tetragonal distortion at room temperature, an α-δ fringe contrast is observed at the twin boundary for reflections which coincide, in the cubic phase, for matrix and twin. In the tetragonal phase, (111) planes parallel to the twin boundary remain parallel for matrix and twin. From structure images the width of the interface in the tetragonal phase is estimated to be less than three unit cells. The distortion of planes near the interface is caused by the presence of the tetragonal phase. For the structural model of an ideally coherent interface, a (Ba-O3) plane is proposed. This preserves the Ti-O octahedra...

Extended defects in hexagonal BaTiO3

Abstract At 1460°C in air, BaTiO3 shows a phase transition from the cubic to the hexagonal phase. Under reducing conditions, however, the phase transition temperature can be lowered to 1330°C. After cooling, the hexagonal phase is preserved at room temperature. Hexagonal BaTiO3 ceramics prepared in a reducing atmosphere were analysed by transmission electron microscopy. Stacking faults in the basal plane extending over several hundred micrometres were found with a density of 106 m−1. The density of dislocations was approximately 108 cm−2 and was much higher than in cubic BaTiO3 ceramics processed by the same heat treatment. Perfect and partial dislocations lying in the basal plane were analysed by their diffraction contrast. Perfect dislocations have Burgers vectors b of〈100〉 which is the shortest lattice vector of this structure. Perfect dislocations dissociate into Shockley partial dislocations. Diffraction contrast analysis revealed that stacking faults have shear vectors R of ⅓〈111〉. Such defects are ...

Disorder in the BiO sublattice of Bi2Sr2Can−1CunO2n+4+z phases

Abstract Defects in the crystal structure of Bi 2 Sr 2 Ca n −1 Cu n O 2 n +4+ z were characterized by high-resolution transmission electron microscopy (HREM) and optical diffraction techniques. It was found that in addition to the well-known intergrowth defects of the n =3 phase, another type of defect concerning a sequence of lamellae (on a nanometer scale) parallel to the c -axis with two different types of crystal structure, the B-centered intrinsic structure and the A-centered faulted structure. Besides these two centered crystal structures, a shear in the b -direction between the BiO layers was observed in the b-c plane. Because of this, non-periodic shear reflections appear, which are compatible only with a primitive Bravais lattice. In these grains regions of different size form domains of incommensurate structure, separated by (001) boundaries. The phase of the displacement field in these domains changes from one domain to another by π/4.7. In addition, the results presented in this paper fully explain the discrepancies observed in the previous determinations of the crystal structure of Bi 2 Sr 2 CaCu 2 O 8+ z .

Lattice defects in single-crystal lithium niobate. II: Electric fields of dislocations and small-angle grain boundaries

Abstract The elastic stresses of small-angle grain boundaries (SAGBs) in non-centrosymmetrical crystals produce electric fields due to the piezoelectric effect. Calculations of these fields for YZ-LiNbO3 suggest that they are responsible for the pinning of ferroelectric 180° domains to SAGBs lying in planes close to (0001). As poling fields (Ep≈.103 Vm−1) are generally two orders of magnitude smaller than the electric field of the SAGB, no polarization reversal takes place and ferroelectric domains remain in the crystal. Attention is paid to the fact that dislocations parallel to some crystallographic directions of high symmetry have no electric fields. These dislocations would show a different interaction with 180° domains. So far this has not yet been observed in lithium niobate but should be taken into account when ferroelectrics belonging to other point groups are investigated.

Lattice defects in single-crystal lithium niobate I. Transmission electron microscopy

Abstract Residual ferroelectric 180° domains in YZ-LiNbO3 wafers have been studied by transmission electron microscopy. In many cases domain walls lie pinned in the plane of a small-angle grain boundary while the other corresponding domain wall stays at a distance of about 20 μm even against an electric poling field. The diffraction contrast of domain walls and of grain boundary dislocations was investigated. Burgers vectors of 1/3[1101] and 1/3[01Tl1] have been identified by means of computer simulation of their contrast Taking into account the piezoelectric effect does not markedly change the simulated dislocation images but is responsible for inner electric fields around dislocations which determine the interaction of small-angle grain boundaries with ferroelectric domain walls. This might explain why the poling field which is usually applied after crystal growth is in many cases not sufficient to obtain single-domain YZ-LiNbO3.

Pinning of magnetic domain walls at dislocations and precipitates in Co5Sm crystals

Abstract A Lorentz–electron microscopic investigation of Co5Sm single crystals reveals that the influence of dislocations on the precipitation and on Bloch walls gives an important contribution to the coercive field in Co5Sm. No pinning of domain walls occurs at basal dislocations. The observed strong pinning at prismatic dislocations is due to long-range interactions and is interpreted in terms of a micromagnetic continuum theory. It is found that for prismatic a-edge dislocations the maximum interaction force on domain walls is about 54 dyn/cm. The deformation strains, which occur because of the coherent precipitation of Co7Sm2 and Co17Sm2, create an interaction force on domain walls of the order of magnitude 1 × 10−5 dyn for a precipitate at the initial stage of precipitation.