Structural change arising from correlated lattice and charge interaction in complex, multi-element, crystals has a profound effect on their physical properties. Examples include charge ordering in complex oxides, polarization nanodomains in ferroelectrics, and vortex matter. A common scheme to these systems is that there exists a distinction between local crystal symmetry and the average, macroscopic symmetry imposed by fluctuations in the crystal lattice. Before we can control and exploit these fluctuation-induced emergent properties, the crucial first step is to fully characterize any correlation that may exist.
This thesis explores the crystallographic aspect of local charge, polarization, and lattice interactions in complex, multi-element, crystals by developing scanning convergent beam electron diffraction (SCBED) based techniques. The applications of SCBED characterization demonstrated here include: ferroelectric BaTiO3 single crystal, relaxor-ferroelectric (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (x=0.08) single crystal, and multi-principal-element alloy Al0.1CrFeCoNi.
First, we show the local crystal symmetry and polarization fluctuations in BaTiO3 single crystals as determined using SCBED. An improved algorithm for CBED symmetry quantification is used to map the ferroelectric domains and local symmetry across the ferroelectric phase transition temperatures. The symmetry in BaTiO3 was found inhomogeneous; regions of a few tens of nanometers retaining almost perfect symmetry are interspersed in regions of lower symmetry. The SCBED results suggest the coexistence of displacive and order-disorder phase transition, affected upon by the local structure.
Next, we examine the local symmetry, polarization nanodomains, and the domain wall (DW) structures in relaxor-ferroelectrics. Nanometer-sized domains having the monoclinic Pm symmetry in PZN-8%PT single crystals are identified by performing SCBED along the [100], [001], and [111] zone axes. Intensity distribution in the (000) disks in the CBED patterns is used to determine lattice-rotation at the precision of ±0.012° by performing SCBED on a standard Si sample. A careful examination of the polarization DWs revealed the presence of lattice-rotation vortices of ~15nm in diameter in PZN-8%PT, which can be attributed to bound charge discontinuity and depolarization fields.
The lattice distortion effect in high entropy alloys (HEAs) is explored as a model of multi element alloys. Lattice distortion is one of the four core effects of HEAs, which results from different atom sizes and influences solid solution hardening. However, so far quantification of lattice distortion effects by X-ray and neutron diffraction has provided contradictory results. Using SCBED, we visualize the sub-nanometer strain fluctuations and local symmetry breaking in single phase Al0.1CrFeCoNi. Our results reveal 10±3nm, disc-shaped, clusters having ~7.1% tensile displacements along directions distributed throughout the specimen; local strain, on the contrary, was found to be fluctuating within ±1.3% and slow-varying over ~50nm. The observed inhomogeneous lattice distortion using scanning electron diffraction thus provides a new perspective on structure and property relations in multi-principal-element systems
